Chng 4. Khi nim c bn ca ha hc tinh th Trnh Hn Ngy Tuyt NhungC s ha hc tinh th NXB i hc quc gia H Ni 2006. Tr 69 140. T kho: Ha tinh th, lin kt trong tinh th, cc loi cht, lin kt ion, lin kt tn d, lin kt cng ha tr. Ti liu trong Th vin in t H Khoa hc T nhin c th c s dng cho mc ch hc tp v nghin cu c nhn. Nghim cm mi hnh thc sao chp, in n phc v cc mc ch khc nu khng c s chp thun ca nh xut bn v tc gi.
Mc lc
Chng 4 KHI NIM C BN CA HO HC TINH TH.............................3 4.1 NHNG YU T XC NH CU TRC TINH TH.................................3 4.1.1. 4.1.2. 4.1.3. 4.1.4. Cu hnh in t ca nguyn t ...............................................................3 Bn knh hiu dng ca nguyn t v ion ................................................4 S phi tr, a din phi tr v gii hn bn vng ca chng....................6 Tnh phn cc ca ion .............................................................................9
4.1.5. nh lut Goldschmidt ..........................................................................11 4.2 CC DNG LIN KT TRONG CU TRC TINH TH ...........................12 4.2.1 Lin kt ion...........................................................................................12 4.2.2 4.2.3 4.2.4 4.2.5 4.3 Nng lng mng ca tinh th ion.........................................................16 Lin kt kim loi ...................................................................................18 Lin kt cng ho tr .............................................................................20 Lin kt tn d ......................................................................................22
CC LOI CU TRC TINH TH TIU BIU..........................................24
4.3.1 Cch thc th hin loi cu trc.............................................................24 4.3.2 Phn loi cu trc tinh th .....................................................................26 4.4 KHI QUT V CC LOI CHT KHC NHAU.....................................33 4.4.1 Kim loi v hp kim..............................................................................33 4.4.2 4.4.3 4.4.4 4.4.5 4.4.6 4.4.7 Mt s hp cht hu c.........................................................................39 Sulfur v mui sulfur ............................................................................41 Halogenur .............................................................................................49 Oxit v hydroxit ....................................................................................53 Carbonat, sulfat v phosphat .................................................................65 Silicat v alumosilicat ...........................................................................71
3
Chng 4 KHI NIM C BN CA HO HC TINH THThnh tu thc nghim ngt mt th k qua chng t tnh ng n ca khi nim v nhm khng gian. Cho n nay cha tng gp tinh th no m khng thuc mt nhm im hay mt nhm khng gian no . Chnh mng khng gian vi nguyn t gp mt ni nt mng sinh ra mng tinh th. Chng ny s ni ti tp tnh ca nguyn t trong mng.
4.1
NHNG YU T XC NH CU TRC TINH TH.NH LUT GOLDSCHMIDT
Khi sp xp theo s ca mt loi mng, nguyn t mang theo cc bn sc ha hc ring. Khi l mt ion, n c th c ha tr no ; tng ng, n c mt bn knh xc nh, v thng mang dng cu. Hn na, n cn chu s chi phi ca nhiu yu t khc nhau khi nm trong trt t ca t chc mng.
4.1.1. Cu hnh in t ca nguyn tNh vt l hc an Mch N. Bohr (1913) pht trin nhng m hnh ph bin t trc bng vic a ra nhng m hnh h mt tri da trn hiu ng quang in. Trong m hnh ny Bohr gi nh rng in t chuyn ng quanh ht nhn trn nhng qu o (hay mc nng lng) trn vi bn knh tu thuc nng lng in t. Ht nhn nguyn t gm proton v neutron (ring hydro ch c 1 proton duy nht trong ht nhn). in t phn b trn cc v (lp) k hiu n, gi l s lng t chnh hay l K, L, M, N Chng hn, n = 1 (K), n = 2 (L), v.v Mc du vy, nguyn l bt nh tn ti trong th gii vi m (W.L. Heisenberg, 1927) cho thy i vi ht c bn nh in t khng th ni v qu o ch xc ca in t khi n chuyn ng quanh ht nhn. Nm 1926, E. Shrodinger tp hp nhng thnh tu c trc trong m hnh mi ca mnh v biu din n thnh mt phng trnh sng vi m hnh l thuyt xy dng trn c s c hc lng t. Bn phn b mt tn sut in t quanh ht nhn, tc l m my in t, c th xem nh bc nh mt in t tp hp t hng triu ln chp ti mt v tr xyz trong khng gian nguyn t. Vy, l thuyt sng ca Shrodinger miu t chuyn ng ca in t thng qua khi nim tn sut gp ca mt in t xc nh trong mt n v th tch (khng gian) nh b. v tr ca in t c xc nh trong khng gian ba chiu cn k n cc s lng t. S lng t chnh n l phng trnh ca khong r t in t ti ht nhn. S lng t l quy nh hnh dng vng khng gian (orbital) thng gp in t. Trng thi khc nhau ca l (0, 1, 2, n1) c k hiu ch tng ng; trong thng gp nht l cc ph lp.
0 s
1 p
2 d
3 f
Dng cu l orbital s, p l dng qu t v nh hng theo trc, cn d l dng orbital thay i. S lng t m hn ch hng ca in t c dng nhng s nguyn t 1 n +1. Khi l = 0 th m ch c mt gi tr m = 0, c ngha ph lp s ch c mt orbital. S lng t s quy nh hng xoay ca in t, bi v n ch c th xoay theo mt trong hai hng, nn s ch c hai gi tr 1 1 v + . in t xoay c xem nh thanh nam chm 2 2
thu nh, n sinh ra t trng khi va chuyn ng trong orbital va xoay quanh trc ca n. Thng thng, do in t xoay theo hng trong nguyn t, n c biu th bng mi tn; mi orbital c hai in t ng vi hai mi tn trong mi : hng ln trn (bn tri) v hng xung di (bn phi). Chng hn, in t duy nht ca orbital 1s ca nguyn t hydro biu th bng mt mi tn hng ln trn; 1s ca nguyn t hli c hai in t biu th bng mi tn hng ln trn v mi tn xung di.
4.1.2. Bn knh hiu dng ca nguyn t v ionKhi nguyn t thiu hoc tha in t th tr thnh ion mang in tch. Ion tch in m (anion) c s d in t, tc l in t vt s cn thit trung ho in tch ht nhn. Ion tch in dng (cation) do khng s in t cn thit trung ho in tch ht nhn. Nguyn t v ion c cu to khc nhau. Khi tp hp li thnh tinh th, chng c lc tng tc thay i. S sp xp ca chng trong khng gian tinh th cng ht sc a dng. V vy, cc cht vi thnh phn ho hc khc nhau thng c cu trc tinh th khng ging nhau. Mt trong nhng c s quan trng nht ca cu trc tinh th l khong cch gia cc nguyn t hay ion tri du k nhau. T , mi nguyn t hay ion trong tinh th lun c mt lc tc dng i vi mi nguyn t hay ion vy quanh. Mi ht c mt vng nh hng, mt cu tc dng quanh n. Bn knh ca cu tc dng y gi l bn knh (hiu dng) ca nguyn t hay ion. Xc nh bn knh nguyn t v ion. Ngay sau khi c nhng kt qu thc nghim v khong cch nguyn t ca nhiu cu trc tinh th, nhng c gng u tin c thc hin nhm xc nh bn knh nguyn t v ion. Phng php phn tch cu trc bng tia X cho nhiu s liu khong cch anioncation vi chnh xc cao, nhng khng th cho thng tin v bn knh ion. Chng hn, khng th gii trc tip n phng trnh vi n n s dng a) v b) di y. Trong khi ch cn bit bn knh ca mt ion, cng tm tt c cc gi tr cn li. Mi u, nhng gi tr tnh c cho bn knh ca ion chlor khng c cng nhn; l do khng lu s ph thuc ca bn knh vo trng thi in tch ca nguyn t (gi tr bn knh hu hiu ca nguyn t ghi trong ph lc 4). Chng hn, bn knh ca nguyn t natri ln hn bn knh ca ion natri Na+. Nm 1920, gi tr bn knh ca mt s ion c xc nh nh vic s dng cc hp cht vi cu trc cng loi NaCl. Trong s ny c hai cp hp cht vi khong cch nguyn t bng nhau:
5
Mg S Mg Se
2,60 2,73
Mn S Mn Se
2,59 2,73
a) b)
Mt khc, cn nh rng anion l ion d in t, nn thng c kch thc ln hn cation l ion thiu in t. i chiu cc gi tr trn c th thy khong cch ion khng i trong mi cp hp cht, d thnh phn cation thay i; chng t i lng ny hon ton ph thuc vo cc anion kch thc ln. Ch cn thay cc anion selen hay lu hunh bng anion oxy, s thy khong cch cation anion tng ln, nu magnesi thay bng mangan:Mg O 2,10 Mn O 2,24
Vic khong cch nguyn t bng nhau hay xp x bng nhau trong cc cu trc sulfur hay selenur ca magnesi v mangan cho php gi nh rng anion trong cc hp cht ny ging cc qu cu tip xc nhau. Bit rng khong OA trn hnh 4.1 bng hai ln bn knh ca anion, khong KA (tnh c khi bit ta xyz ion) bng mt na cnh hnh vung, d dng tm c bn knh ion: (OA)2=(OK)2+(KA)2=2(OK)2 T , bn knh ca anion lu hunh (2RS)2 = 2(2,60)2; tc l 1,93. T nhng gi tr bn knh anion va xc nh v khong cch nguyn t trong cc cu trc ca nhng hp cht tng ng c th ln lt tnh c bn knh cation:Mg S Mg O Mn O Ca O 2,60 2,10 2,24 2,38 Mg2+ O2 Mn2+
Hnh 4.1. Xc nh bn knh ion theo phng php Lande (1920)
2,60 2
= 1,83. Tng t, bn knh ca anion selen bng
2,73 2
=
0,77 1,33 0,91 1,05
Ca2+
Ph lc 3 lit k gi tr bn knh hu hiu ca cc ion (xp theo h thng tun hon cc nguyn t ho hc) ph bin trong th gii khong vt. Xt c im cc nguyn t v nhng bin thin ca bn knh ion lin quan vi nhng c im y, c th khng nh cc quy lut sau: - Theo chu k (hng ngang) ca h thng tun hon bn knh cc ion gim, trong khi ho tr ca chng tng, chng hn:Na+ RA () 0,98 Mg2+ 0,74 Al3+ 0,57 Si4+ 0,41
- Theo nhm (ct dc) s hiu cc nguyn t tng th bn knh ion tng theo, v d:
Li+ RA () 0,68
Na+ 0,98
K+ 1,33
Rb+ 1,49
Cs+ 1,65
Trong h lantan (cc nguyn t t him ho tr ba) vi s th t tng t 57 (lantan) n 71 (luteci) c hin tng gim bn knh ion: La3+ 1,04; Lu3+ 0,80. Hin tng ny nh hng ti gi tr bn knh ca hng lot kim loi khc. V d, n lm xut hin cc i nguyn t c gi tr bn knh ion bng nhau, mc du chng thuc hai chu k khc nhau: zirconi hafni, niobi tantal v.v... Hin tng gim bn knh cng bt gp nhm cc nguyn t cui ca h thng tun hon bt u t actini (s hiu 89). Nhng c im mang tnh quy lut va k lm xut hin kh nng: nguyn t ca mt s nguyn t c mt cng nhau trong mt v tr tinh th hc ca cu trc vt rn. y l cc nguyn t phn b dc ng cho t pha trn bn tri xung pha di bn phi ca bng tun hon (ph lc 3). Ni cch khc, chng c th c th thay th ng hnh cho nhau trong tng khong vt, khi hi nhng iu kin cn thit, trc ht l do chng c bn knh ion xp x bng nhau (xem chng 5). - Kch thc ion thuc cng mt nguyn t ho hc c th bin i tu thuc in tch ca n: cation vi in tch ln hn th c bn knh nh hn v ngc li, cation in tch cng nh thng c bn knh cng ln. V d:Ha tr Bn knh, Mno+ 1,29 Mn2+ 0,91 Mn3+ 0,70 Mn4+ 0,52 S2
So 1,04
S6+ 0,29
1,82
4.1.3. S phi tr, a din phi tr v gii hn bn vng ca chngS phi tr ca mt nguyn t trong cu trc tinh th n cht l s nguyn t gn nht vy quanh n. Trong tinh th ion, s phi tr ca ion l s ion tri du, gn nht vy quanh n. V d: trong cu trc tinh th halit mi ion natri c su ion chlor gn nht vy quanh. Ngc li, mi ion chlor nm gia su ion natri gn nht. Nh vy, trong khong vt ny c hai loi ion u cng c s phi tr su. Ion vi bn knh cng nh th c s phi tr cng thp: bn knh ion natri bng 0,98 th s phi tr ca n trong cu trc NaCl bng su; vi bn knh 0,79, ion km c s phi tr bng bn. C hai hp cht u c cng thc thuc dng AX, tc l mng ca chng cha hai loi nguyn t s lng nh nhau. Trong cc hp cht dng khc, mi loi nguyn t c s phi tr ring. Chng hn, trong cu trc tinh th fluorit CaF2 s phi tr cation calci bng tm, ca anion fluor bng bn. l hp cht dng AX2; cn hp cht dng A2X, chng hn argentit Ag2S c s phi tr bn ca cation bc, tm ca anion lu hunh. iu kin cn cu trc bn vng l cc qu cu ion ngc du phi tip xc nhau. Nu cu anion tip xc nhau, cn cu cation qu nh s dch chuyn t do trong khong trng gia cc anion, cu trc tr nn km bn vng (hnh 4.2). Trong trng hp ny, iu phi xy n l cc ion t phn b li, dn n cu trc vi s phi tr nh hn nhng bn vng.
7
Hnh 4.2 S bin i t cu trc bn vng vi s phi tr cao sang cu trc vi s phi tr thp hn
Nm 1922, gii hn bn vng cho cc s phi tr khc nhau c xc lp, cn c trn t s bn knh gia cation v anion. Di y, trn v d NaCl s din gii php tnh v gii hn bn vng cho mt s phi tr. Trong cu trc ny, ion natri c s phi tr su, ngha l su anion chlor nm trn nh ca hnh bt din, cation natri nm ti tm hnh. Hnh 4.1 l mt ct qua tm cc cation v anion. Nu gi RX l bn knh ca cation v RA l ca anion, th: 4RX = (2RX + 2RA) 2 hay l RA: RX = 2 1 = 0,41. Gi tr ny l gii hn di ca bn vng thuc s phi tr su. Trong trng hp anion c bn knh nh hn so vi cation, t l RA : RX = 1 : 0,41 = 2,41. y l gii hn trn ca s phi tr su. Nhng trong khong RA : RX t 0,41 n 2,41 cng c cc gii hn trn 1,37 v di 0,73 ca s phi tr 8. T l bn knh ion tng ng cc s phi tr v hnh phi tr khc nhau nh sau.RA : RX 0 0,15 6,45 S phi tr 2 3 4 6 8 Hnh phi tr Gnh cn Tam gic u T din u Bt din u Lp phng
0,15 0,22 4,45 6,45 0,22 0,41 2,41 4,45 0,41 0,73 1,37 2,41 0,73 1,37
Tuy vy, RA: RX ln hn 1 ch hin thc i vi s phi tr 8, v cation ln nht l Cs+ th bn knh ch t 1,65, cn anion vi kch thc nh nht l F th bn knh bng1,33.Trong trng hp ny, RA : RX = 1,24 v lot gi tr gii hn trn ca RA : RX khng tn ti i vi ion n (gm mt nguyn t). Chng c th c ngha no khi p dng cho ion phc nh [Co(NH3)6]3+ v.v (xem thm 4.1.5). Hy tham kho cc halogenur di y, s thy nhng gii hn hnh hc ghi bng trn xc thc n u.Hp cht RA: RX Hp cht RA: RX LiI 0,31 KCl 0,73 LiBr 0,35 NaF 0,74 LiCl 0,38 (CsI) 0,75 NaI 0,45 RbBr 0,76 NaBr 0,50 RbCl 0,82 LiF 0,51 (CsBr) 0,84 NaCl 0,54 (CsCl) 0,91 KI 0,60 KF 1,00 KBr 0,68 RbF 1,12 RbI 0,68 CsF 1,24
Phn ln cc hp cht trn u c cu trc loi halit vi s phi tr 6, tr ba hp cht trong ngoc c s phi tr 8. i vi chng, c ba u c RA : RX > 0,73, quy tc gii hn hnh hc c khng nh. T chui hp cht NaFCsF c th nhn thy khng nht thit cht kt tinh lun c c phi tr cao nht, ngay c khi chng c RA : RX ph hp. ng ra, i vi mi cht cc gii hn di v trn c bn cht khng nh nhau. Mt khi gii hn di (0,41 ca s phi tr 6) b vt qua, cu trc s km bn vng do anion tip xc nhau. Nu RA : RX > 0,73, s tip xc ion cha xy ra cho n khi RA : RX = 2,41, m trong khong 0,41 2,41 cn c cc gii hn bn vng ca s phi tr 8.
Chc chn, ch ring lun gii hnh hc s khng cn c cho phi tr mi, khi gi tr RA: RX t 0,73. Vn cn c soi xt di gic ni nng ca cu trc. Tuy vy, vic su hp cht bng trn vi loi cu trc halit u c RA: RX > 0,73 khng chng t s thiu cn c ca gii php hnh hc trong vic s phi tr ph thuc t s bn knh ion. Nh trn ni, cc trng hp ny khng ging cc ng cu trc khc ca NaCl l ba halogenur liti vi RA : RX < 0,41. C th tm hiu cc bit l ny khi kho st hin tng phn cc ca ion. a din phi tr ca mt cation l a din nhn (tm im) cation y lm tm v ly (tm im) cc anion vy quanh lm nh. Nh vy, y l a din tng tng, hnh thnh do ni tm im cc anion gn cation trung tm nht v vy quanh . Trong cu trc tinh th ca mui n (halit), a din phi tr c dng bt din u (hay hnh tm mt). C th ni, trong trng hp ny ion natri c phi tr bt din. Nu ni tm im ca bn nguyn t lu hunh phi tr quanh nguyn t km trong cu trc tinh th ca sulfur km, th a din phi tr c dng mt t din u (hnh bn mt). Khi lp phng ng vi s phi tr tm, hnh tm mt lp phng (hnh 4.3) ng vi s phi tr mi hai. Cn lu , mt s phi tr c th ng vi nhng a din phi tr khc nhau. Cu trc tinh th ca khong vt molybdenit MoS2 chng hn, vi s phi tr su nguyn t molybden ging natri trong mui n, n c hnh phi tr khng phi bt din, m lng tr ba phng (xem hnh 4.12). Khi lp phng dng Tomson cng c s phi tr 8 nh khi lp phng; hnh phi tr ny ch c mt cp mt vung song song.Sphi tr adin phi tr RA : RX
Hnh 4.3 a din phi tr v cc s phi tr tng ng
Khng th nhm ln a din phi tr vi a din tinh th. Hn na, khng phi mi a din phi
tr u c i xng tng ng vi 32 nhm im.
9
T tng ca Pauling v a din phi tr khng ch c ngha phng php lun: n c xem nh phng tin ph tr trong Roentgen tinh th hc. Lch s pht trin tinh th hc chng kin nhiu i thay trong nhn thc v tinh th. Nu lc u, tinh th c m t nh s xp cht ca nhng phn t dng a din (Hay R.Y., 1784), th v sau, n c xem nh h thng cc phn t xp theo trt t ca mng ba chiu gm cc nt im (Bravais A., nm 1866). By gi, tm im phn t a din phi tr l ni ca cation; ng vi chng l cc nt im ton hc thuc mng khng gian, cn cu anion th chia x v nhng a din (phi tr) khc nhau. iu ny gip m t v ti lp cu trc tinh th vi s h tr ca cc m hnh a din thng thng.
4.1.4. Tnh phn cc ca ionKhi tham gia thnh to tinh th ion thng c xem nh c dng cu. Nhng khi nm trong in trng ca cu trc tinh th, cu ion c th s bin dng. Di tc dng ca in trng y, cc in t ca ion, nht l in t vng ngoi gn kt yu, d dng b cun ht v mt pha. Ht nhn ca ion mang in dng th b cun ht v pha khc. Kt qu, trng tm ca in tch dng v ca in tch m, vn trng nhau trong cu ion, gi y tch ri nhau mt khong nht nh: ion tr nn mt lng cc. Hin tng ny gi l s phn cc ca ion. Nu gi l l khong cch gia hai in tch e m v dng ca ion, th el l momen lng cc: i lng biu th tnh phn cc ca ion. Momen lng cc t l thun vi hiu th ca trng in E: el = E vi l i lng khng i, th hin tnh b phn cc ca ion. Tnh cht ny mnh hn nhng nguyn t vi kch thc ln v vi lin kt yu trong v in t. Thng thng, bn ca lin kt in t xc nh ch yu trong cch gn kt nguyn t ca hp cht. Cho nn, nguyn t ca cng mt nguyn t ho hc s bc l tnh b phn cc khng ging nhau, khi chng tham gia to nn nhng hp cht khc nhau. V d: ion chlor trong NaCl b phn cc mnh hn so vi ion ny trong HCl. hiu r hn bn cht ca hin tng, hy a thm khi nim v nng lc gy phn cc ca ion ny i vi ion khc. S ph thuc ca tnh b phn cc v nng lc gy phn cc ca ion i vi bn knh ion Ri ca chng din bin r rt trong dy nguyn t kim loi kim:Li Ri () 0,69 0,06 1,7 Na 0,98 0,19 1,0 K 1,33 0,91 0,6 Rb 1,49 1,90 0,5 Cs 1.65 2,85 0,4
S liu dn cho thy, tnh b phn cc ca ion t l thun vi bn knh ca n, cn nng lc gy phn cc th t l nghch. Cation vi bn knh nh v in tch ln thng c kh nng gy phn cc mnh. Bn thn chng hu nh khng b phn cc. Cation vi 18 in t lp ngoi nh ng, bc, vng, km, cadimi, thu ngn cng c nhng c tnh y. Anion ho tr thp c tnh b phn cc mnh nht. i vi ion ca nhiu nhm khc, thng k di y cng cho thy quy lut y. Theo chiu dc t di ln v chiu ngang t tri sang phi: gi tr bn knh ion v tnh b phn cc gim, nng lc gy phn cc tng. Trong phn t, tc dng phn cc n phng lm hnh thnh lng cc v co rt khong cch gia cc ion. Trong cu trc tinh th cn tnh
n tc dng ng thi t cc ion phi tr i xng; chng gy phn cc t nhiu pha ln ion trung tm. Trong trng hp ny khng nht thit c lng cc hnh thnh, nhng nht nh khong cch ion phi co rt v do s phi tr thng gim theo. O2
He F Cl Br I Ne Ar Kr Xe
Li
+
Be
2+
B
3+
Na+ K+
Mg2+ Ca2+
Al3+ Sc Y+ La3+ 3+
S2 Se Te2 2
Rb+ Cs+
Sr2+ Ba2+
Nhn y, hy ni v s ph thuc ca khong cch AX i vi s phi tr. Hin tng phn cc lun c ch trong mi tinh th mc khc nhau. N xut hin mnh m nht trong phn t ring l, chng hn dng AX vi s phi tr 1. Khi s ny tng th c th thy trc s tng theo ca khong AX. Trong v d v hai bin th a hnh ca NH4Cl; khong cch (NH4)+Cl tng t 3,27 ln 3,35,v s phi tr t 6 tng thnh 8 (cu trc t loi NaCl i thnh loi CsCl). Cp hp cht sau cng cho thy s ph thuc y. Vi cu trc loi NaCl, SrO c s phi 6 ca Sr, th khong SrO bng 2,57. Cn SrZrO3 vi loi cu trc perovskit (CaTiO3 vi Ti nh lp phng, Ca tm, O trung im ca cnh) c phi tr 12 ca Sr, th khong SrO bng 2,89. Nh vy, nh hng co rt khong cch ca phn cc i vi tng ion cn ph thuc vo s phi tr ca n. iu ny gy kh khn cho vic nh gi nh hng ny i vi khong cch nguyn t. Bng sau y do Goldschmidt khi qut v s ph thuc ca khong cch ion vo s phi tr c th l mt gii php. S phi tr 6 ph bin nht trong s hp cht ion, cho nn khong AX i vi s phi tr 6 ly lm 100%; nhng phn ln kim loi li c s phi tr 12, v th khong AA i vi s phi tr 12 ly lm 100%.i vi tinh th ion S phi tr 12 8 6 4 AX () 112 103 100 94 i vi tinh th kim loi S phi tr 12 8 6 4 AA () 100 98 96 88
Ion kh b phn cc thng to thnh cu trc cc tinh th i xng cao; trong quanh cation/anion l cc anion/cation phi tr u n, nh vn thng thy trong loi cu trc NaCl. Nu thay ion natri bng ion bc gy phn cc mnh v chlor bng cc halogenur, d dng nhn thy s bin thin khong cch AgX do bc gy phn cc i vi halogenur:Hp cht AgF AgCl AgBr AgI AgX, 2,46 2,77 2,88 2,99 RAg+ RX, 2,46 2,94 3,09 3,33 Khong cch co rt, % 0 5,8 6,8 10,3 Loi cu trc Halit Halit Halit Sphalerit
11
Khong cch AgX co rt mc tng dn cho ti khi cu trc chuyn i ti AgI (vi RAg : RX = 0,51) t s phi tr 6 sang s phi tr 4. Khong cch khc thng ca cu trc cho php kt lun v s bin dng (do hin tng phn cc) ca v in t. Vai tr ca phn cc s tr nn rt ng k khi cc ht tng tc l ion gy phn cc mnh v ion d b phn cc. Kt qu nhn c l nhng nhm chc (radical) bn vng (CO3)2, (NO3), (SO4)2, (PO4)3, v.v .Trong cc hp cht carbonat v nitrat, khong cch nguyn t gia carbon/nit v oxygen ln lt bng 1,26 v 1,22, tc l nh hn c bn knh ion oxy (1,36). Lin kt ho hc trong cc nhm chc ny mnh ti mc chng khng b phn hu trong dung dch. Cu trc tinh th cho thy cc nhm chc c th sp xp xen k vi cc ion khc nh mt ht vt cht c lp. V d trong cu trc tinh th calcit CaCO3 (hnh 4.18). iu tng t c th ni v cc cu trc lp nh CdCl2, CdI2 v MoS2 (xem 4.3.2). Lp Cd/Mo nm xen gia hai lp I/Cl/S v gy phn cc mnh i vi chng. Lc lin kt bn trong tp ba lp ny mnh hn hn so vi lc gn cc tp ny vi nhau, ko theo tnh ct khai dc theo (0001) ca cc pha rn ny.
4.1.5. nh lut GoldschmidtNhng iu lun gii t u chng l nhm gii thch nhng yu t nh hng n cu trc tinh th. Theo Goldschmidt, cu trc tinh th xc nh bi s n v cu trc trong mng, tng quan kch thc gia cationanion v c tnh phn cc ca ion. Nn xem n v cu trc l nguyn t hay ion, i khi l nhm nguyn t, tc l phn t hay ion phc. Ion n trong cu trc tinh th l n v cu trc dng cu. Trong tinh th nhiu cht ion phc khng c dng cu. Trong s anion thng gp nhiu hn, nhng cation phc cng khng him. T gc ho hc tinh th ch nhng ion c i xng khc i xng ca hnh cu mi gi l ion phc. Ion n c th cha mt hay nhiu proton th hnh dng khng thay i so vi hnh cu; bi v proton c kch thc khng ng k. Chng hn ion OH v ion + O2 khng khc nhau my. Cc ion nh SH, SeH, TeH v NH2 cng nh cc ion NH4 v + PH4 u mang dng cu. Mc du chng c hnh vi hi khc trong mng tinh th v i xng ca chng suy gim trong vng nhit thp. Trong anion phc (cng gi l radical) lin kt gia nguyn t trung tm (nhn radical) v cc nguyn t bin c bn cht trung gian gia lin kt ion v lin kt cng ho tr. Gi d gia cc nguyn t ny ch c lc ht tnh in tc ng th kh c th ni v s hnh thnh radical. V d trong mng tinh th spinel Al2MgO4, nhm t din MgO4 khng phi l radical theo quan im ha hc: bn cht cng ho tr khng ng k trong lin kt gia Mg2+ v O2. Khng ging nh c tnh ca nhm SO2 , nhm MgO4 khng th di chuyn trn vn t 4 hp cht ny sang hp cht khc m khng bin i. Cc kt qu phn tch Fourier cho thy gia cc nguyn t lun duy tr mt in t kh ln khi chng lin kt nhau bng lc cng ho tr. iu ny ph hp vi lun thuyt Lewis, theo in t chung gia cc nguyn t to nn dng lin kt cng ho tr. ng quan tm trong s cc ion phc l radical n nhn. Trc ht l ion vi thnh phn AX3, trong X thng l oxy cng c th l lu hunh. Radical c dng tam gic u vi
Xem thm quy tc Pauling cui 4.2.1 v 5.5.9. Hiu ng chn ca ion.
2 nhn l nguyn t A. V d BO3 , CO3 v NO3 . Trong hng lot trng hp nguyn t nhn nh ln khi mt phng ca cc nguyn t oxy. Lc , ion c dng thp ba phng; l trng hp cc radical halogenat ClO3 , BrO3 v IO3 .
Radical AX4 thng to ion vung hay t din. Nhng radical t din nh SiO4 , PO3 , 4 4 2 SO4 , ClO4 v.v c vai tr ln trong t nhin. Nhiu t din ghp li thnh radical a nhn. Chng hn hai t din to nn radical nh pyrophosphat, pyrosulfat, v.v Gc ho tr SOS, POP, SiOSi thng c gi tr dao ng trong khong 110140 (Linton H., Truter M.R., 1961). Nu v s cc radical n nhn ghp li (trng hp) theo mt chiu khng gian, anion s n c dng chui. Radical hnh tam gic ca borat ghp li thnh chui metaborat (BO2 )n . Tng t, cc ion metasilicat, metaphosphat, v.v vi cc t din ghp li thnh chui vi cc chu k tun hon thay i, gm 2, 3, 4, t din. V d (Si2O6), (Si3O9), (Si4O12). Cc chui ny c th khp hai u li cho ion dng vng kn. V d vng ba Si3O6 , vng bn 9 4 P4 O12 . Tinh th lng tr su phng ca khong vt beryl Be3 Al2Si6O18 l silicat o vng (gm 6 t din vi nhn silic). Diopsit CaMg(SiO3 )2 cha radical chui t din silicoxy. S trng hp c th din bin theo hai chiu khng gian to nn cc radical dng l nh 4 Si4 O10 (mica, chlorit), hay ba chiu nh trong cc dioxit silic t nhin hay trong cc feldspat.
4.2
CC DNG LIN KT TRONG CU TRC TINH TH
Tinh th l nhng thc th bn vng do cc ht vt cht tng tc vi nhau m to dng nn. Trong tinh th ca cc cht khc nhau, lc gn kt cc nguyn t (phn t hay ion) vi nhau c th mang bn cht khng ging nhau. Lin kt trong cu trc tinh th c 4 dng chnh: lin kt ion l dng thnh hnh trong cc hp cht v c v khong vt (cn gi l hp cht ion), lin kt kim loi thng c trong tinh th kim loi v hp kim, lin kt cng ho tr gn cc nguyn t thnh phn t v thnh hp cht nguyn t, lin kt tn d gp ch yu trong hp cht hu c khi gn cc phn t vi nhau.
4.2.1 Lin kt ionLc tng tc tnh in gia cc ion mang in tri du lm nn lin kt ion. Chng hn, trong cu trc tinh th halit, lin kt ca anion chlor v cation natri c dng ion. Bi v v in t ca anion v cation u tch in m, nn c th gi nh rng trong hp cht loi ny, cc ht phi duy tr mt khong cch hp l, va nhm trnh mt lc y cho v in t ca hai ion. Nu coi ion nm trong trng thi tng tc, c phm vi nh hng vi bn knh hiu dng xc nh, th cu trc tinh th c xem nh khi cu xp cht. Tuy vy, lun gii v s phn b ion trong tinh th, cn lu khng ch yu t hnh hc thun tu, m c iu kin trung ho in tch tng th ca ton cu trc (xem li 4.1.4). Bn trong tinh th halit, vy quanh mi ion chlor ch c 6 ion natri, mc du tng quan kch thc ht cho php sp t
13
ti 12 cation xung quanh mi anion. Quanh mi cation natri cng c 6 anion chlor. S lng ion ho tr mt (Cl v Na+) bng nhau m bo trung ho in tch: trong iu kin hai ion tri du u cng mt s phi tr. Nu trong NaCl ion cc loi u c phi tr 6, th CsCl vi cng t lng 1 : 1 li c s phi tr chung l 8. T l bn knh gia cation v anion i khc. l nguyn nhn khin hai hp cht tng t nhau v bn cht ho hc ny kt tinh thnh hai loi cu trc khc nhau. Nu tng quan kch thc v nng lc ca ion l nhng yu t xc nh s phi tr trong cc hp cht n gin, th i vi loi hp cht phc tp phi p dng thm nhng quy tc khc (Pauling L., nm 1929). Quy tc Pauling a) a din phi tr ca cation do cc anion vy quanh n to nn. Khong cch cation anion o bng tng bn knh ca chng. S phi tr ca cation xc nh ch yu bng t l bn knh ca n vi anion .Cu trc tinh th c th xem nh s gn kt a din phi tr cc loi ca cation; trong , mi anion tr nn mt ht trung gian, ni cc a din y vi nhau. b) Quy tc ho tr tnh in. Trong cu trc tinh th bn vng, tng nng lng ca cc mi lin kt ni mt cation vi cc anion gn k ng bng in tch ca cation y. Lc lin kt tnh in (o bng gi tr tuyt i) l in tch ho tr ca ion (z) chia cho s phi tr (n) ca n, tc l z : n. Gi tr nhn c gi l ho tr tnh in, l s o lc lin kt m cc ion lin k nhn c t n. Trong NaCl chng hn, ion natri c 6 ion chlor vy quanh trong a din phi tr bt din v lc lin kt mi ion chlor nhn t n, ho tr tnh in, l 1/6. Nh vy, 6 mi lin kt ni ion Na+ trung tm vi 6 ion Cl gn nht quanh n hon ton trung ho in tch ca cation natri trung tm. Anion chlor cng c 6 cation natri gn k, cho nn ho tr tnh in ca mi mi lin kt cng bng 1/6. Nh th, in tch ca anion chlor c trung ho bi 6 mi lin kt ni ion trung tm vi 6 cation natri (phi tr bt din). Quy tc ho tr tnh in cho php nh gi bn cht lin kt a din phi tr trong cu trc tinh th. Trong cu trc bn vng, tng gi tr cc ho tr tnh in t cc cation trung tm ca a din phi tr ng bng in tch ca anion gia cc a din phi tr. Chng hn, trong cu trc tinh th grosular Ca3 Al2(SiO4)3, cation Ca2+ phi tr lp phng (s phi tr bng tm), Al3+ phi tr bt din (s phi tr bng su) v Si4+ phi tr t din (s phi tr bng bn). Nh th, cc lc lin kt m ion oxy nhn t cation Ca2+, Al3+ v Si4+ ln lt bng 2/8 = 1/4, 3/6 = 1/2 v 4/4 =1. trung ho in tch 2 ca n th ion oxy phi thuc v hai a din lp phng ca Ca2+, mt bt din ca Al3+ v mt t din ca Si4+ (2 1/4 + 1/2 +1 = 2). y l lin kt c s ca cc a din phi tr trong grosular. Tinh th c gi l ng cc, nu tt c cc mi lin kt ca n u c bn nh nhau. Khong vt thuc nhm spinel chng hn, c cng thc tng qut AB2O4, trong , A l cation ho tr hai nh Fe2+ hay Mg2+ v B l cation ho tr ba nh Al3+ hay Fe3+. Nhng hp cht loi ny tng c gi l aluminat v ferat. Danh php ny cho thy mt nhm chc 2 tn ti trong cu trc, v d CO3 . Trong thc t, cc d liu ca phng php nhiu x tia X chng t ion A phi tr bn v B phi tr su. T , lc lin kt oxy nhn t A (ho tr tnh in) bng 2/4 = 1/2 v t B l 3/6 = 1/2. Nh vy mi mi lin kt u cng mt bn vng (ho tr tnh in ca A v B u c chung mt gi tr l 1/2) v tinh th nhm ny l cc oxit ng cc.
Tham kho 4.1.5. nh lut Goldschmidt
Khi cation c nh, in tch cao, phi tr vi anion c ln, in tch thp, th nhm chc bn chc s hnh thnh. V d: carbonat v nitrat. Gi tr tnh c v bn ca cc lin kt bn trong cc nhm chc ny cho thy, ho tr tnh in tng lun ln hn mt na in tch ca anion. iu ny c ngha l trong nhm chc anion lin kt vi cation bng mt lc ln hn so vi lc lin kt m n c th c i vi cc cation khc. Chng hn, trong nhm carbonat, C4+ phi tr ba vi O2, do ho tr tnh in 4/3 ln hn mt na in tch ca anion oxy. l nhm chc hnh tam gic, n v cu trc c s ca khong vt carbonat. Mt v d khc l nhm chc sulfat, S6+ phi tr bn vi O2, ho tr tnh in 6/4 = 3/2 > 1. V gi tr ny ln hn mt na in tch ca anion oxy, sulfat to nn mt anion phc lin kt bn vng, trong oxy gn vi lu hunh mnh hn so vi mi lin kt gia n vi cc cation khc trong cu trc. y l n v cu trc hnh t din, nn tng ca mi hp cht sulfat. Nhng hp cht loi ny c cu trc khng ng cc/d cc. ng nhin, nu nhm chc c coi nh n v cu trc, trong cc hp cht loi ny tt c cc mi lin kt ca nhm chc vi cc cation u c chung bn vng, ging nh mi lin kt ca mt tinh th ng cc. Tng t, trong barit, mi mi lin kt ca Ba vi nhm chc SO2 u nh nhau v bn. Tinh th vn c gi l ng cc, mc d bn 4 cnh lin kt yu hn ca Ca v Ba vi cc nhm chc, cn c mt cc lin kt mnh CO v SO. Hy xt mt trng hp khc, trong lc lin kt gia cation trung tm vi cc anion phi tr ng bng mt na in tch ca anion. Ti y, mi anion c th lin kt vi mt cation khc bt k, cng vng chc nh lin kt vi cation trung tm. Cation khc ny c th l cng mt nguyn t. y l trng hp nhm chc ca silicat vi oxy nh t din v silic nm ti tm im. Lin kt gia silic v oxy c ho tr tnh in bng 4/4 = 1, ng mt na ho tr ca anion oxy. Nh th, t din SiO4 c th lin kt vi ion silic khc v nh th s c hai t din gn vi nhau, thng qua mt oxy chung, to nn nhm chc (Si2O7)6. Cng bng cch y, cc t din SiO4 c th gn kt (trng hp) vi nhau to thnh chui, lp hay khung, tu s lng anion oxy m mi t din gp vo trng hp l 2, 3 hay c 4. Chnh mc trng hp y lm nn s a dng ca silicat (xem hnh 4.36).
Tham kho 4.1.4.Tnh phn cc ca ion v 5.5.9.Hiu ng chn.
15
Hnh 4.4 T din (hng trn) v bt din (hng di) gn kt qua nh (a), qua cnh (b) v qua mt (c)
c) a din phi tr thng gn vi nhau qua nh chung. Nu cc a din gn kt qua cnh, qua mt chung, cu trc tr nn km bn vng. Bi v, lc ny khong cch cation cation co rt lm cho lc y gia chng tng. Bng sau cho thy, trn v d t din v bt din, khi chng gn nhau qua cnh hay qua mt chung, khong cch gia cc cation gim so vi khi a din gn nhau qua nh (xem thm hnh 4.4). Lc y cation cation pht sinh cng tr nn mnh m, nu cation c ho tr cao v bn knh nh. Chng hn, khi t din vi Si4+ ti tm; c bit nu t din gn kt qua cnh hay mt chung, khong cch gim ln lt cn 58% hay 33%.Khong cch cationcation a din phi tr T din Bt din a din chung nh 1 1 a din chung cnh 0,58 0,71 a din chung mt 0,33 0,58
d) Khi tinh th cha trong cu trc nhiu loi cation, nhng cation ho tr cao v s phi tr nh thng c xu hng khng gn cc a din phi tr vi nhau. Trong nhng trng hp khc, khi cc a din gn kt nhau qua cnh chung, cnh y d dng thu nh, in tch m dn ti ni y v cc cation th gin ra mt khong cch ln hn. Kt qu, lc y cationcation c khc phc mt phn.
Hnh 4.5 Cu trc tinh th amphibol mt nghing (C2/m) th hin bng a din phi tr t din T, bt din M1, M2, M4 v lp phng bin dng M4 (theo Papike J.J. v nnk, 1969)
e) Mt hp cht c th cha trong thnh phn nhiu cation a dng v ha hc, th mi nhm ca chng c th gm nhng cation ging nhau v mt tinh th hc. Bi vy, trong cu trc ca hp cht, cc v tr tinh th hc dnh cho chng ch l mt s hn ch. iu ny th hin trong cu trc amphibol (hnh 4.5). y, tuy thnh phn gm nguyn t nhiu nguyn t, cc v tr khc bit nhau v mt tinh th hc th hn ch v s lng. Mi v tr c th cha ion thuc cc nguyn t khc nhau. Hnh cho thy v tr t din T (T1 v T2), bt din M (M1, M2 v M3 c kch thc v hnh dng khc nhau cht t), v tr M4 (hnh phi tr khng u n ca s.p.t. 8), v tr A vi kch thc ln, m hnh phi tr khng u n v v tr ca nhm OH. V tr t din l ca Si4+ v Al3+. V tr M1, M2 v M3 dnh cho Mg2+, Fe2+, Mn2+, Al3+, Fe3+ v Ti4+. V tr M4 c th do Ca2+ v Na+ chim. V tr A c th l ch ca Na+ hay K+. Cn nhm OH cng chung mt v tr vi Cl hay F. Nh vy, 13 ion thuc nhiu nguyn t phn b trong 5 v tr tinh th hc khc nhau [gm t din, bt din, lp phng bin dng, v tr A rng ln v v tr OH]. Tnh trng hn ch v v tr tinh th hc cn biu hin s loi anion tham gia phi tr cation. Su anion thuc phi tr bt din thng ch c hai loi (4 anion oxy v 2 anion hydroxil).
4.2.2 Nng lng mng ca tinh th ionKhi nim nng lng mng ng vai tr ng k trong vic gii thch nhiu c tnh ca tinh th ion. Nng lng ny l hm ca in tch cc ion tham gia vo cu trc, ca gi tr bn knh ca chng v o bng cng cn thit phn hu 1 gam mol ca cht, tc l bin n thnh nhng ion ring l. C th xc nh nng lng mng bng phng php thc nghim. Nng lng tnh c chnh l nng lng c gii phng trong mt lot qu trnh ni tip nhm hnh thnh tinh th: bin nguyn t t do thnh ion ri tp hp chng thnh cu trc tinh th. V nguyn l kt tinh l qu trnh to nhit; cc ion t do vn c nng lng d tr; khi kt tinh th nng lng
17
y c gii phng. Chng hn, khi cho kh chlor tc dng ln kim loi natri hnh thnh tinh th NaCl, nng lng gii phng l 97,7kcal/gam mol. Lng nhit ny khng ph thuc cch thc hnh thnh tinh th. Qu trnh ny c th tri qua my giai on: to kh natri, bin natri nguyn t thnh ion Na+, bin chlor phn t thnh chlor nguyn t, ri chlor nguyn t thnh Cl. Hai ion tri du ny tng tc cho NaCl s gii phng lng nhit tng ng. Theo Born M. v Haber F. (1919), lot phn ng trn c vit di dng sau:U=+J+ D E+Q 2
vi l nhit thng hoa kim loi, J th nng ion ho kim loi, D nhit phn li phn t phi kim, E nng lng do phi kim thu in t, Q nhit gii phng do phn ng. Phng php thc nghim ny cho php xc nh nng lng mng ca hp cht n gin vi chnh xc t 5%. Nng lng mng cng c xc nh bng phng php ca Born M. v Lande A.(1918), cn c vo lc tng tc ca cc ion tch in tri du. Lc ny tnh bng in tch ion v bng khong cch r gia chngK= z1ez2e r2
vi z1e v z2e l in tch ca cc ion tri du to ra tinh th (e l in tch in t, z1 v z2 l ho tr nguyn t cng o bng gi tr tuyt i). Nu khong r ln v hn th lc K bng khng. khong cch gii hn lc tng tc gia cc ion tri du mang du m, ng vi lc ht; ion tip cn nhau n mt khong cho php ng vi trng thi lin kt bn vng. Lc mang du dng l lc y gia hai ion tch in cng du v khng mt hp cht bn vng no hnh thnh khong r bt k no. Nh vy, hp cht bn vng ng vi tng nng lng mang du m, y chnh l iu kin hnh thnh tinh th ion. Nng lng mng l nng lng tng tc ca mi ion trong mng v c gii thch nh sau. N bng cng sn sinh khi cc ion t v cc tip cn nhau sp xp thnh cu trc tinh th. Cng tnh bng lc K khng i nhn cho on ng. Khi lc bin i cng vi khong cch th h thc tnh cng c dng tch phn. Nh vy, cng sinh ra khi hai ion tip cn nhau n khong R bng:U= z1z2e2 R
hay i vi ng lng phn t gam N ca hp cht:U=N z1z2e2 R
Bi v ion sp xp khng ging nhau trong nhng cht khc nhau v trong tinh th khng ch c lc ht m cn c lc y gia cc ion, nn nng lng mng tnh theo cng thc sau:U=N az2e2 n 1 R n
vi a l hng s Madelung ph thuc loi cu trc; cn n l i lng lin quan n tnh n hi ca tinh th.
Capustinsky ch ra rng, tnh ton nng lng mng gn ng cho mt cht, c th khng cn xc nh hng s Madelung a; v hng s ny t l thun vi s ion trong n v cng thc n. i vi hp cht n gin, ng a ra mt cng thc gn ng:U = 256,1 m.z1z2 rA + rX
trong , rA + rX l tng bn knh cc ion. Nm 1943, cng thc ny c ng sa li v c dng sau:U = 256,1 m.z1z2 rA + rX 0,345 1 rA + rX
4.2.3 Lin kt kim loiKim loi c nhng c tnh nh khng thu quang, nh kim, dn nhit, dn in v.v... iu ny l do nguyn t to nn chng bng dng lin kt c bit, gi l lin kt kim loi. Ngay t nm 1900, Drude P. gi nh rng nhng c tnh ny lin quan ti cc in t t do c mt trong tinh th. Chng bt khi cc nguyn t kim loi v chuyn ng trong khng gian gia cc ht mang in t dng. Lc lin kt phn b u n theo mi hng (c i xng ca hnh cu). Khc vi tinh th ion vi s lng xc nh ca cation v anion do nhu cu trung ho in tch, trong cu trc kim loi vn c s trung ho gia in tch dng ca nguyn t v in tch m ca in t chuyn ng xung quanh. V vy, hp cht to nn t nhng kim loi khc nhau thng khng tun theo quy tc ho tr m ch ph thuc nhng yu t hnh hc. Trong tng nguyn t kim loi in t chim nhng mc nng lng xc nh, nhng trong tinh th kim loi c s chuyn ng t do ca in t t nguyn t ny sang nguyn t kia. Tc dng ca lc lin kt kim loi vi i xng dng cu to cho nguyn t kim loi mt xu hng c trng: chng sp xp thnh tinh th theo nguyn l xp cu cht nht, tc l vi phi tr ti a l 12. Kim loi c ba loi cu trc c trng:
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Xp cu theo lut lp phng vi phi tr 12. Xp cu theo lut su phng vi phi tr 12. Xp cu theo s mng tm khi lp phng vi s phi tr 8.
* Hai lut xp cu c bn Nu xp cc qu cu cng bn knh trn mt phng, sao cho chng u tip xc nhau (cht nht), th mi qu cu s c 6 qu khc vy quanh (hnh 4.6,a). Hy xp kht hai lp cu ny ln nhau c hnh 4.6,b. Lp th ba c th xp chng kht ln hai lp kia theo hai cch khc nhau. Tm ca qu cu lp th ba nm ngay trn tm cu lp mt (xem li hnh va dn). Chng cng c th nm ti v tr trung gian: di mi qu cu ca lp trn khng c cc qu cu ca lp di (hnh 4.6,c). Trong trng hp u, tun t cc lp c th biu din di dng cng thc sau: ABABAB y l lut xp cu su phng, vi A l lp 1, B l lp 2. Lp th 3, th 5, th 7 v.v... u lp li lp th nht. Lp th t, th su, th tm v.v... u lp li lp th hai. Trng hp hai s c cng thc vi dng: ABCABCABC Trong tun t ny, tm cu lp bn nm i lp vi tm cu lp mt, lp nm i lp vi lp hai, v.v... y l lut xp cu lp phng. Hai lut chnh ny u c cht cao nht: 74,05%. Loi cu trc th ba ca kim loi tng ng vi mng lp phng tm khi c cht ch t 68,1% . c tnh ca tinh th kim loi nh nh kim, dn nhit, dn in v.v..., u do khi in t t do phn b gia cc ht nhn nguyn t, chng duy tr trng thi chuyn ng c trong th nng chy. i vi nhng tinh th c cu trc cng loi v c cng s in t ho tr, khong cch nguyn t cng ln th im nng chy v im si cng thp, h s n nhit v h s n hi cng cao. Trong s nhng kim loi cng cu trc v khong cch nguyn t xp x bng nhau, kim loi no c s in t ho tr ln th lc lin kt ca n cng mnh; im nng chy, im si v cng cao; h s n nhit v h s n hi thp. V d: cu trc ca kim loi Mg v Zr l lut xp cu su phng vi khong cch nguyn t 3,2; nhng s in t ho tr ln lt bng 2 v 4, cho nn chng khc xa nhau v nhit nng chy (657oC v
Nguyn l xp cu cn p dng m t cu trc vi cc n v cu trc khng c dng cu. Tham
kho 4.4.2. Mt s hp cht hu c.
1860oC). H s n nhit v h s n hi ca Zr nh bng mt na ca Mg, cng ca Zr ln gp 3 ln cng ca Mg. Kim loi lun c tnh do, tc l kh nng dch chuyn ca mt phn tinh th so vi phn kia. D dng v thun li l dch chuyn song song vi phng ca mt cc lp cu. Trong lut xp cu su phng, cc lp song song vi mt (0001), tc l vung gc vi trc chnh ca tinh th. Trong lut xp cu lp phng c ti bn hng phn lp. S dch chuyn xy ra theo hng vung gc vi mt trong bn trc bc ba (hay vi mt trong bn ng cho ca khi lp phng). Kim loi thuc lut xp cu su phng ch vi mt hng phn lp c do thp hn so vi kim loi thuc lut xp cu lp phng (tham kho 5.5.4). c im ca dng lin kt kim loi dn n s a dng ca hp kim. Chng hn, vng v bc c cng loi cu trc theo lut xp cu lp phng v cng gi tr bn knh nguyn t. Bi vy, chng c th thay th lin tc cho nhau trong cu trc tinh th. iu ny cng l nt c trng cho cc cp kim loi sau y: molybden wolfram v paladi platin. Vi bn knh chnh lch 12,5%, cc nguyn t ng v vng thng khng th thay th nhau mi t l. Trong nhng iu kin nht nh, chng c th to thnh cc hp kim vi nhng t lng khc nhau. Hnh 4.7 l nhng v d ca cu trc hp kim loi ny vi t l Cu : Au l 3 : 1 v 1 : 1.
4.2.4 Lin kt cng ho trTrong nhng hp cht ion, nguyn t ca cc nguyn t khc xa nhau v bn cht ho hc. Khi to thnh hp cht, mt s d dng mt mt hay nhiu in t ho tr ca chng. S khc li d dng thu nhn nhng in t . Trong hp cht nguyn t (hp cht vi dng lin kt cng ho tr) xu hng nhng v xu hng thu in t ca mi nguyn t th hin gn nh nhau. Khi nm ti nhng v tr n nh trong mng tinh th, cc nguyn t phi c lp v ngoi cng bo ho in t.H H H C H C H H
Mt lot nguyn t c kh nng t ti lp v n nh trn bng cch dng chung mt hay nhiu i in t. l nhng nguyn t thuc nguyn t ging nhau hoc gn nhau v tnh in m (ph lc 4), tc l khng khc bit nhau v bn cht ho hc. Trong cng thc phn t ca cc cht loi ny, mi mi lin kt cng ho tr tng ng mt i in t dng chung. V d: H2, CCl4, NH3, CH4, C2H4. Mi cp in t dng chung cng thng biu hin bng mt nt gch.
21
Mt trong nhng iu ng k, lm cho lin kt cng ho tr khc hn lin kt ion, l ch mi nguyn t trong cu trc ch to mt s hn ch cc mi lin kt quanh n. V d, trong trng hp hydro, mi nguyn t ch lin kt vi nguyn t th hai cng loi; n khng thu nhn thm nguyn t th ba no khc. Trong cc cht loi ny, mi nguyn t ch to thnh s mi lin kt k ti a l: k=8N (N l s hiu nhm ca bng tun hon cc nguyn t ho hc). Chng thuc nhng nguyn t ca cc phn nhm ph vi N bng 4, 5, 6, 7 (xem thm ph lc 2). Theo quy tc ny, d dng nhn bit c im cu trc tinh th ca nhiu n cht. Chng hn cu trc tinh th hydro v cc halogen (nhm 7) gm nhng phn t ch cha hai nguyn t: s mi lin kt bng 8 7 = 1. Trong cu trc tinh th ca cc nguyn t thuc nhm ph su, nhng phn t ca chng thng c dng vng kn hay dng mch (hnh 4.8). V d: mi nguyn t lu hunh hay selen ch to thnh hai mi lin kt cng ho tr. Tng t, mi nguyn t thuc phn nhm ph bn (C, Si, Ge, Sn) u c kh nng xy dng bn mi lin kt cng ho tr vi bn nguyn t cng loi. Do lin kt cng ho tr u c hng v to vi nhau nhng gc vi gi tr nht nh, nguyn t lin kt vi nhau bng lc cng ho tr thng c s phi tr thp. Ring carbon trong kim cng (hnh 4.9), cng nh lu hunh hay km trong sphalerit, 4 mi lin kt ca mi nguyn t u c hng xc nh: y l lin kt nh hng; gc gia chng bng 109o28. Nhng nu trong trng hp nh lu hunh (hnh 4.8) lin kt cng ho tr thc hin bng s giao ho ca cc in t ho tr (c thn trong orbital) thuc phn lp 3p4 vi nhau, th trong nhiu trng hp khc in t giao ho li thuc cc phn lp khc nhau (sp, dp, sp2, dp2). Trong trng hp ny, ngay trc khi xy ra lin kt thng c hin tng phn b li in t gia cc phn lp. Tr li vi carbon, cu hnh in t ca n:1s2
2s
2
2p
2
Bnh thng, vi 2 in t c thn phn lp 2p , nguyn t ch to 2 mi lin kt cng ho tr vi ho tr 2, v d trong CO. Nhng, ho tr 4 vn thng gp carbon: trc khi ho hp, nguyn t phn b li in t, chuyn chng vo trng thi kch thch. Mt trong 2 in t ca phn lp 2s2 phi chuyn vo orbital trng ca phn lp 2p2. Cu hnh in t ca carbon l 1s2 2s2 2p2 v sau khi lai ghp tr thnh:2
1s
2
2(sp )
3
2(sp )
3
2(sp )
3
2(sp ) Hnh 4.9. Cu trc tinh th kim cng
3
Nhng orbital lai ghp 2(sp3) ny ca nguyn t to vi nhau mt gc xc nh v hng v pha cc nh ca t din u vi tm l nguyn t carbon. Chng xen ph cc
orbital tng ng ca nguyn t C khc to nn tinh th kim cng. Khong cch CC rt ngn, gi tr ca n (1,444) thng nh hn khong cch tng ng ca lin kt ion. Chng hn, i vi sphalerit, to thnh bn mi lin kt, nguyn t lu hunh b ra su in t, nguyn t km ch b ra hai in t. Cho nn hai trong bn mi lin kt s th hin bng mi tn thay cho nt gch. Cu trc kim cng (hnh 4.9, 4.14), cng nh cu trc sphalerit (gn nh cng loi cu trc) u c nt chung l mi nguyn t c bn nguyn t khc vy quanh; trong trng hp kim cng, mi nguyn t b ra bn in t v thu v bn: cp in t ca mi mi lin kt l thuc hai nguyn t khc nhau. Tuy nhin, to thnh mi lin kt cng ho tr, hai in t ca n khng nht thit phi xut pht t cc nguyn t khc nhau.\ C \ \ C \ Zn \ S \
Tu ln ca hiu gi tr in m (ph lc 4) ca chng, hai nguyn t A v X c th cho mt hp cht AmXn bng dng lin kt vi bn cht tri ca mt trong hai dng l ion hay cng ho tr. Nhiu tc gi nh Pauling, Povarennykh v.v..., m t tng quan trn bng th (hnh 4.10). Kho st trng thi lin kt ho hc trong s ln hp cht t nhin cho thy: Hnh 4.10. ng cong ph thuc ca t l (%) lin kt ion vo hiu gi tr in m ca anion vad cation trong cu trc
Tt c cc fluorur u c lin kt gn nh thun tu ion. Lin kt cng ho tr ch mc t 2% trong KF n 20% trong AlF3. Cc chlorur kim loi kim c t 27% n 33% bn cht cng ho tr trong lin kt. Phn ln cc oxit c lin kt ch yu dng ion, tr thch anh SiO2 v pyrolusit MnO2. Hai cht khong vt ny c lin kt cng ho tr tri hn: ln lt bng 54% v 65%. Sulfur ch yu l nhng hp cht nguyn t: lin kt cng ho tr mc t 68% (trong argentit Ag2S) n 88% (trong molybdenit MoS2). Selenur, tellurur, arsenur, antimonur c lin kt vi bn cht cng ho tr cn cao hn.
4.2.5 Lin kt tn dTrong s n cht, ch c cc nguyn t nhm 8 ca h thng tun hon (He, Ne, Ar, Kr, Xe) mi kt tinh bng lc lin kt tn d. Lc tn d hay van der Waals rt yu ny lin kt cc nguyn t bo ho in t lp ngoi cng. N cng l lc gn kt cc phn t vi nhau (ch : lc lin kt cc nguyn t bn trong phn t li c bn cht khc, lin kt ion
23
hoc cng ho tr). Mt s kh ln cht kt tinh hnh thnh nh lin kt tn d: l hu ht cc cht hu c, hydro, oxy Vi lc lin kt van der Waals, ht (nguyn t kh tr, phn t, v.v...) c khuynh hng tp hp quanh n s lng ln nht nhng ht cng loi to thnh tinh th. V vy, y l dng lin kt khng c hng v khng bo ho. So vi lc bn trong (gn kt cc nguyn t) tng phn t, lc van der Waals nh hn nhiu ln. Nhng chnh dng lin kt ny l nguyn nhn ca nhiu tnh cht vt l quan trng ca vt rn loi ny: im nng chy, cng, h s gin n nhit, v.v... Chng thng c im nng chy thp, cng nh v gin n nhit ng k. Bng cch so snh nhit nng thng hoa ca mt hp cht phn t vi nng lng phn li ca phn t cht , d dng nhn thy lc tn d khng ng k so vi lc lin kt khc. Chng hn, hydro rn c nng lng thng hoa bng 0,5kcal/mol, trong khi nng lng phn li ca phn t hydro khong 100kcal/mol. Nu em nng lng mng ca mt s cht loi ny so snh vi nng lng mng ca cc hp cht ion, th s thy s khc nhau cng ln. Sau y gii thiu vi nt s lc v mt dng lin kt, trung gian gia cc lin kt tn d v ion. l lin kt hydro. Nhng bc x cc mnh thng dng xc nh cu trc tinh th cng khng gip pht hin nguyn t hydro, do nng lc nhiu x nh yu ca n. Ch c th on bit lin kt hydro bng cc du hiu sau: Nu hai nguyn t mang in m ni vi nhau bng mi lin kt hydro (tc l c nguyn t hydro ng gia) th khong cch gia chng thng nh i mt cch khc thng. Gc HOH ca lin kt hydro bng 109o.
Nng lng ca lin kt hydro dao ng trong khong kh ln: 5 10kcal/mol; khong cch nguyn t (OO chng hn) thay i t 2,54 (trong KH2PO4) n 2,76 (trong nc ). Khong cch cng nh lin kt cng mnh. Nng lng lin kt hydro nh hn nng lng lin kt cng ho tr hay ion 10 ln nhng li 10 ln ln hn nng lng lin kt tn d. Khi gc lin kt bng 109o, dng lin kt hydro c v ging lin kt cng ho tr. Nhng lin kt hydro y li ging lin kt ion. Nguyn t H mt in t duy nht bin thnh proton, c xu hng thu ht mt lng ti a cc ion tri du, nhng ch cn hai nguyn t mang in m (oxy chng hn), H+ qu nh b b ph hon ton. Trong thc t, khong H O thng c coi l c nh (khong bng bn knh ca O2). Tuy nhin, kt qu nghin cu bng ph hng ngoi cho thy khong cch HO trong mt phn t nc bit lp bng 0,8 (ngn hn 0,4 so vi khong cch bnh thng). Khi kt tinh thnh mng nc , proton (cc dng) ca phn t nc ny ht oxy (cc m) ca phn t nc kia. Lc ht nh hn lc ht tnh in gia cc ion trong phn t nc . V vy, khong cch gia hai nguyn t oxy trong nc kh ln. Cho nn khng th ni proton nm cch u hai nguyn t oxy; mi lin kt hydro khng i xng. Trong trng hp nc , proton c th khng dnh lu hn vi mt nguyn t oxy no, m lun chuyn t nguyn t ny sang nguyn t kia. Xung quanh mi phn t nc c bn mi lin kt hydro, tc l mi phn t b vy quanh bi bn phn t nc khc. Sau khi xt nhng dng lin kt ho l ch yu c th nhn thy rng mt hp cht bt k khng ch c mt dng lin kt n thun. Lin kt ha l trong tinh th thc thng c tnh cht trung gian gia hai, ba dng v ch t nhiu c bn cht ca mt trong nhng dng lin
kt k trn. Hin nay ngi ta cha c mt phng php hu hiu no biu din chnh xc mt dng lin kt thc. Nh th, chng hn mt hp cht c gi l ion nu dng lin kt ca n c tnh cht nghing v dng ion. Cng v l do mi s phn loi cc cht da trn c s lin kt ho l u khng ng vng c. Tuy nhin, iu khng lm gim ngha to ln ca lun thuyt v lin kt ho l i vi ho hc tinh th.
4.3
CC LOI CU TRC TINH TH TIU BIU
i tng xem xt di y khng phi cu trc tinh th ca tng cht c th. Trong h thng kho st y, loi cu trc tp hp cc cht khc nhau v ho hc, v dng lin kt trong mng, nhng cng chung mt trt t phn b ht vt cht. Mt trong nhng cht ng cu trc c bit n sm nht s dng gi tn cho c loi cu trc. ng nhin, loi cu trc khng bao hm chi tit c th; nh kch thc mng chng hn, v iu ch trng l cch sp xp ca ht trong phm vi mng c s. i khi, loi cu trc cn bao hm nhng cu trc gn ging, sai khc t nhiu v i xng.
4.3.1 Cch thc th hin loi cu trcMi cch th hin cu trc c u im ring, c hiu qu truyn t ring ty trng hp. C 4 cch th hin cu trc (u l s ca c s) sau y: a) H thng nt im, ng hn l chm m hay khuyn trn vi tm im ch th nhn ca ht vt cht. M hnh cu trc kim cng nh trn hnh 4.9 l v d. Nu cch n gin ny cn hiu qu i vi cu trc ca n cht, th i vi hp cht tng i phc tp, n khng gip phn bit cc loi ht vt cht vi bn knh khc nhau chng hn. b) H thng cc qu cu kch thc thay i (hnh 4.22), tu thuc bn knh ht vt cht cc loi. Nu cn, s dng cc mu sc phn bit loi ht. Thng thng, mu dng cho oxy, vng m cho silic v.v... Cch ny cng c nhc im ring, nht l khi hp cht c nhiu loi kch thc ht; ht nh b thng b khut sau ht ln. Trong hai cch trn, cc ht thng ni nhau bng ng lin hay ng t, m nht khc nhau, nhm tng hiu qu th hin hnh khi v biu din mi lin kt ha l. c) H thng cc a din phi tr: cu trc tinh th phc tp thng biu din di dng a din phi tr cc loi, thng gp l cc hnh bt din u v t din u. Trong khi cu anion xp cht, ng vi n qu cu anion l n khoang bt din v 2n khoang t din; chng c th cha cu cation bn knh ln lt l 0,41 v 0,22
25
(coi bn knh anion l n v). Hnh 4.11 cho thy cc khoang t din v bt din sp xp thnh chui (song song trc chnh) theo quy lut ca 2 lut xp cu. Nu trong xp cu su phng (hnh 4.11,a) tng cp chui khoang t din nm xen k gia cc chui khoang bt din, th xp cu lp phng (hnh b) s xen k 2 : 1 ny cho thy ngay trong tng chui. Theo quy tc ha tr tnh in, cc cation c khc nhau s khng chim ht s khoang trng y, chng chim tng phn theo t lng quy nh trong thnh phn ho hc v theo nhng s ring. Nhng a din phi tr cha cation ny s gn vi nhau to thnh s cu trc cho mi cht. Ngoi nhng a din thng thng, cu trc c th c nhng a din khc. V d hnh lp phng, hnh tm mt lp phng v.v... Hnh 4.12 gii thiu m hnh cu trc ca khong vt molybdenit MoS2 xy dng bng cc khi lng tr ba phng, chng l a din phi tr ca nguyn t molybden (xen gia cc lp ny l ny l lp bt din trng).
Hnh 4.12 Cu trc tinh th ca khong vt molybdenit MoS2 th hin bng lng tra din phi tr ca nguyn t Mo (theo Belov N.V., nm 1947)
i vi nhiu trng hp n gin, din t hiu qu mt loi cu trc, ch cn s dng vi ba cu n gin (xem 4.3.2), m khng nht thit nh n hnh v. d) Cch y khng lu, nhu cu th hin cu trc tinh th trn mt phng lm xut hin hnh chiu c s ca n. Mt chiu ny (l mt hnh v) i qua tm ca c s. Trn , cc vng trn c tm ng vi v tr nguyn t. ng knh vng trn thay i, t l thun vi khong cch t tm nguyn t n mt chiu. Nguyn t nm pha trn mt chiu th hin bng vng m, nguyn t pha di bng vng thanh nt. Nhng ht nm ti mt chiu th biu din bng vng trn nh hay bng mt nt im.
Hnh 4.13 Nguyn t A, B, C, D th hin bng khuyn trn trn mt chiu nm ngang
Hnh 4.13 cho bit cch th hin nguyn t A, B, C, D bng vng trn (hnh bn phi) mt chiu dng song song vi cc cnh y ca t din u ABCD v i qua tm ca n (hnh bn tri). Nguyn t cch u mt chiu AA = BB = CC = DD, cho nn cc vng trn ng vi bn nguyn t cng bng nhau, i vng AB m hn CD, v chng pha trn mt chiu, v d: cu trc kim cng (hnh 4.14). Nguyn t carbon nh lp phng v tm hai y th hin bng tm ca 8 vng trn ln (hnh bn phi). Bn nguyn t ti tm cc mt bn ng vi 4 vng trn nh; tm ca chng nm trn mt chiu. Bn nguyn t cn li nm trn trong lng ; 2 pha trn, 2 pha di mt chiu v cng cch u mt chiu mt
khong bng 1/4 cnh . Ti hnh bn phi chng l bn vng trn vi ng knh bng 1/2 ca vng ln
Hnh 4.14 Cu trc tinh th kim cng Bn tri: m hnh mng, bn phi: th hin bng khuyn trn trn mt chiu nm ngang
4.3.2 Phn loi cu trc tinh thCu trc tinh th c th phn loi theo hp cht ho hc. n cht kim loi v phi kim hp thnh nhm A. Nhm B bao gm cc hp cht gm hai thnh phn hoc phc tp hn, cng c th cha nhm chc. Chng hn, cu trc ca hp cht dng AX thuc nhm B, ca hp cht AX2 thuc nhm C, cu trc hp cht dng AnXm thuc nhm D. Nhng hp cht c hn hai loi nguyn t m khng cha nhm chc th thuc nhm E. Cha nhm chc khc nhau l cu trc thuc cc nhm F, G, H, K; cu trc silicat thuc nhm S. H thng phn loi ny c nhc im ring; nhng hp cht khc nhau v loi cu trc v v tnh cht th tp hp trong mt nhm. Mt nguyn tc phn loi khc da vo dng lin kt ho hc: cu trc vi lin kt kim loi, cu trc vi lin kt cng ho tr, v.v... Nhc im ca h thng ny rt r: phn ln hp cht c trng bng lin kt trung gian. By gi, nu lu mi tng quan ngc gia kch thc ca c s v dng quen ca tinh th (xem li mc 3.3.4), th c th xp cc cu trc tinh th vo ba nhm chnh: Cu trc ng thc; Cu trc lp v dng lp; Cu trc chui v dng chui.
Nhm mt tp hp nhng cu trc h lp phng v nhng cu trc cc hng khc vi tc b a c hay hay hay xp x bng 1 (tc l c cc thng s cnh gn l a a b a c b c + + + 2 2 2 2 2 2
bng nhau). Nhm hai gm cc cu trc vi t l c/a ln hn mt, nh hn mt l c/a ca cc cu trc nhm ba (xem hnh 4.6 v 4.4.1). a) Cu trc ng thc
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Loi cu trc ng Cu hay l lut xp cu lp phng vi nhm khng gian Fm3m l nn tng ca s ng loi cu trc ng thc. Nguyn t ca nhiu kim loi (ng, vng, bc, ch, niken, cobalt, bch kim, st, nhm, scandi, calci, stronti, nguyn t t him, v.v..., xem ph lc 2), cng nh mt lot hp kim, u xp theo lut ny. Loi cu trc halit NaCl. C s ca cu trc l lut xp cu lp phng ca anion Cl. Cation Na+ chim ht s khoang bt din. Ion dng ny cng xp theo s ca ion m. S phi tr ca hai loi ion u bng 6. Tt c khoang t din u b trng. Cu trc ca cht khong vt pyrit FeS2 (xem hnh 1.8,b); nguyn t st chim v tr ca natri, cp nguyn t lu hunh S2 (dng gnh cn, ho tr 2) nm ti ch ca chlor. Nhm dng trc S2 nh hng ln lt theo hng bn trc bc ba ca lp phng. Nhm khng gian ca pyrit Pa3 khc hn ca halit Fm3m. Mng lp phng tm mt tr thnh nguyn thu, mt nh trt thay cho mt gng. Cu trc calcit CaCO3 (xem hnh 4.18) cng c th xem nh cng loi, bi v cation Ca2+ sp t ging nh Fe2+ trong pyrit v Na+ trong halit. 2 Anion CO3 hay S2 tuy khng c dng cu nh Cl nhng li ging n v trt t sp xp. 2 Loi cu trc halit c trng cho halogenur ca kim loi kim (tr cesi), ca bc (tr AgI); cc oxit, sulfur v selenur ca kim loi kim t; cc sulfur, selenur v tellurur ca km, mangan, cobalt, nickel, st, bch kim, v.v (xem cc bng 4.2, 4.9, 4.10 v 4.17). Loi cu trc sphalerit ZnS. Cng nh trong cu trc halit, c s ca cu trc sphalerit l xp cu lut lp phng ca nguyn t lu hunh. Khc vi halit, trong sphalerit nguyn t km chim khoang t din; ch mt na s khoang ny b chim. Chnh v th, tm nghch o v trc xoay bc bn bin mt: nhm khng gian ca sphalerit l F 3m. Khoang t din cha km nh hng song song nhau, ngc vi hng ca khoang trng. Nu c hai loi nguyn t thay bng nguyn t carbon th cu trc thu c s l ca tinh th kim cng (xem hnh 4.14). Loi cu trc sphalerit ZnS cng ph bin trong cc hp cht AX, thuc loi ny c halogenur ng, sulfur, selenur, tellurur ca beryli, km, cadimi, thu ngn cng nh mt s arsenur v antimonur (ca Ca v Al). L bin th a hnh h su phng ca sphalerit, vurtzit c cng thnh phn ha hc v cng phi tr t din ca km, nhng nguyn t lu hunh ca n li xp theo lut xp cu su phng (xem cc bng 4.7 v 4.8). Loi cu trc fluorit CaF2. Tng t sphalerit, cu trc fluorit cng c nn mng l lut xp cu lp phng, nhng do nguyn t calci to nn. Cc nguyn t ny gin rng ti mc khoang t din c th cha nguyn t c ln nh fluor, v chng b chim ht, khng ging trng hp sphalerit, s phi tr ca calci bng 8, ca fluor 4. Nhm khng gian gi nguyn Fm3m (hnh 4.15). Loi cu trc fluorit v fluorit ngc (A2X) thng gp trong sulfur, halogenur v oxit (xem cc bng 4.1, 4.12, 4.16 v 4.20). Loi cu trc st (-Fe) hay l mng lp phng tm khi vi nhm khng gian Im3m l s cu trc ca nhiu kim loi ngoi st: crom, vanadi, molybden, wonfram, niobi, titan, ziriconi, bari, kim loi kim v s ln hp kim (xem ph lc 2). Nu coi 2 loi nguyn t l ng nht, tc l thuc cng mt nguyn t ho hc, th cu trc ny theo s mng lp phng tm khi. Ni cch khc, trong loi cu trc clorur cesi CsCl, nguyn t ca mi nguyn t sp xp theo s mng lp phng nguyn thu; hai loi nguyn t ny lng vo
nhau v cch nhau 1/2 ng cho khi ca mng. S phi tr ca c hai loi nguyn t u bng 8, a din phi tr l khi lp phng. Nhm khng gian Pm3m. Loi cu trc ny bao gm cc halogenur cn li ca cesi (tr fluorur cesi thuc loi cu trc halit), halogenur ca tali (tr fluorur tali), ca amoni (nh NH4Cl, NH4Br v NH4I) v hng lot hp kim (AgCd, AgLi, AgMg, AlCo, AlFe, BeCo, BeCu, v.v...) (xem bng 4.11). Loi cu trc spinel MgAl2O4. Nguyn t oxy xp theo lut xp cu lp phng, magnesi nm ti khoang t din, nhm ti khoang bt din. c s cha 8 n v cng thc. Nguyn t Mg ch chim 8 trong s 64 khoang t din, nguyn t Al chim 16 trong 32 khoang bt din. Cng thc ng vi mng c s l Mg8 Al16O32. Phn ln khoang cc loi b b trng, do nhm i xng khng gian ca loi cu trc l Fd3m (mt nh trt d nm thay mt gng v tr to ), i xng thp hn so vi i xng ca lut xp cu. Dc trc bc ba cation kim loi phn b trn cc lp gm hai loi. Xen gia cc lp nhm phi tr bt din l lp nhm v magnesi vi t l Al : Mg = 1 : 2. Loi cu trc ny bao gm khong 60 hp cht (nhm spinel) vi cng thc tng qut AB2X4. V d: hercynit Fe2+ Al2O4, gahnit ZnAl2O4, galaxit MnAl2O4, chromit Fe2+Cr2O4, v.v Trong nhm spinel mt s hp cht c cu trc hon i gia A v B. Cng chim v tr A vi cation ha tr hai c c 50% cation ha tr ba. S cation cn li nm ti v tr B. Chng hn, magnetit Fe3+(Fe2+Fe3+)O4 v magnesioferrit Fe3+(Mg Fe3+)O4 (xem hnh 6.36 v bng 4.22). Vi cng thc ha hc ng dng ca spinel, cu trc forsterit Mg2SiO4 gn ging cu trc ca n. Nguyn t oxy xp theo lut (khng phi lp phng) gi su phng. Nhng, vi s gp mt ca cation cc loi, olivin khng c cu trc lp; 1/8 khoang t din do nguyn t silic chim. Nguyn t magnesi chim mt na s khoang bt din (xem 6.1). Mt phn ln silicat khung c th lit vo nhm cu trc ng thc, in hnh l cristobalit, bin th SiO2 nhit cao. Cu trc ny c th suy ra t cu trc kim cng (hnh 4.14); nm ti ch ca carbon l nhm t din SiO4 (silic nm ti tm t din, oxy ti nh, xem 6.11 v hnh 4.16). Ni cch khc, nguyn t silic xp theo hai s ca mng lp phng tm mt (a v c, xem cui mc 4.4.1); hai FLP ny cch nhau 1/4 ng cho khi. Nguyn t oxy nm trung im khong SiSi, nhn s phi tr 2. Silic c phi tr t din vi 4 oxy (xem thm hnh 4.36). Ging kim cng, cristobalit cng kt tinh thnh bt din (xem chi tit 6.12). Nu b qua nhng tnh cht vt l ph thuc vo i xng, v d: tnh ct khai, hiu ng ho in, p in; th c im ca cu trc ng thc l thng ng hng i vi hng lot tnh cht. Chng hn, tc truyn sng sng, truyn nhit, dn in, gin n nhit. b) Cu trc lp v dng lp
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Khc vi cu trc ng thc, cc cu trc lp c c im d hng r nt. Trong lut xp cu su phng t l thng s gia trc ng v trc ngang c : a = 1,633; mt mng (0001) lun l mt c mt ht ln nht. Bi v mt ht ca cc mt vung gc vi trc c v song song vi n l khc nhau, nn t l c/a cng ln hn 1,633 th c im d hng ca lut xp cu su phng cng r nt. Chng hn, cu trc cc kim loi km v cadimi c t s c/a ln lt bng 1,86 v 1,89; s bin dng ca xp cu su phng lm cho cu trc phn lp cng mnh (hnh 4.27). Loi cu trc magnesi Mg hay lut xp cu ...ABABAB... l s cu trc chung cho hng lot nguyn t kim loi nh Be, Os, Rh, Sc, Gd, Y, -Cr, Cd, Ti, Zr, v.v... v nhiu hp kim (xem thm 4.4.1). C s ca
Hnh 4.18 Cu trc tinh th ca calcit
nhiu loi cu trc tinh th ca cc hp cht. Trong , khoang t din v khoang bt din do cc nguyn t kch thc nh v trung bnh chim mt phn theo nhng s ring. Chng hn, cu trc corindon -Al2O3 l lut xp cu hai lp bin dng i cht ca nguyn t oxy. Nguyn t nhm chim 2/3 s khoang bt din, khoang t din b trng hon ton. Cc khoang bt din trng nm xen gia cc cp khoang cha nhm, nu kho st dc theo cc trc tinh th hc (hnh 4.17). Hematit Fe2O3, ilmenit FeTiO3, geikielit MgTiO3, pyrophannit MnTiO3 u thuc loi cu trc corindon. Cu trc calcit CaCO3 rt ging cu trc loi halit. V tr do calci chim gi trong calcit 2 tng t v tr ca natri trong halit. Thay th ch cho chlor l nhm chc CO3 . Bi v nhm chc ny khng c dng cu, cu trc calcit khc vi cu trc halit ch c s lp phng ca halit b bin dng. N bp dt dc mt trc ba v bin thnh mt thoi R, nh t vi gc 102o (thay cho 90o). Nu ion hnh cu l ch giao nhau ca bn trc xoay i xng bc ba th calcit ch gi li mt trc . Trc chnh ny xuyn tm nhm chc v vung gc vi mt phng ca chng. Dc trc bc ba ny, cc tam gic ca nhm chc xen k ngc chiu nhau (hnh 4.18). Cc nhm hnh phng ny song song vi nhau, gy nn c im d hng r rt ca cu trc v ca cc tnh cht (xem thm 6.13.1). Thuc loi cu trc ny l mt lot carbonat (Mg, Zn, Co, Fe, Mn, Cd, v.v...), nitrat (Na, Li), mt s borat ca t him v.v... Cu trc lp ca iodur cadimi CdI2 xy dng trn c s lut xp cu hai lp ca iod. Nguyn t cadimi chim mt na s khoang bt din, lp bt din ny nm xen gia cc lp khoang bt din rng. Lc gn kt cc tp bt din vi nhau l lc lin kt tn d. Loi cu trc ny c trng cho hng lot iodur (Mg, Ca, Mn, Fe, Co, Pb), bromur (Mg, Mn, Fe, Cs), sulfur, selenur, tellurur (Ti, Zr, Sn, Ta, Pt), hydroxit nh brucit Mg(OH)2 (xem hnh 4.19 v cc bng 4.3, 4.14 v 4.23). Tng t, cu trc lp ca chlorur cadimi CdCl2 cng c cc tp bt din ca Cd, xen gia chng l tp bt din rng; nhng cc nguyn t Cl xp theo lut lp phng. Loi cu trc ny bt gp trong mt s halogenur khc (xem bng 4.15). Gibcit Al(OH)3 vi nhm ho tr ba chim 2/3 s khoang bt din c coi l mu hnh l O hai bt din. Cn brucit vi cation chim ht hnh phi tr bt din th to thnh l O hai bt din ca silicat lp. Sau y l mt s v d.
Hnh 4.19 Cu trc tinh th ca CdI2
Nhm SiO4 trong cu trc cc khong vt ny gn kt vi nhau theo cch ring, to nn nhm chc dng l (hnh 4.20) vi cng thc [Si4O10]4. Cc l t din u c hai mt khc nhau; nu pha y t din trung ho in tch, th pha nh (vi ion oxy cn d in tch 1) ca chng tp trung tt c in m ca nhm chc. L ba bt din (Al+OH)2+ ca kaolinit trung ho in tch 4 ca n, bng cch gn ngay vi n t pha nh t din v cng vi n to ra tp hai l TO.
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Hnh 4.20 Cu trc tinh th kaolinit Kaolinit Muscovit Al2[Si2O5](OH)4 KAl2[AlSi3O10](OH)2
Hnh 4. 21 Cu trc tinh th ca pyrophyllit Pyrophyllit Phlogopit Al2[Si4O10](OH)2 KMg3[AlSi3O10](OH)2
Cc tp ny gn vi nhau bng lin kt yu. Trong pyrophyllit, cu trc bao gm cc tp ba l TOT lin kt trc tip vi nhau (hnh 4.21). L Al+OH kp gia hai l nhm chc; chng u hng mt l tch in m v pha l Al+OH d in tch dng. Trong ba l ca cu trc muscovit v phlogopit, 1/4 nguyn t silic b nhm thay th; in tch m d s do cation K+ trung ho. Bng cch , n gn cc tp ba l vi nhau (xem thm 6.8, 6.9, 6.10 v cc hnh 6.24, 6.25 v 6.26). Tinh th vi cu trc lp hay dng lp thng c biu hin d hng r nt. Song song vi mt lp, thng l hng ct khai hon ton v ch s khc x t gi tr cao nht, cn theo hng vung gc th thp nht. Theo hng ny h s gin n nhit v co c cng cho gi tr cao nht. Thng thng, vi cu trc lp in hnh, tinh th c cng thp v dng quen hnh tm. c) Cu trc chui v dng chui Loi cu trc rutil TiO2 (mt trong nhng bin th a hnh ca dioxit titan). Nguyn t titan phi tr bt din, c 6 nguyn t oxy vy quanh. Mi oxy li nm gia 3 nguyn t titan, ng vi phi tr tam gic u. Cc khi bt din titanoxy gn vi nhau bng cnh chung, to nn chui song song vi trc bc bn i xng. Cc chui ny gn vi nhau qua nh ca bt din. Nhm khng gian P42/mnm (hnh 4.22). Cng loi vi cu trc ny l stishovit, khong vt vi s phi tr 6 khc thng ca Si4+ v do trng lng ring cao nht so vi cc bin th a hnh khc ca silica. Kiu ghp song tinh ph bin ca rutil ph hp vi c im cu trc ca n: chui bt din c th ghp vi nhau bng cnh bt k ca a din phi tr, to song tinh dng khuu in hnh (hnh 4.23). Thuc loi cu trc rutil c cc khong vt nh cassiterit SnO2, platnerit PbO2, pyrolusit MnO2, cc fluorur ca Mg, Zn, Mn, Fe, Co, Ni, Pd (xem bng 4.13) v nhng hp cht vi cng thc AX2 khc.
Hnh 4.22 Cu trc tinh th ca rutil
Hnh 4.23 S gn kt song tinh dng khuu ca rutil
Loi cu trc nickelin NiAs. C s ca cu trc l lut xp cu hai lp do nguyn t arsen to ra. Nguyn t nickel chim ht s khoang bt din gia 6 qu cu arsen, cc khi bt din cha nickel gn vi nhau qua mt chung to nn chui ko di theo hng trc i xng bc su. Dng chui ca cu trc th hin s khc nhau v khong cch gia cc nguyn t nickel NiNi: 2,52 dc hng c so vi 3,61 dc hng a. Cng loi vi cu trc ny c pyrrhotin Fe1 xS (x khng vt qu 0,2). Cu trc thuc loi sai hng Frenkel; st ho tr hai khng th chim ht s khoang bt din do nhu cu trung ha in tch ca h. Hin tng ny gy ra s thiu ht in tch dng; v vy, mt lng tng ng Fe3+ vo thay cho Fe2+. iu ny s ni r hn chng V. Nhiu ng cu trc khc ca nickelin lit k trong bng 4.4. Loi cu trc chui in hnh thuc v pyroxen v amphibol (hnh 4.24). Trong pyroxen cc t din SiO4 kt ni nhau qua nh chung, to nhm chc [Si2O6]4 dng chui dch dc ko di v hn dc trc tinh th hc chnh. Trong amphibol, nhm chc [Si4O11]6
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l chui kp gm 2 chui n pyroxen (xem cc hnh 4.5, 4.36, 6.10 v 6.11). Cc cation ho tr hai nh Ca2+, Mg2+, Fe2+ hay cp cation ho tr mt v ba nh Na++Al3+, Li++Al3+, Na++Fe3+ lm nn phn cation trung ho in tch m ca hai nhm chc. i vi cu trc chui, nt c trng l dng quen ca tinh th hnh lng tr gi bn phng trong pyroxen v gi su phng trong amphibol; gc gia cc mt thuc hnh n {110} ln lt gn bng 90o v 120o. Theo hng thng gc vi chui, h s n hi v gin n nhit thng ln hn theo hng song song vi chui. Ch s khc x theo hng dc theo chui thng ln hn, tc l truyn sng sng thng nh hn so vi hng vung gc.
4.4
KHI QUT V CC LOI CHT KHC NHAU
4.4.1 Kim loi v hp kim phn bn phi bng h thng tun hon cc nguyn t ho hc l cc nguyn t phi kim. Phn ln cc nguyn t ha hc l kim loi (khong 75), m in hnh l cc nguyn t nm phn tri ca bng. Ch yu, chng c cu trc n gin vi i xng lp phng v su phng. Theo nhng quan nim hin i, kim loi v ion c hnh dng gn vi ht cu. Li do tnh v hng ca dng lin kt ca chng, kim loi c xu hng kt tinh theo s xp cht ca cc lut xp cu. Vic nguyn t ca mng kim loi c i xng dng cu c chng minh bng php phn tch bn (mt in t) Fourier. thng hiu cu trc cc pha kim loi cn lm r vai tr ca bn knh nguyn t. T nm 1926, Goldschmidt nhn thy gi tr bn knh ca cng mt nguyn t ph thuc vo phi tr; bn knh ca ht vi phi tr 12 c gi tr cao nht, phi tr 8 bn knh gim 3% gi tr; trong trng hp phi tr 6 v 4 bn knh cn gim na (xem 4.1.4). Khong cch nguyn t trong cu trc kim loi v hp kim c th coi bng tng bn knh ca hai ht lin k. Trong nhiu trng hp i khi khong cch nguyn t nh hn tng bn knh ca chng. Cc pha ny xy ra hin tng co rt khong cch nguyn t, m nguyn nhn c th nm s thay i trng thi ion ha. Ngay c nhng pha hp kim thuc cng mt loi cu trc vn c s chnh lch ng k v khong cch nguyn t gia gi tr o c trong cc pha tng ng v gi tr tnh ton tng bn knh. Tr mt s bit l, cu trc ca tt c cc kim loi c xc nh (xem ph lc 2); bn knh nguyn t ca chng lit k trong ph lc 4. Cu trc tinh th kim loi Nhng loi cu trc ph bin nht ca kim loi bao gm: Xp cu su phng (ABABAB), l loi cu trc magnesi. Xp cu lp phng (ABCABC ), l loi cu trc ng. Mng lp phng tm khi (Ilp), l loi cu trc -st.
cht (t l phn trm khng gian do cu nguyn t chim) v s phi tr cao chng t nguyn t kim loi vi lin kt v hng v khng bo ho c xu hng thu ht s lng ln nht cc nguyn t cng loi (xem thm 4.2.3). Trong trng hp kim loi vi lut xp cu lp phng (mng lp phng tm mt) s vic r; mi nguyn t nm cch nhng khong nh nhau ti 12 ht phi tr. Cng c th ni nh vy i vi phi tr 8 ca kim loi vi mng lp phng tm khi. Xp cu su phng c coi l cht nht, khi t s c/a = 1,633. Nu tng quan thng s mng c gi tr
khc, nh vn thy trong thc t, th khong cch nguyn t ca phi tr 12 s c 2 gi tr. Nhng ngoi hai trng hp ca Zn v Cd, chnh lch ny khng ln (xem sau). i vi nhiu kim loi, hin tng a hnh kh c trng. Chng hn, ceri c th c 2 loi cu trc xp cu lp phng v su phng ty nhit thay i, st c cu trc lp phng tm mt v tm khi, v.v Kim loi thuc cc phn nhm t IA, IIA, IB n nhm VIIIB ca h thng tun hon u kt tinh theo mt trong ba loi cu trc k trn. Ring trong s cc kim loi ca cc phn nhm khc v mangan, ngoi cc loi cu trc y, cng bt gp mt s ln loi cu trc khc.
-Mn c loi cu trc xp cu lp phng bin dng. Nguyn t u c s phi tr 12, nhng chng phn ho thnh hai loi ng vi cc khong cch nguyn t 2,36 v 2,67. -Mn c cu trc tng t vi lut xp cu lp phng, nhng bin dng dc trc xoay bc bn khin cho c s c cnh thng ng a co rt: a/a = 0,93 (hnh 4.25).Kim loi indi c cu trc cng loi nhng t l a/a = 1,08.
Hnh 4.25 Cu trc -Mn thuc loi ABCABC bin dng dc trc trc xoay L4. mng tm mt ABCDEFGH chuyn i thnh mng tm khi MBNCHQGP vi th tch gim 50%
Hnh 4.26 Cu trc kim loi thy ngn. Hg xp theo lut xp cu ABCABC bin dng dc trc L3 ng: mt thoi c thng s gc 7242 (b), bnh thng l 60 (a)
Theo nguyn l Bravais th hai cu trc ny chuyn t h lp phng sang h bn phng. mng c s chuyn t tm mt sang tm khi vi th tch gim mt na.
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Cu trc ca thu ngn: V nguyn tc, mng lp phng tm mt c th biu din di dng mt thoi (nguyn thu), vi gc gia cc cnh l 60o. mng tm mt ca thu ngn l mt thoi c gc gia cc cnh l 72o32 (hnh 4.26). Cu trc loi xp cu su phng thng bin i theo trc chnh lm thay i t l thng s mng c/a, m khng ko theo s bin ng v i xng. Hnh 4.27 l mng ca cu trc tun theo lut xp cu su phng l tng: c/a = 1,633, tc l khng c chnh lch v khong cch gia cc nguyn t k nhau, d chng cng lp hay khc lp, c li y lun bng tng bn knh ca cp nguyn t k nhau. i chiu vi trng hp ny l hai cu trc sai khc nhiu nht. l beryli, km v cadimi vi c/a ln lt bng 1,57; 1,86 v 1,89. Mt lot nguyn t ho hc ng cu trc, khc nhau v tng quan thng s mng vi cc gi tr c/a thay i so vi 1,633. Ngoi km, cadimi v beryli, phn ln trong s 28 nguyn t c c/a xp x nh hn gi tr l tng. Nhng sai lch ny gi v s bin dng ca ht cu nguyn t sang elipsoit trn xoay. Cu trc tinh th hp kim Nguyn t kim loi thuc cc nguyn t khc nhau c th khng tng tc vi nhau, c th rn ln th lng. Trong thi cc khc, hai kim loi c th phn ng vi nhau to thnh hp cht ho hc xc nh. Gia hai gii hn , nhng h trung gian c th tn ti: Kim loi tan vo nhau trng thi lng, khi chuyn sang trng thi rn chng to eutecti; Gia hai kim loi c th hnh thnh dung dch cng mi t l thnh phn.
V mt hnh hc, hnh 4.28 l s kt hp theo nhng cch khc nhau ca hai loi ht. S lng tng i ca hai loi trong c 4 trng hp l nh nhau, nhng s phn b ca chng li khc nhau. Trong trng hp A, nguyn t hai loi phn b hon ton ngu nhin; tn
sut gp ca nguyn t trng v en l nh nhau ti im bt k. Trng hp ny tng ng vi trng thi hon ton khng trt t. N khc hn trng hp hon ton trt t D; l cu trc tinh th ca nhiu hp cht v c. Gia hai trng thi ny c th c hai trng hp trung gian. Trng hp B c th c nguyn t phn b theo trt t tm gn; nguyn t en v trng ch duy tr c phi tr trt t trong tng phm vi nh hp. Trn B ny khng tm thy i nguyn t en no nm cch nhau c li nh nht. Nhng thng tin hin c v cu trc vi m ca pha dung dch cng cho thy chnh s phn b y c trng cho phn ln pha kt tinh ny. Trng hp C khng ch c trt t tm gn, trong cn c trt t tm xa. Tinh th pha hp kim phn ln c cu trc c trng ny. iu cho thy gia dung dch cng v hp cht khng c ranh gii r rt; trong t nhin v trong phng th nghim vn c th bt gp dung dch cng vi trt t cao v vn c hp cht khng duy tr c trt t hon ho.
Hp kim vi loi cu trc ng (Cu). Kim loi c n vi chc nguyn t ng cu trc vi Cu (xp cu lp phng), cha k mt s ch khc rt t so vi loi cu trc ny. Chng hn mangan v thu ngn. Hp kim c th cng kt tinh theo s ny, y t lng gia cc nguyn t kim loi c th thay i trong gii hn ln. Da vo cu trc ca kim loi ng (hnh 4.29,a) c th nhn din cu trc ca hng lot hp kim. Trc ht, hy lu cch tnh s nguyn t ng trong mng lp phng tm mt ca n. Cu ti nh l chung cho 8 , nn n ch c 1/8 v 8 Cu cc nh ch c (1/8) 8 = 1 nguyn t Cu () thuc mng cho. Cu tm cc mt ch tnh 1/2 v 6 mt tnh (1/2) 6 = 3 Cu (). Lng nguyn t Cu tnh trn mng c tng l: () + () = 1 + 3 = 4. Trong cu trc hp kim Cu3 Au (hnh 4.29,b), nguyn t ca hai kim loi phn b ti v tr ca mng lp phng tm mt. Nh vy, cu trc khng cn thuc mng tm mt: bc tnh tin ph dc ng cho ca cc mt bin mt; n thuc mng lp phng nguyn thu. Kim loi vng nm ti nh , vi to im u 000 v s bi l 1.
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Nguyn t ng, vi s bi l 3, chim v tr ti tm ca cc mt; tc l chng chim ti 3/4 v tr ca mng tm mt lp phng. Cu trc ca hp kim Pt7Cu (hnh 4.29,c) c nguyn t platin (vi s bi bng 7) chim ti 175% s v tr ca 2 h im quy tc thuc mng lp phng tm mt. 25% v tr cn li l ca nguyn t ng: chng nm ti nh ca mng vi s bi l 1. Gi d hai loi nguyn t trong hp kim CuAu (hnh 4.29,d) thuc mt nguyn t ho hc, th cu trc s thuc mng lp phng tm mt. Nhng d dng nhn thy hai loi nguyn t xp thnh lp xen k nhau; trt t cc lp hon ton ging nhau. Nu xt trn mng, nguyn t ng nm nh v tm cc mt y ca ; cn nguyn t platin chim v tr tm cc mt bn. Mi loi nguyn t chim 50% s im lut xp cu ABCABC lm cho i xng chuyn sang h bn phng; khng phi do mng bin dng, m do chnh s phn b ca cc loi nguyn t trong hp kim. Cu trc PtCu (hnh 4.29,e) c th suy ra t cu trc kim loi thu ngn. Cc cnh mng ca PtCu u tng gp i. Tng t cu trc ca thu ngn, mng ca cu trc hp kim ny l kt qu bin dng ca lp phng tm mt (n b bp dt theo hng ca mt trong 4 trc bc ba; 3 trong 4 trc do bin mt; cn i xng th tr thnh ba phng). Gi d, c th ng nht hai loi nguyn t ca hp kim, th c th thy rng cc ht cng loi xp theo lut ABCABC bin dng. Khong cch nguyn t cng lp th ln hn khong cch cc ht khc lp. Vy, ngoi s bin dng ca mng, i xng ca cu trc tr thnh ba phng cn do chnh s sp xp cc ht. Nguyn t thuc hai nguyn t phn b xen k nhau theo tng lp vung gc vi trc bc ba. Hp kim vi loi cu trc st (-Fe)
Hnh 4.30 Loi cu trc -Fe (a) ca cc hp kim Cr2Al (b); NaTl (c) v Fe3Al (d)
Fe c cu trc thuc mng lp phng tm khi; nguyn t st u c s phi tr 8, hnh phi tr lp phng, s bi bng 2 (hnh 4.30,a). Cu trc tinh th hp kim Cr2Al. Thot nhn, c th thy hnh 4.30,b gm 3 tm khi ging Fe nu coi hai loi nguyn t l thuc mt nguyn t. Thc ra, nguyn t mi loi phn b trn cc lp ring vi cng mt trt t sp xp. Cc lp song song, cch u nhau, u vung gc vi trc chnh v trch nhau 1/2 ng cho mt y mng. Hai lp crom lin k nm xen k vi tng lp nhm n l. S bi ca 2 h im ln lt l 4 v 2, trong khun kh ca nhm khng gian P4/mmm. Cu trc tinh th hp kim TlNa v AlFe3. Hy tng tng ct lp phng ca mi cu trc bng 3 mt trc giao ti tm ca . Nh th, c th ni mi hnh l tp hp ca 8 tm khi lp phng nh, gi d hai loi nguyn t ca mi hp kim l thuc mt nguyn t ho hc. Trn thc t, cc ht ca mi cu trc u nm trn cc im ca 4 v tr c bit thuc mng tm mt lp phng (FLP). Trong cu trc TlNa v AlFe3, nguyn t mi loi chim 2 h im nh sau (hnh 4.30,c v d). Tl : 000 v Al :111 1 113 ; Na : 00 v 444 2 444
113 1 313 ; Fe : 000; 00 v 444 2 444
Nhn chung, trong cu trc n gin ca hp kim loi ny, nguyn t ca mi nguyn t c th chim mt phn, hay chim ht cc im ca mng FLP theo bn v tr c bit nh sau: a) V tr 000 bao gm nh v tm cc mt ca . b) V tr1 2
00 bao gm im gia ca cc cnh lp phng.
Hai v tr sau nm trn cho khi, mi v tr 4 im chia u cho 4 ng cho, c th l: c) V tr d) V tr111 444 113 444
v .
Hy im li cu trc cc hp kim va m t. Cu3Au: Tt c cc nguyn t ca hp cht Cu3Au ch chim ht mt h im ca mng FLP; trn hnh 4.29,b cc nh lp phng ng vi 1/4 s im trong v tr a) l ca Au, 3/4 s im cn li l dnh cho Cu. Pt7Cu: Ton b nguyn t Cu trong Pt7Cu (hnh 4.29,c) ch chim 1/4 s im ca a), 7/4 s im cn li ca hai h im FLP, tc l 3/4 s im ca v tr a) v tt c cc im ca v tr c), u l ch ca Pt. CuAu: Mi loi nguyn t trong CuAu nm ti 1/2 s im ca v tr a), sao cho cc lp ca mi loi nguyn t u xen k nhau, nh trn hnh 4.29,d. TlNa: mt na s nguyn t Tl chim ht v tr c), na kia chim ton b h im ca v tr a); Na chia u cho 2 v tr: b) v d) cn li (hnh 4.30,c).
39
AlFe3 : Trong AlFe3, nguyn t ca n phn b trn c 4 v tr c bit ca FLP. Nhm chim d), st chia u cho 3 v tr a), b) v c) (hnh 4.30,d). Qu trnh trt t n nh trong pha hp kim Nhng nghin cu chi tit cc pha rn trong h CuZn cho thy trong vng nhit cao pha -CuZn c cu trc hon ton khng trt t. Nhit h thp, trt t t gi tr rt cao v hon ton c th lit n vo loi cu trc CsCl. Qu trnh bin i trng thi y t khng trt t sang trt t v ngc li din bin trong mt khong nhit xc nh, m khng phi t bin mt gi tr nhit nh trong bin i a hnh. trt t cng thp, bn thn n cng d b ph b. Trt t cao c tc dng gy cn tr i vi s xut hin ca tnh trng khng trt t. Nu tng nhit mt cch tun t th lc u c th thy trt t gim chm v khng ng k. Nhit tng tip tc th tnh trng mt trt t tng nhanh dn v cui cng, trt t bin mt hu nh t bin. Chng hn, trt t bt u gim mnh trong pha CuZn khi nhit t khong 390C v kt thc sau gn 20C. Bradley A.J. v Jay A.H. (1932) nghin cu chi tit kha cnh cu trc ca qu trnh trt t ho trong h FeAl. Tr li mng lp phng, nu c 4 v tr a, b, c v d u cha nguyn t Fe th y l hnh nh 8 tm khi ca cu trc -Fe ghp li. Khi thnh phn gm 50% nguyn t Fe v 50% nguyn t Al v Al chim cc im ca v tr a) v b); Fe chim cc im ca v tr c) v d), th y l 8 mng ca hp cht FeAl kt tinh theo loi cu trc CsCl. Nu nguyn t Al thm nhp dn vo pha -Fe tinh khit th pha hp kim trng thi nhit cao vi thnh phn nguyn t Al t 0 25% l mt dung dch cng in hnh, Al phn b ngu nhin trong cu trc Fe. Tn sut gp Al trong cc v tr a), b), c) v d) t l thun vi lng Al ho tan. Bt u t 25% Al tr i, Al cng nhiu, dung dch cng cng tr nn trt t trong cu trc. Nguyn t Al bt u chim c) v d), y Fe sang a) v b). Qu trnh ny kt thc khi thnh phn nguyn t nhm t 50%, trt t t gn 100% ng vi cu trc FeAl. Nu s nung t t ch thch hp, trt t c th n sm hn. Bt u t 18% thnh phn nguyn t, nhm bt u y st chim v tr c). Vi 25% thnh phn nguyn t nhm, loi cu trc mi ra i vi cng thc Fe3 Al. Tuy vy, trt t cha ti 100%: mt phn v tr c) vn cn c st. Hm lng nhm c tng cng tip th cc nguyn t ny s y ht st khi v tr c), lc ny Al vt qu 25% thnh phn nguyn t. Chng xm ln v tr d), b dn v tr c). Tn sut gp Al c) v d) nh nhau khi hm lng Al t 38%. Cu trc trt t FeAl c c khi thnh phn Al ln hn 38%.
4.4.2 Mt s hp cht hu cCc hp cht hu c thuc loi cu trc phn t. Dng lin kt gn nguyn t thnh phn t thng mang bn cht cng ho tr. Phn t tp hp nhau to nn tinh th bng lin kt van der Waals. y l lin kt v hng; c th thy trc rng phn t trong tinh th c xu hng tp hp quanh n s lng ln nht cc phn t k cn. Mc du phn t khng c dng cu, chng vn c th xp cht trong cu trc. Phn ln hp cht hu c c phn t vi s phi tr 12. Cn c cc s liu v hnh dng v kch thc phn t, cng nh v th tch mng c s, cht ca nhiu cu trc c xc nh. i lng ny dao ng trong khong t 60 n 80%. R rng, nguyn l xp cu ni chung c th p dng cho cc cu trc hu c. Phn t thng c dng bt k, khng u n; khi xp cht, ch li ca ht ny lp
vo ch lm ca ht kia. Trong iu kin nht nh, vi bn nng t xoay quanh trc ring hay quanh tm im, phn t tr nn c dng ng, dng cu v i xng cao hn. Cu trc tinh th metan CH4. Nu nguyn t carbon nm ti tm ca phn t t din u, th 4 nguyn t hydro ti nh; iu ny m bo cho s o n nh (khong 109) ca gc ha tr HCH. i xng cao v kh nng t xoay, phn t metan lm nn nhm khng gian Fm3m ca cu trc tinh th, s bi n=4, thng s mng a = 5,89 ( nhit 180C). Tm im ca phn t phn b ti cc nt ca mng lp phng tm mt. Cu trc tinh th Hnh 4.31. Cu trc phn t adamantan adamantan C10H16 c xc nh rt sm, cng do phn t ca n c i xng cao vi nhm im 4 3m (hnh 4.31). Trong khun kh lp phng (ng t) mi cp CH2 chia nhau nm trng trn hai mt gng vung gc nhn trc 4 lm giao tuyn; cc ht carbon ca chng phn b ti v tr a) ca LLP. Cn 4 cm CH chia u cho 4 trc xoay bc ba v dng trc ca chng trng vi 4L3, nguyn t carbon nm v tr c) ca FLP. Mi nguyn t carbon ca cm CH2, vn c 2 mi lin kt vi 2 hydro, li c 2 lin kt vi 2 carbon ca 2 cm CH. Mi carbon ca cm CH ( c 1 mi lin kt vi hydro) lin kt vi 3 nguyn t carbon ca 3 cm CH2. Vy, mi nguyn t carbon ca phn t c 4 mi lin kt c trng, nh hng u n v to nn nhng gc ho tr HCH n nh. Phn t adamantan sp xp song song vi nhau, tm im ca chng phn b theo s ca mng lp phng tm mt. Kch thc mng a = 9,426; s bi im n = 4. Cu trc tinh th benzen C6H6. Cu trc cc cht thm c nghin cu k hn so vi cc lp hu c khc. Tt c cc nguyn t carbon ca phn t C6H6 u nm trn mt mt phng (hnh 4.32,a). Vic phn t ca n nm ti tm i xng chng t s cn bng ca cc lin kt gia carbon vi nhau. Khong CC thay i t 1,374 n 1,382. Gc ho tr CCC t 11928 n 12042. Cc thng s hnh hc ca cu trc nh sau: a = 7,460; b = 9,666; c = 7,034. Nhm khng gian Pbca (hnh 4.32,b).
Hnh 4.32. Cu trc phn t benzen (a) v hnh chiu mng tinh th trn (010) (b)
41
Cu trc ca parafin C29H60. Cng nh trong tt c cc hp cht bo, nguyn t carbon xp thnh chui dch dc, vi khong CC chng 1,54, vi gc CCC c trng 109,5 (hnh 4.33,a,b). Trong tinh th, cc chui ny xp kht, song song vi trc c (hnh 4.33,c). Mng trc thoi ca parafin c kch thc nh sau: a = 7,45; b = 4,95; c = 77,2. Nhm khng gian Pnma. Phn t parafin trong tinh th thng dao ng nhit xung quanh trc c. Tip cn im si, chng xoay quanh trc v c dng ng; do i xng tinh th t trc thoi bin thnh su phng.
4.4.3 Sulfur v mui sulfury l hp cht ca kim loi vi lu hunh, selen, tellur v bn kim (arsen, antimon v bismut). Cc bn kim ny kt hp vi lu hunh to anion phc ca mui sulfur. So vi Se, Te v bn kim, S l anion quan trng nht v ph bin hn c trong t nhin. cha (g/t) ca cc nguyn t ny trong ca Tri t (theo Goldschmidt) nh sau:S 520 Se 0,09 Te 0,0018 As 5 Sb 1 Bi 0,2
Trong iu kin t nhin, S thng gp trong hai trng thi ho tr: 2 trong anion S2 v +6 trong [SO4]2 ca sulfat. Do vy, linh ng ca S trong v Tri t xc nh bng oxy ho ca n; trong mi trng kh S cho sulfur, iu kin oxy ho n cho sulfat. Qu trnh oxy ho ca lu hunh din bin theo trnh t sau: S2 S22 So S4+ S6+ Bn knh ca S6+ (0,34) rt gn vi ca P5+ (0,35) v Si4+ (0,39); t c th thy kh nng thay th ng hnh gia [SO4]2, [PO4]3 v [SiO4]4. Do S ho hp to thnh hp cht vi Fe, Ni, Cu, v.v, S tch khi dung th silicat ngay giai on u ca qu trnh phn d magma. Lu hunh gp trong m khong kh thnh v nhit dch vi khi lng ln nht. y l ngun S chnh cung cp cho sulfur ca hng lot kim loi (Cu, Zn, Pb, Ni, Co, Mo, Hg, Ag, v.v). S cng tn ti trong sn phm hu magma vi hm lng ln, chng t n l thnh phn ca kh ni la. Dung nham ni la qun o Hawai cha ti 5,5% SO3. Kh H2S ng vai tr quan trng trong thnh to sulfur. Khi nhit tng, n khng bn vng v phn hu thnh hydro v lu hunh; trong nc phn li ging phn t nc theo s :H2S H+
+
(SH)
;
(SH)
H
+
+
S
2
cha trung bnh ca S trong magma 520g/t; trong trm tch 1300 g/t; trm tch st cha t hn v ct kt cha nhiu hn gi tr ny. Nc lc a cha S nhiu hn hai ln so
vi nc bin. Ngun lu hunh cung cp cho nc l nh khong vt sulfur b oxy ho. Theo hm lng trong nc bin th lu hunh ng hng th t, sau Cl, Na v Mg; n tn ti dng sulfat, nhng nhng bin kn thiu oxy nh Bin en th H2S l dng tn ti ca S. Ngun gc ca S c th phn bit bng thnh phn ng v. Lu hunh c 4 ng v bn vng l S32, S33, S34 v S36 vi ph bin ln lt l 91,1%, 0,74%, 0,4% v 0,0016%. T l S32/S34 c gi tr khng i trong thin thch 22,2; bin ng cc ngun khc. Trong trng hp t l ny c gi tr ln hn 22,2 th S c ngun gc sinh hc, S c l l ny nh hn 22,2 l S hnh thnh t ngun gc v c. Tp tnh ca lu hunh di su kh ging ca selen v tellur. Sulfur st y rt giu Se do c thay th ng hnh Se S. Cng nh Se v Te, S to khong vt c lp v tp trung trong m khong nhit dch. Selen v lu hunh u b thc vt hp th, mt s cha Se n 15000g/t. Khc vi S, Se khng tp trung trong nc bin. c im ho hc tinh th Sulfur v mui sulfur thuc hng cc hp cht n gin, cha S, Se, Te, As, Sb v bn cnh chng 20 nguyn t di dng cation. Quan trng nht l 6 nguyn t Ni, Co, Fe, Cu, Ag v Pb. Chng u c i lc vi S, phn no vi As (Ni, Co v Fe) hoc vi Te (Cu, Ag, Au v Hg). Theo c im a ho cc cation ny thuc loi siderophil v chalcophil. Kim loi ca sulfur v mui sulfur c phi tr bt din v t din; ngoi ra l phi tr tam gic, lng tr, v.v V d, galen PbS, nickelin NiAs c phi tr bt din; sphalerit ZnS, argentit Ag2S c phi tr t din ca kim loi (tham kho 4.3.2). Ngoi ra, molybdenit MoS2 v covelin Ag2S c cu trc lp. Mui sulfur c cu trc phc tp hn, cu trc mt s trong chng c th suy ra t cc cu trc n gin ca sulfur (Hellner, 1958). Sulfur thnh phn n gin th c cu trc vi i xng cao, cn mui sulfur vi thnh phn tng i phc tp th tinh th c i xng thp. Phn ln sulfur kt tinh h lp phng, s khc su phng, s cn li trc thoi, t nht l sulfur mt nghing. Tri li, cc mui sulfur phn ln kt tinh h mt nghing hay trc thoi. Ch 10 mui sulfur ca Cu, Ag v kim loi him nh Ge, Sn v V thuc h lp phng. Thng thng, khong vt sulfur c tinh th hon thin, nht l sulfur cha cation Fe, Ni, Co, Zn v Pb. Mui sulfur thng gp dng tp hp, thng kt khi hay kt ht, i khi dng tp hp si. Sulfur, selenur v tellurur H thng tun hon cc nguyn t ha hc cho thy S, Se, Te v Po nm cng nhm ph VIb vi O; chng u c lp v in t ngoi cng ging nhau. Mc du vy, xt ring hnh vi ha hc tinh th, chng c nhiu im khc xa oxy. C th tm li gii cho iu ny cu to cc lp v in t bn trong v s chnh lch ln v kch thc ht. So vi oxy, cc nguyn t ny c gi tr in m thp hn nhiu, nn chng khng to nhiu hp cht ion vi kim loi. Nhng hp cht n gin m anion S2 , Se2, Te2, Po2 to vi kim loi kim v kim th cha hn l nhng hp cht ion. Vi bn cht chalcophil ca lu hunh v cc nguyn t cng phn nhm, cc hp cht y gn nh nhng cht ng cc vi cc mi lin kt dng trung gian gia cng ha tr v kim loi. V d, mt s sulfur kim loi vi loi cu trc halit v fluorit. Khong vt galenit PbS mang thuc tnh ca kim loi; nh c mu xm c trng, nh kim mnh v dn in tt; nhng n li gin, khng rn c. Tp tnh ny rt c trng cho nhiu sulfur, selenur, tellurur. Kim loi phn nhm IA thng cho cc sulfur, selenur, v.v... vi loi cu trc fluorit ngc khng mu, tan trong nc v tan trong nc, S d thy phn cho SH v H2S (xem
43
bng 4.1). Khong cch ht khng cho thy s chnh lch gia cc gi tr thc nghim v l thuyt, nht l i vi cc ht kch thc khng ln. Nhm khng gian Oh5 = Fm3m, Z = 4, phi tr 8 4Bng 4.1 Sulfur, selenur, tellurur v polonur kt tinh loi cu trc fluorit ngc Hp cht a, A B, thc nghim Li2S Li2Se Li2Te Na2S Na2Te 5,708 6,005 6,504 6,526 7,314 2,47 2,00 2,82 2,83 3,17 A B, l thuyt 2,55 2,66 2,87 2,80 3,12 Na2Po K2S K2Se K2Te Rb2S 7,473 7,391 7,676 8,152 7,65 Hp cht a, A B, thc nghim 3,24 3,20 3,32 3,53 3,35 3,13 3,24 3,45 3,26 A B, l thuyt
Kim loi phn nhm IIA, tr Be, u to vi lu hunh v nguyn t cng phn nhm nhng pha rn thuc loi cu trc halit (xem bng 4.2). Khong cch ion khng c s chnh lch nhiu gia gi tr l thuyt v thc nghim. Cc hp cht tng t ca beryli thuc loi cu trc sphalerit vi phi tr t din v lin kt ch yu l cng ha tr. Khng nghi ng rng Be khc hn cc kim loi IIB kia. Trong s sulfur ca kim loi phn nhm IIIB c LaS kt tinh theo loi cu trc halit. mng lp phng cha khng La, chng phn b ngu nhin trong khoang bt din do S to ra; dung lng ca n ng vi cng thc La2S3. Nhm khng gian Oh5 = Fm3m, Z = 1, phi tr 6 6Bng 4.2 Sulfur, selenur, tellurur v polonur kim loi kim kt tinh loi cu trc halit Hp cht a, A B, AB, Hp cht a, thc nghim l thuyt MgS 5,1913 2,59 2,55 SrS 6,0079 MgSe 5,463 2,73 2,66 SrSe 6,2320 CaS 5,6836 2,84 2,86 SrTe 6,660 CaSe 5,924 2,96 2,96 BaS 6,381 CaTe 6,357 3,18 3,17 BaTe 7,000 CaPo 6,51 3,26 BaSe 6,589 A B, thc nghim 3,00 3,12 3,33 3,19 3,50 3,29 AB, l thuyt 3,01 3,12 3,33 3,19 3,51 3,30
Tng t Sn, Ta v Pt, kim loi ca phn nhm IVB cng cho cc disulfur ging nh TiTe2, tc l kt tinh theo loi cu trc lp CdI2 (xem bng 4.3). Dng lin kt ca chng u mang tnh trung gian gia ion v cng ha tr. S bin ng v t lng thng xy ra trong mt s pha, nh TiS2, TiSe2 v TiTe2; hm lng Ti tng tun t chuyn dn sang loi cu trc nickelin NiAs vi cng thc ln lt TiS, TiSe v TiTe. Nhm khng gian D3h2= R m, Z = 1, phi tr 6 3Bng 4.3 Sulfur, selenur v tellurur kt tinh loi cu trc iodur cadimi CdI2 Hp cht TiS2 TiSe2 a, 3,412 3,548 c, 5,695 5,998 Hp cht CoTe2 NiTe2 a, 3,8921 3,869 c, 5,3744 5,307
TiTe2 ZrS2 ZrSe2 VS2 SnS2 -TaS2
3,764 3,66 3,80 3,355 3,646 3,319
6,526 5,85 6,19 6,134 5,880 6,275
RhTe2 PdTe2 PtS2 PtSe2 PtTe2
3,92 4,0365 3,544 3,731 4,018
5,41 5,1262 5,029 5,072 5,211
Nhm khng gian D6h4 = P63/mmc, Z = 2, phi tr 6 6Bng 4.4 Sulfur, selenur, tellurur kim loi kt tinh loi cu trc nickelin Hp cht VS VSe Vte TiS TiSe TiTe CrS CrSe CrTe MnTe MnTe (20) a, 3,367 3,587 3,813 3,30 3,548 3,842 3,462 3,691 3,963 4,146 4,1429 c, 5,825 6,009 6,132 6,44 5,998 6,403 5,765 6,031 6,17 6,709 6,7031 Hp cht FeS FeSe FeTe CoS CoSe CoTe NiS NiSe (20) NiTe PdTe RhTe a, 3,450 3,643 3,816 3,374 3,6294 3,8921 3,428 3,6613 3,965 4,1521 3,99 c, 5,882 5,970 5,673 5,187 5,3006 5,3744 5,340 5,3562 5,365 5,6719 5,66
Phn nhm VB c vanadi v phn nhm VIIIB c phn ln kim loi tham gia thnh phn ca monosulfur, monoselenur v monotellurur. Chng u kt tinh theo loi cu trc nickelin (bng 4.4). Thuc loi cu trc ny cn c CrS, CrSe v CrTe. Khi S tng trong cu trc th CrS cho pha mi mt nghing phc tp hn. Cr2S3 kt tinh theo loi cu trc nickelin vi nhiu v tr b trng. Cng phn nhm vi Cr phi k thm Mo v W; chng ch c trong disulfur v diselenur. V cu trc molybdenit MoS2 vi phi tr lng tr ba phng do 6S to nn th trn y ni n (xem hnh 4.12). ng cu trc vi molybdenit cn c tungstenit WS2 v WSe2 ; chng cho cc vi tinh mm, mu xm en dng graphit. Phn nhm VIIB c Mn tham gia vo cc pha MnS v MnS2 vi cc selenur v tellurur ng cu trc tr MnTe. Khong vt alamandit mu lc thuc loi cu trc halit l mt trong ba bin th a hnh ca MnS. Hai bin th khc mu tht ng cu trc vi sphalerit v vurtzit. MnS2 c cng cu trc vi pyrit cng tn ti trong t nhin. Khc vi n, MnS2 c nh
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kim cng, mu nu v ct khai hon ton; nguyn nhn ca s khc bit ny cn cha bit r. Reni to mt vi sulfur, trong ReS2 kt tinh dng vy mng lc gic, c l thuc loi cu trc molybdenit. Sulfur, selenur v tellurur ca nguyn t h st v platin l nhng hp cht quan trng, s ln c trong t nhin. Ht thy, chng c tnh bn kim v kt tinh theo nhng loi cu trc tng ng vi c im y. Tr NiTe, monosulfur, monoselenur v monotellurur ca kim loi h st u thuc loi cu trc nickelin (bng 4.4), chng u c s thiu ht kim loi gn ging pyrrhotin. V nhiu phng din, tt c chng u tng t cc pha hp kim. Nhm khng gian Th6 = Pa3, Z = 4, phi tr 6 4 (1+3)Bng 4.5 Sulfur, selenur, tellurur kim loi kt tinh loi cu trc pyrit Hp cht MnS2 MnSe2 MnTe2 FeS2 NiS2 NiSe2 CoS2 CoSe2 a, 6,107 6,430 6,951 5,417 5,676 5,9604 5,535 5,8611 Hp cht RhS2 RhSe2 RhTe2 RuS2 RuSe2 RuTe2 OsSe2 OsSe2 OsTe2 a, 5,585 6,002 6,441 5,58 5,933 6,372 5,6075 5,933 5,369
Loi cu trc rt quan trng, thng c mt trong hp kim l pyrit FeS2 ni trn. Fe c 6S gn nht vy quanh, S c 3Fe v 1S to phi tr t din, khong S S = 2,05 (xem bng 4.5). ng cu trc cn c nhiu hp cht khc cha nguyn t phi kim phosphor, arsen, antimon v bismut. Thnh phn pyrit rt n nh, trong khi diselenur nickel c s bin thin gii hn ca kim: NiSe1,952,05. Cobaltin CoAsS cng cu trc nh gersdorfit NiAsS; mng tinh th ca chng rt gn vi ca pyrit (1/2 S2 thay bng As) vi s phn b ngu nhin ca lu hunh v arsen. St c sulfur khc ni trn l pyrrhotin. Fe1xS vi x = 0 0,2 l pyrrhotin dng dung dch cng khuyt tt (xem 5.2.3), nhit nng chy gia 1190C v 400C l dung dch cng hon ton, vi thnh phn t Fe1xS n FeS. 0 254C l trng bn vng ca pyrrhotin mt nghing vi thnh phn Fe7S8. Bin th a hnh khc ca FeS2 l marcasit (bng 4.6) vi khong S S trong S2 bng 2,21 v vi phi tr ging nh trong pyrit. Arsenopyrit FeAsS vi loi cu trc marcasit, As y thay chn ca S. Monosulfur cobalt kt tinh theo nickelin, disulfur cobalt theo pyrit, Co9S8 theo pentlandit (Fe,Ni)S2 (hnh 4.34), Co3S4 theo spinel. Tt c, chng u mang tnh bn kim, u dn in tt. Ring CoSe v CoTe u c cu trc ca nickelin, cn CoSe2 v CoTe2 mang cu trc ln lt ca pyrit v marcasit.
Monosulfur nickel nhit cao c loi cu trc loi nickelin, khi thiu ht Ni n cho bin th mt nghing. Cn pha ba phng (mt thoi) l khong vt milerit; pha nhit thp ny c phi tr 5 cho c hai loi nguyn t. NiSe cng thuc loi cu trc nickelin. NiS2 v NiSe2 thuc loi cu trc pyrit. Pentlandit (Fe,Ni)S2 l khong vt rt quan trng. mng lp phng ln thit lp trong lut xp cu lp phng ca kim S. Kim loi chim gi c hai loi khoang t din v bt din (b), ring khoang t din tp hp li thnh cm tm ci vi i xng ring m3m (a); tm ca cm ny v ca bt din u phn b theo lut xp lp phng tm mt (c), ging loi cu trc halit (hnh 4.34). Theo Milovsky v Kononov (1985), hnh bt din ca Fe v Ni c th nm trong nhn ca cm tm (?). Cng thc cu trc ca pentlandit vit thnh (Fe4Ni4)IV(Fe,Ni)VIS8. Nhm khng gian D2h1 = Pnnm, Z = 2, phi tr 6 4 (1+3)Bng 4.6 Sulfur, selenur, tellurur kim loi
