25.T.Sekiya.pdf

Journal of Non-Crystalline Solids 144 (1992) 128-144 North-Holland

]OURNA L OF

NON-CRYSTALLINE SOLIDS

Raman spectra of MO1/z-TeO 2 (M = Li, Na, K, Rb, Cs and T1) glasses

Takao Sekiya, Nor io Mochida, Atsushi Ohtsuka and Mamoru Tonokawa a Division of Materials Science and Chemical Engineering, Faculty of Engineering, Yokohama National University, Tokiwadai 156, Hodogaya-ku, Yokohama-shi 240, Japan

Received 22 August 1991 Revised manuscript received 23 March 1992

The structure of MOI/2-TeO 2 (M = Li, Na, K, Rb, Cs and T1) binary glasses has been studied by means of Raman spectroscopy. The glasses having low alkali content have a continuous network constructed by sharing corners of TeO 4 trigonal bipyramids (tbp's) and TeO3+ I polyhedra having one non-bridging oxygen atom (NBO). In the glasses containing 20-30 mol% alkali oxide, TeO 3 trigonal pyramids (tp's) having NBOs are formed in the continuous network. When alkali content exceeds 30 mol%, isolated structural units, such as Te2 O2- ion, coexist in the continuous network. The fraction of TeO 4 tbp's decreases with increasing alkali content. The glasses, which contain nearly 50 mol% alkali oxide, are composed of a continuous network constituted by TeO3+ 1 polyhedra and TeO3 tp's, and of isolated structural units, such as Te20 z- and TeO 2- ions. The structure of thallium tellurite glasses having less than 30 tool% T101/2 is similar to that of alkali tellurite glasses containing equal amounts of MO1/2. The fraction of TeO 3 tp's having NBOs in the thallium tellurite glasses, when TIO~/2 content is equal to or higher than 40 mol%, is larger than that in the corresponding alkali tellurite glasses. In the 66T101/2. 34TEO2 glass, most of tellurium atoms are in a form of isolated TeO~- ion. A new hypothesis is also given for a mechanism for the basic structural changes in the tellurite glasses.

1. Introduction

It was indicated in a previous paper [1] that TeO 2 glass is composed of TeO 4 trigonal bipyra- mids (tbp's), in which one of the equatorial sites is occupied by a lone pair of electrons, and that most of the tellurium atoms are connected at vertices by Te-eqOax-Te linkage. The TeO 2 glass has an unique structure as a consequence of the the structural unit and its connecting style differs from the conventional glass formers, such as

1 Present address: Foundation for Promotion of Material Science and Technology of Japan, Kamisoshigaya 3-11-1, Setagaya-ku, Tokyo 157, Japan.

Correspondence to: Dr T. Sekiya, Division of Materials Sci- ence and Chemical Engineering, Faculty of Engineering, Yokohama National University, Tokiwadai 156, HodogayaTku, Yokohama-shi 240, Japan. Tel: +81-45 335 1451. Telefax: + 81-45 331 6143.

B203, SiO2, GeO 2 and P205. It is expected that TeO 2 may have a structural role differing from the conventional glass formers in binary glasses which contain network modifiers. So far, several investigations have been made to determine the structure of alkali tellurite glasses [2-7]. Mochida et al. [2] showed that the primary structural unit of tellurite glasses having high TeO 2 content is a distorted TeO 4 tbp and that the fraction of TeO 3 trigonal pyramids (tp's) increases with increasing mono- or di-valent cation content. A neutron diffraction study of L i20-TeO 2 glasses per- formed by Noev et al. [3] showed that, in addition to the TeO 4 tbp's, there exist regrouped tetrahe- dra in which each of the tellurium atoms has three oxygen atoms in equatorial sites and one oxygen atom in an axial site. It was proposed by Nishida et al. [4] that an introduction of Na20 into TeO 2 matrix results in a change of the glass matrix from a three- or two-dimensional network

0022-3093/92/$05.00 1992 - Elsevier Science Publishers B.V. All rights reserved

T. Sekiya et al. / Raman spectra of MO1/2-Te02 (M = Li, Na, K, Rb, Cs and Tl) glasses 129

structure to a lower dimensional one. An infrared study of LiC1-Li20-TeO 2 glasses performed by Yoko et al. [5] and by Tanaka et al. [6] suggested that, on addition of Li20 to TeO2, the bond strengths of Te-axO bonds become weaker and the TeO 4 tbp network breaks up accompanied by creation of non-bridging oxygen atoms (NBOs) in both Te-eqO and Te-axO bonds. Yoko et al. [7] proposed a mechanism for the change of the coordination number of Te 4+ from 4 through 3 + 1 to 3 and from 4 to 3 in LiX-Li20-TeO 2 (X = F and Br) glasses.

In the present study, the structure of MO~/2- TeO 2 (M = Li, Na, K, Rb, Cs and T1) binary glasses has been investigated by means of Raman spectroscopy. It is expected that the coordination state of tellurium atom in tellurite glasses may be determined by investigating several binary tellu- rite glasses having typical network modifiers. In this report, the symbol NBO is used for represen- tation of the oxygen atoms forming Te=O, Te-O- and their resonating bonds.

2. Experimental procedure

Reagent-grade TeO2, thallium carbonate and alkali carbonates were mixed in the required compositions and then melted in Au crucible for 10-30 min in the temperature range from 700 to 800C. The glass specimens were obtained by quenching the crucible in air or by dipping the bottom of crucible into ice-cold water. Crystalline specimens were obtained by gradually cooling the melts in the furnace. All the crystals were identi- fied by X-ray powder diffraction. Hygroscopic specimens depending on composition of alkali oxides were sealed in glass ampules. Physical properties such as density, d, glass transition tem- perature, Tg, deformation temperature, Td, and thermal expansion coefficient, a, were measured on samples of RbOl/z-TeO 2 and CsO1/z-TeO 2 glasses.

Raman spectra were measured in the wave- number range from 20 to 1000 cm -a with Ar + laser (514.5 nm, 200 mW) and R-800 laser-Ra- man spectrophotometer with triple-monochrome- ter supplied by Japan Spectroscopic Co., Ltd.

(JASCO). The spectra were observed at an angle of 90 to the exciting light. The intensities of Raman spectra of glass specimens with differing compositions were corrected by an internal stan- dardization technique with a standard of 35.2T101/2 64.8SIO 2 (mol%) glass. A small disk pellet was prepared from a mixed powder of each specimen and the standard glass. Raman spec- trum was measured while the disk pellet was rotating. Further, the Raman spectrum was de- convoluted into symmetric Gaussian functions. In order to confirm the peak deconvolution, the following procedure was used. A pair of polarized spectra, lib and I, was deconvoluted in the wavenumber range from 420 to 880 cm-a and was simulated by the minimal number of peaks, tak- ing the depolarization ratio, p(=l/Iii), into consideration. Generally, the Raman peaks differ from one another in the depolarization ratio [8]. The deconvolution parameters, peak positions, intensities, half widths and depolarization ratios, were optimized by the least-squares method. Fi- nally, the peak intensities were optimized for intensity-normalized Raman spectrum while the other parameters were fixed.

The analysis of normal vibration was carried out with the Cartesian symmetry coordinates method [9,10]. A simple Urey-Bradley force field [11] was used as the internal potential field. For TeO 2- ions which have C3v symmetry, Raman peaks have already been assigned by Siebert [12]. The force constants of stretching of valence bond, K, bending of bonds, H, and repulsion between non-bonding atoms, F, were optimized by a brute force method [13]. The initial value of the force constant for the optimization of stretching was

Table 1 Optimized force constants and calculated frequency values of TeO~- ion

Force constants

(mdyn/A.) Frequencies (cm 1)

A I E

obs. a) calc. obs. a) calc.

K(Te-O ) = 4.183 H(o-_xe_o- ) = 0.379 758 736 703 721 F(o- Te_o- ) = 0.250 364 329 326 353

a) Ref. [12].

130 71. Sekiya et aL / Raman spectra of MO1/2-TeO 2 (m = Li, Na, K,, Rb, Cs and Tl) glasses

given by Siebert [12], and the other initial values were assumed to be zero. The program of opti- mization [9] was developed for NEC PC-9801 series computer by the group of this laboratory. The results for the force constants are shown in table 1.

3. Results

3.1. MO1/2-TeO 2 (M = Li, Na, K, Rb and Cs) glasses

Figure 1 shows the intensity-normalized Ra- man spectra of NaO1/e-TeO2 glasses. The Ra- man spectra of MO1/2-TeO 2 (M = Li, Na, K, Rb and Cs) binary glasses containing equal amounts of MO1/2 are similar to each other. For example, the spectra of 30MO1/2 70TeO 2 (M = Li, Na, K and Rb) glasses are shown in fig. 2. Figure 3 shows examples of peak deconvolutions of the spectra of 30NaOa/e'70TeO 2 and 50NaO1/2.

T

_M=, J _

M=

M=Rb

1000 800 800 400 200 Wavenumber (cm -~)

Fig. 2. Intensity-normalized Raman spectra of 30MO1/2. 70TeO 2 (M = Li, Na, K and Rb) glasses.

T 4~

C-

O3

C--

1000 800 600 400 200 Wavenumber (cm -I)

Fig. l. Intensity-normalized Raman spectra of xNa01/2- (100-x)TeO 2 glasses.

50Ten 2 glasses. Five peaks are assigned at about 780, 720, 665, 615 and 450 cm -1 in the range from 420 to 880 cm-a, as is similar case for Ten 2 glass [1]. In order to make a quantitative judge- ment of the agreement between observed and simulated spectra, the R-value was defined as

Ei l / / (observed) - / / (calculated) [ R=

F.i l l i(observed) I

where I i is intensity at position u i. The averaged and maximal R-values were 0.02 and 0.03, respec- tively. A good agreement was obtained between the observed and simulated spectra. In the case of the binary glass system, shifts of the peak positions are small. The five peaks are named A, B, C, D and E (fig. 3). The composition depen- dences of individual peak intensities are shown in fig. 4. All the peaks observed in the range from 420 to 880 cm -a are attributed to the vibrations of coordination polyhedra of tellurium. It is seen in fig. 4 that peak intensities are reduced with respect to tellurium content of these binary

T. Sek~,a et al. / Raman spectra of MO1/2-TeO 2 (M = Li, Na, K, Rb, Cs and Tl) glasses 131

glasses. The ordinate shows the relative intensi- ties when the intensity of peak C of TeO2 glass is taken to be 1.00.

The relative intensity of peak E decreases with increasing alkali content. Peak C has a maximum intensity at about 5 mol%, and the intensity de- creases similar to that of peak E. The intensities of peaks A and B decrease until alkali content reaches about 10 mol%, and then increase with further increase of alkali content. Peak B has a maximum between 20 and 30 tool%. The change of intensity of peak D is similar to that of peak B.

In the composition range of alkali content > 30 mol%, the intensities of peaks A, B and D increase, while the intensities of peaks C and E decrease. These results indicate that new peaks may appear at the same wavenumber positions as peaks A, B and D on the addition of modifier. The new peaks are named peaks A', B' and D'.

The intensities of new peaks, I A, and Iw, are calculated from the formulas

IAr,gl =/A ,g l -- ( /A ,Te / / /E ,Te) /E ,g l ,

IB' ,gl = IB,gl -- ( IB ,Te / IE ,Te) IE,mgl,

where IA,gl, IB,gl and IE,gl are the intensities of peak A, B and E of binary glasses, respectively, and IA,Te, IB,Te and IE,Te are the intensities of peak A, B and E of TeO 2 glass, respectively. It is assumed that the intensities of the vibrations due to a continuous network composed of TeO 4 tbp's contribute to intensities of peak A and B in proportion to the composition dependence of the intensity of peak E. Taking the assignment of peak E into consideration (discussed below), it is supposed that, under this treatment, the esti- mated intensities of peaks A' and B' are equal to or smaller than exact intensities. The composition

~ f (a) cLO,5

.- , ' ' , . . , ...;~ ' .

o.ot

T

c

C

1.0

' (b )

c~. o.5

' ' - . . . . ~ ' ~ " ; " ~ "

0.0 " ~');~"

_ . J

i

r T

1000 800 800 400 200 1000 800 600 400 200 Wavenumber (cm -I) Wavenumber (cm -I)

Fig. 3. Polarized Raman spectra and deconvolution of xNaO1/2.(lOO-x)TeO 2 glass using a symmetric Gaussian function; (a) x = 30 and (b) x = 50. The observed spectra of I H and I , resolved Raman peaks (solid lines) and a sum of the peaks (broken lines)

are shown. The p is depolarization ratio. The observed and simulated results are indicated by points and a solid line, respectively.

132 T. Sekiya et al. / Raman spectra of MO1/2-ZeO 2 (M = L4 Na, 1~ Rb, Cs and TI) glasses

>" 1.4 I/1 C

1.2 -

-o 1.0'

0.8 r

la) 2.0 ; ~0.4

1.8 -~ ~0.3 ~" /9 d \ / J o \ 1.6 - o\ X~0"2 / / o

o\ u e / / u:peak A' I X =01 ~ ^ .'.

~o rr~ ' //~o o:peak B' \ 0t - - ' ' ~ ' i

_~ \ 0 10 20 30 40 ,~v ~XL iO,n Content (moP/,)

vX X a:peak A \ o:peak B

0.6 ~ v ~ v:peakC , , ,~ . -u~_~o z~ :peak D

0.4 i ~'~P7 - ~ 'v o:peak E ( N O

0.2 \ z~ ' - - ' - '~z~~z '

0 0 1'0 2'0 3'0 4'0 LiOn/2 Content (mol*/o)

2.0

,.8

1.6

>" 1.4 l.a c-

1.2 -

1.0'

~o 0.8 cr

0.6

0.4

0.2

0 0 NaOl/2

(b)

\ / / \ ;o.: / /

\ ~n, / / a:P eakA' " o~ ~ ~" /u z o : peak B'

- \ o- /o/

1'0 2'0 3'0 4'0 s'o / V~NaOl /2 Content (moP/,)

\ \ [] :peak A ~oVN o : peak B

X~ v:peak C /~a:~,~--ta~T.-- z~: peak D

1'0 2'0 3'0 4'0 5'0 Content ( tooll, )

2.0

1.8

1.6

>, 1.4

c

1.2 c-

-o 1.0 )

-~ 0.8 c~

0.6

(c) 0.4

~0.3 o/ ~o

o~ .0 .2

k_ ~ / / []:oeak A' \ 5,0, o:peak B,

v .~ c~ 0 6z'Z~I'0 2'0 3'0 4'0 /~ ' - ' '~o KO~z2 Content (motl,)

X \~ [] :peak A

\ \ o :peak B \v \ v :peak C

o~---...i'~X z~ :peak 0 ~, ~_...~a-------~'~m o :peak E ~ ~ -o

2.0

1.8

1.6

1.4 C

1.2 c"

-o 1.0 E l

-~ o.8

0.6

(d)

0.4

0.4

~0.3

0.2

gO.1

00 10 20 30 40 RbOln Content ( mol/o )

0.4 ~~=- - - - " - o ~ ~ ~o

0.2 0.2 . ~

O0 I'0 2'0 3'0 4'0 O0 10 20 30 4 ~ KOl/2 Content (mol*/.) RbOll2 Content (mol*l.)

u : peak A o:peak B v :peak C zx : peak D o:peak E

Fig. 4. Composition dependences of the intensities of Raman peaks A, B, C, D and E for MO1/z-TeO 2 glasses; (a) M = L: (b) M = Na, (c) M = K, (d) = Rb and (e) = Cs. The insets show the composition dependences of peaks A' and B'.

T. Sekiya et al. / Raman spectra of MO1/e-TeO2 (M = Li, Na, K, Rb, Cs and Tl) glasses 133

2.0

1.8

1.6

1.4

1.2 r"

~ 1.0' I1J

~ o.8 n-

0.6

0.4

(e)

\o

>,0.4 u :peak A' = o :peak B' 80.3 /

0.2 "O

u

.~0.1 13 ,~/O

0 1'0 2'0 3'0 Cs01/2 Content

( moP/ )

u:peak A o:peak B v:peak C

O ." - - - - - -O ~-------D ~:peak D ~....____~ o:peak E

0.2

o o

CsO~12

i 10 2'0 3'0

Content ( moP/. ) Fig. 4. (continued).

dependences of intensities of peak A' and B' are shown in the insets in fig. 4. The intensity of peak A' increases with increasing alkali content. The change of intensity of peak B' has a maximum at about 30 mol%. The detail of the peak assign- ments is discussed below.

3.2. T lO] /2 -TeO 2 glasses

The intensity-normalized Raman spectra of T101/2-TeO2 glasses are shown in fig. 5. The shape of Raman spectra of the glasses having TIO1/2 content up to 30 mol% is similar to that of binary alkali tellurite glasses containing equal amounts of MO1/2. The peak deconvolution indi- cates the presence of five peaks in the range from 420 to 880 cm -1, similar to the spectra of alkali tellurite glasses; with increasing T101/2 content, the peak at about 450 cm-1 becomes weaker and the peak at about 780 cm -1 shifts toward lower wavenumber. The peaks at about 275 and 720 cm -1 become more intense, and the peak and about 275 cm-~ shifts toward higher wavenum- bet. In the spectrum of 66T101/2-34TeO 2 glass

T >-

09 C

4-) C

1000 800 G00 400 200 Wavenumber (cm -~)

Fig. 5. Intensi~-normalized Raman spectra of xTIO1/2"(100- x)TeO 2 glasses.

sample, the peaks are located at about 100, 320, 680 and 725 cm-1.

The Raman peaks of T101/z-TeO 2 glasses were more intense than those of alkali tellurite glasses. The color of the glass specimens changes from pate yellow through yellow to red with in- creasing T1OI/2 content. It seems that T101/2- TeO 2 glasses having higher T1Oa/2 content have resonance- or preresonance-Raman effect. The resonance- and preresonance-Raman effects are observed when the exciting frequency lies in and is close to the electronic absorption band of the compound, respectively [8,14]. Therefore, no de- tailed discussion is made with respect to the composition dependence of the peak intensities in TIOa/z-TeO 2 glass system.

3.3. Properties

The composition dependences of molar vol- ume, Vm, glass transition temperature, Tg, and thermal expansion coefficient, a, are shown in

134 7". Sekiya et al. / Raman spectra of MO1/2-Te02 (m = LL Na, K, Rb, Cs and Tl) glasses

30 _o~O- ~ - - D []-

"O ~ '~- -6 ~ . E o NaOll2 ~ - -%

a. KOl/2 ~! .x " [] RbOv2

2C 0 Cs01/2 AgOv2 TlOv2

o 2'0 4'0 6'0 MOv2 content (mo l%)

Fig. 6. Composition dependences of molar volume, Vm, of MO1/2 -TeO 2 (M = Li, Na, K, Rb, Cs, Ag and T1) glasses.

figs. 6, 7 and 8, respectively. The Vm, Tg and oz of binary glasses containing Li, Na, K, Ag and T1 are due to ref. [2]. The molar volumes change linearly with the content of MO1/2. With increasing MO1/2 content, the glass transition temperatures

30.0

d~ "O v

25.0 E, x

20.0

: LiOl/2 o : NaOv2 c/ /#"

:KOv2 / / [] : RbO1/2 / / O : CSOl/2 / / : AGO1/2 /,,, / /d :TIOv2 __4/,~" ,~

/ / /

/1

15c 2'o 4'o 6'o MOll2 content (moP/o)

Fig. 8. Composition dependences of thermal expansion coeffi- cient, a, of MOI/a -TeO 2 (M = Li, Na, K, Rb, Cs, Ag and T1)

glasses.

decrease and thermal expansion coefficients in- crease linearly.

300

250

200

150

1000

:LiO~/2 o : NaO1/2 zx : KO1/2 [] : RbOvz o CsOv2 AgOv2 TlOlt2

2'0

\ 6'0

MOv2 content (mol/o)

Fig. 7. Composition dependences of glass transition tempera- ture, Tg, of MO~/2 -TeO 2 (M = Li, Na, K, Rb, Cs, Ag and TI)

glasses.

4. Discussion

4.1. Assignment of Raman peaks

Assignments of Raman peaks observed in the spectra of the glasses are made on the basis of Raman spectra of MO1/2-TeO2 crystals and TeO 2 glass. Few investigations for Raman spec- tra of MO1/2-TeO: crystals have been made. The present authors investigated Raman spectra of o~-TeO 2 crystal [1]. In this section, a relation- ship between Raman peaks observed in spectra of binary tellurite crystals and their structural units is discussed in order to assign Raman peaks observed in spectra of binary tellurite glasses.

The coordination states of tellurium atoms are classified to TeO 4 tbp, TeO 3 tp and their inter- mediate states which are found in TeO 2 crystals and binary tellurite crystals. The Te-O distance in the TeO 4 tbp ranges from 0.185 to 0.195 nm for two equatorial sites and from 0.205 to 0.215 nm for two axial sites. In the TeO 3 tp, the Te-O

T. Sekiya et al. / Raman spectra of MOl/2-Te02 (M = Li, Na, K, Rb, Cs and Tl) glasses 135

T r

c "

Li2Te03 7121689

N~Te03

2

IC~Te03 L TI~Te03 J

1000 800 600 400 200 Wavenumber (cm -' )

Fig. 9. Raman spectra of M2TeO 3 (M = Li, Na, K, Rb, Cs and T1) crystals.

distance is in the range 0.185-0.200 nm. The Te-O distance in the intermediate states is di- vided into two groups: shorter three bonds of 0.185-0.200 nm and a longer bond of 0.220-0.260 r im.

4.1.1. M2TeO 3 (M = Li, Na, K, Rb, Cs and Tl) crystals

Figure 9 shows the Raman spectra of MzTeO 3 (M = Li, Na, K, Rb, Cs and T1) crystals. These crystals contain isolated TeO 2- ion [15-19]. The isolated TeO 2- ion has a trigonal pyramidal structure, in which three Te-O bonds are equiva- lent because of a resonating effect between one Te=O and two Te-O- bonds [12]. Raman spectra of KzTeO3, RbzTeO 3 and Cs2TeO 3 crystals are very similar to one another. Each of them has a

strong sharp peak at about 780 cm-1 and weak peaks in the range from 250 to 350 cm -1. Thiimmel and Hoppe [17] showed that KzTeO3, RbzTeO 3 and CszTeO 3 crystals are isomorphous and can be described as hexagonal. On the other hand, Loopstra and Goubitz [18] concluded that CszTeO 3 crystal has a hexagonal unit cell and that TeO 2- ion has C3v point symmetry. Four normal vibrations, vl(A1), uz(A1), ~,3(E) and u4(E), are Raman active. According to the result of normal vibrational analysis of the TeO 2- ion, the peak at about 780 cm-1 is assigned to sym- metric stretching vibration of TeO 2- ion (v 1) and the peaks at 250-350 cm-1 are assigned to sym- metric bending vibration of TeO 2- ion (re).

In LizTeO 3 crystal, two strong peaks are ob- served at 725 and 798 cm-1. These two peaks are assigned to Te -O- stretching vibrations [2]. The decrease in the symmetry of isolated TeO3 z- ion causes an increase in the peak intensity of the vibration originally associated with E species [20]. The peaks observed in the range from 310 to 410 cm-1 is assigned to bending vibrations of TeO z- ion.

In Na2TeO 3 crystal, the Raman peaks as- signed to Te -O- stretching vibrations appear in a lower wavenumber region. The averaged length of Te -O- bonds of TeO 2- ion in NazTeO 3 crystal is slightly larger than that in Li2TeO3, KzTeO3, RbzTeO 3 and CszTeO 3 crystals. The distance between the NBO and the adjacent tel- lurium atom is distributed in the range from 0.291 to 0.310 nm in NazTeO 3 crystal [16]. It is assumed that there is a weak interaction between these atoms. As a result, the Te-O- bond is weakened by this weak interaction.

In T12TeO 3 crystal, the Raman peaks assigned to v I and v 2 of TeO32- ion are observed at 730 and 293 cm -1, respectively. The peaks due to Te -O- stretching vibrations shift to a lower wavenumber region because of partial covalency of Tl + ion, compared with K +, Rb + and Cs + ions in KzTeO3, RbzTeO 3 and CszTeO 3 crystals, re- spectively.

4.1.2. M2Te205 (M = Li and Cs) crystals Raman spectra of M2Te205 (M = Li and Cs)

crystals are shown in fig. 10. CszTezO 5 crystal

136 T. Sekiya et aL / Raman spectra of MO1/2-ZeO 2 (M = Li~ Na, 1~ Rb, Cs and TI) glasses

t c- CD

.4-) c-

645 I

~-L i2Te205

3-k i2Ye20s

725

;s2Tea0s 1000 800 600 400 200

Wavenumber (cm -~ )

Fig. 10. Raman spectra of M2Te205 (M = Li and Cs) c~stals.

contains isolated Te20 ~- ions [18]. Each tel- lurium atom forms a TeO 3 tp and has two NBOs by resonating between Te---O and Te-O- bonds. The distance between one of the NBOs and adja- cent tellurium atom of neighboring Te2 O2- ion is in the range of 0.268-0.270 nm. Two intense, sharp peaks are observed at 725 and 776 cm -1, and a broad peak is observed at 260 cm-1 in the Raman spectrum of Cs2T%O 5 crystal. The in- tense peaks are assigned to Te -O- stretching vibrations. A weak interaction between one of the NBOs and tellurium atom of adjacent Te2 O2- ion will cause the Te-O- stretching vibration at 725 cm-1 to shift to a lower wavenumber region, as is the case for Na2TeO3 crystal. The peak at 260 cm -1 is assigned to a bending mode of TeO 3 tp having NBOs.

a-Li2Te205 crystal [21] has (Te202-)= sheet structure, in which TeO3+ 1 polyhedra having Te-O- bond are connected at the three vertices. The Raman spectrum of this crystal shows two intense peaks at 645 and 785 cm-1. The peak at 785 cm-1 is assigned to Te -O- stretching vibra-

tion of the TeO3+ 1 polyhedron. The peak at 645 cm -1 is assigned to antisymmetric vibration of the Teui+~-O-- -Tem+ I linkage. The Roman nu- meral subscript stands for the coordination state of tellurium atom. Two types of Te -O-Te link- ages are formed in a-Li2Te205 crystal: one is Ten i+ i -O- - -Tem+ I and the other is Te ln+i -O- Teiii+ I linkage which is formed by oxygen-shar- ing of almost equivalent Te -O bonds. The former linkage is similar to the Te-eqOax-Te linkage in paratellurite. The peak at 645 cm -1 is compared with that at 649 cm -1 and is assigned to com- bined effect of usiTeO 4 and antisymmetric vibra- tion of Te-eqOa~-Te linkage in paratellurite [1]. The difference in polarizability between Te m + i- O (Te-eqO) and Ten i+r - -O (Te -~O) bonds may cause a change in Raman peak intensity of Te iii + i -O- - -Tein I (Te- eqOax-Te) linkage. The peaks observed in the range from 270 to 400 cm-1 are assigned to symmetric stretching (and bending) vibrations of Te ln+i -O- - -Te ln+i and Te ni + i- O -Te iii + i linkages.

In I3-Li2Te205 crystal [21], a pair of TeO3+ 1 polyhedra is connected by a common edge to form a Te206 unit. Each tellurium atom has one NBO. The Raman spectrum of I3-Li2Te205 crys- tal shows two strong peaks at 691 and 817 cm-1. The peaks at 691 and 817 cm -~ are assigned to antisymmetric vibration of Tem + i-O- - -Teill + i linkage and Te-O- stretching vibration, respec- tively. These peak positions do not correspond to those in the spectra of binary glasses and are observed in the higher wavenumber region com- pared with those of o~-Li2Te205 crystal. The vi- brations are localized because of the edge sharing of TeO 3 + 1 polyhedra and the peaks, which have similar vibrational modes, are shifted to a higher wavenumber region. This tendency has been found for polymorphic forms of TeO 2 crystals, paratellurite and tellurite [1].

4.1.3 Cs2Te4O 9 crystal In the Raman spectrum of Cs2Te409 crystal

(fig. 11), two intense peaks are observed at 672 and 728 cm -1, and a less intense peak at 470 cm -1. According to Loopstra and Goubitz [18], Cs2Te409 crystal contains two different tellurium atoms, Te(1) and Te(2), whose coordination states

T. Sek&a et al. / Raman spectra of MO1/2-TeO 2 (m = Li, Na, K, Rb, Cs and Tl) glasses 137

T f -

O3 ~o C

'28 /

Cs2Te40s

1000 800 800 400 200 Wavenumber (cm -~)

Fig. 11. Raman spectra of Cs2Te409 crystal.

are TeO3+ t and TeO3+ 2 polyhedra, respectively. However, we think that the coordination state of Te(2) is not TeO3+ 2 polyhedron but TeO 3 tp having a weak interaction because of the reasons discussed below.

(a) The relationship between the bond length and the bond strength for various tellurite crys- tals is given by Philippot [22] with the equation,

S = 1.333(r/0.1854) -5.2,

where r is the bond length (nm) and S is the bond strength. The Te-O distances and calcu- lated bond strengths in Cs2Te40 9 crystal are shown in table 2. In the case of Te(2), the bond strengths for two longer Te -O bonds are very small compared with those for three shorter bonds. It seems that the two longer bonds be- tween Te(2) and oxygen atoms can be neglected, although weak interactions may exist.

Table 2 Te-O bond lengths and bond strengths in Cs2Te40 9 crystal

Te-O bond length Te-O bond strength (nm)

Te(1)-O(3) 0.1854 1.333 0(2) 0.1870 1.275 0(4) 0.2020 0.853 0(2') 0.2315 0.420

Te(2)-O(1) 0.1842 1.378 0(4) 0.1956 1.009 0(5) 0.1978 0.952 0(3) 0.2403 0.346 O(1') 0.2513 0.274

The numerals of atoms are taken from ref. [18].

(b) The composition dependence of the molar volume gives information about the coordination state of the individual cations. The molar volume of Cs2Te40 9 crystal is larger than the interpo- lated value with polymorphic forms of TeO 2 crys- tals and Cs2Ye205 crystal. We suggest that Te(2) has a small coordination number because an in- crease of the coordination number of tellurium atom causes a decrease of the molar volume.

(c) The glass formation range of the CSOl/2- TeO 2 system is smaller than that of other alkali tellurite glasses. Cs2Te40 9 crystals precipitate when the melt containing 25-40 mol% CsO1/2 is cooled. We suggest that an addition of Cs + ion results in cleavage of Te -O-Te linkage and the glass formation is hindered in this composition range.

We assume that the coordination state of Te(2) is TeO 3 tp for the above-mentioned reasons. Cs2Te409 crystal is composed of Te(1)O3+ 1 poly- hedra having Te=O- bond and of Te(2)O 3 tp's having Te-O bond, and has a (Te402-)= ladder- like structure containing Ye(1) i i i+ i -O- - - Te(1)in+i , Te(1)m+i-O-Te(2)ni and Te(2)ii I- O-Te(2)in linkages. The NBOs may interact weakly with adjacent Te(2). The Raman peak at 728 cm-t is assigned to the stretching vibrations of Te=O and Te-O- bonds. These vibrations are shifted to a lower wavenumber region because of the weak interaction. The peak at 672 cm-1 in Cs2Te40 9 crystal, compared with the peak at 649 cm-1 in paratellurite, is assigned to the antisym- metric vibration of Teill+ I -O - - -Te l I I + I linkage. The peak at 470 cm-1 is assigned to symmetric stretching (and bending) vibrations of Tern+ I- O- - -Te ii1 + i and Te lI1 + I(nI) -O-Te nI linkages.

4.1.4. TeO 2 glass The Raman spectrum of TeO 2 glass is also

deconvoluted into five peaks in the wavenumber range 420-880 cm -1. Our previous work based on the normal vibrational analysis of o~-TeO 2 crystal showed that the five peaks are assigned to us2TeO 4 (and Vs2+asTeO4) , Usl+asTeO4, UslTeO4, uasTeO 4 and 6stTeO4 (and 6s2TeO4), respectively, and that the former four and the other are as- signed to vasTe-eqO~-Te and vsTe-eqO~-Te , respectively [1]. It was indicated that, in TeO 2

138 T. Sekiya et aL / Raman spectra of MO1/2-TeO 2 (m = Li, Na, I~ Rb, Cs and Tl) glasses

.=.

'S

0

0

I

I

-4

"~ '~1 ,.# ~ O~ O~ 0~',

' ~ ~ ~

-[ o7

e-,

m

L~

7

6

L7 E

7

oo~ LLo + + I

I

O~

7

6 7~

'.or I + II

I

I

0 ~

?v

+' ,1~

+

oo ~ ~ c~ 0 ~ ~ ea j~ ~ ~ a3

E

.4

Z

..=

o~

~.~0

=~ ~

T. Sekiya et al. / Raman spectra of MO1/2-Te02 (M = Li, Na, K, Rb, Cs and Tl) glasses 139

glass, TeO 4 tbp's are formed by most of tellurium atoms and connected at vertices by forming Te- eqOax-Te linkages [1]. The five peaks correspond to the peaks A -E observed in the spectra of binary glasses. We suggest that vibrations of the continuous network of TeO 4 tbp's contribute to intensities of five peaks observed in the spectra of binary glasses.

4.1.5. Assignment of Raman peaks observed in spectra of binary glasses

We assume that a peak observed in the spec- trum of a binary glass with the same wavenumber as a peak not only in a binary crystal but also in TeO 2 glass is due to the same structure and vibrational mode. A relationship between Raman peaks observed in the spectra of binary glasses (peaks A -E and at about 270 cm-1) and those of alkali tellurite crystals and TeO 2 glass are given in table 3. In the spectra of MO1/2-TeO 2 (M = Li, Na, K, Rb and Cs) glasses, the deconvoluted peaks are assigned on the basis of the assign- ments of binary crystals and TeO 2 glass as fol- lows.

(a) At the frequency of peak A (about 780 cm-1), the vibration of the continuous network composed of TeO 4 tbp's is overlaid with peak A'. The peak A' is assigned to the Te-O- stretching vibration of TeO3+ 1 polyhedra or TeO 3 tp's. In this case, NBO has little interaction from adja- cent tellurium atoms.

(b) The peak B at about 720 cm-1 is divided into two parts. One is due to peak B' and the other due to the vibration of the continuous network composed of TeO 4 tbp's. The peak B' is assigned to stretching vibrations between tel- lurium and NBO of TeO3+ 1 polyhedron and TeO 3 tp in which the NBO interacts with adja- cent tellurium atoms. The frequencies of Te -O- and Te--O stretching vibrations are independent of the coordination state of tellurium atoms in TeO3+ 1 polyhedra and TeO 3 tp's, and are depen- dent on the surroundings around the NBOs. In silicate glass, the stretching vibration of NBO is localized [23]. We assume that a weak interaction between adjacent tellurium and NBO weakens Te-O- and Te=O bonds and causes the stretch- ing vibrations of both bonds to shift toward a

lower wavenumber region, when NBO is adjacent to other tellurium atom. We suggest that the existence of peak B' is due to NBOs interacting with adjacent tellurium atoms. The peak B' is hardly possible to be assigned to Te -O- stretch- ing vibration of distorted TeO 2- ions observed in Li2TeO 3 crystal, although it appears in the same wavenumber region as peak B', as is seen in table 3. Because, in binary glasses, no peak is observed in the frequency range of bending vibrations of distorted TeO~- ion.

(c) The peak C at about 665 cm-1 is assigned to the antisymmetric vibrations of Te -O-Te link- ages constructed by two unequivalent Te -O bonds. The Te iv -eqOax-Te lv , Teni+i(iii )- O---TeIii+i, Teiv-eqO---Telii+ I and Te iv- a~O-Tem+I0n~ linkages are examples of this type of Te -O-Te linkages. Generally, in binary tellu- rite crystals, either or both of tellurium atoms in this type of Te -O-Te linkage must form TeO 4 tbp or TeO3+ 1 polyhedron [18,21,24-28]. We suggest that the existence of peak C indicates a continuous network structure containing TeO 4 tbp's and TeO3+ 1 polyhedra. Therefore, the in- tensity of peak C will probably reflect the fraction of TeO 4 tbp's and TeO 3 + 1 polyhedra.

(d) The peak D at about 615 cm -1 also in- cludes the peak D' and reflects the vibration of the continuous network composed of TeO 3 tbp's. The position of peak D' corresponds to that of the peak observed at 633 cm -1 in Cs2Te40 9 crystal.

(e) The peak E at about 450 cm-1 is assigned to the symmetric stretching (and bending) vibra- tions of Te -O-Te linkages, which are formed by vertex-sharing of TeO 4 tbp's, TeO3+l polyhedra and TeO 3 tp's. We suggest that the existence of peak E indicates a continuous network consisting of TeO n (n = 4, 3 + 1 and 3) polyhedra.

(f) The broad peak observed at about 270 cm -1 is assigned to the bending vibrations of TeO 3 tp's having NBOs. The appearance of this peak together with peaks A' or B' indicates the existence of TeO 3 tp having NBOs.

The assignments the Raman peaks for the TIO1/2TeO 2 glasses are the same as those for alkali tellurite glasses. With increasing T101/2 content, the peaks assigned to stretching vibra-

140 T. Sekiya et aL / Raman spectra of MO1/TTeO 2 (M = Li, Na, K, Rb, Cs and Tl) glasses

tions between tellurium and NBOs are shifted to lower wavenumber because of the partial cova- lency of T1 + ion [29]. The Raman spectrum of 66T101/z'34TeO 2 glass is similar to that of T12TeO 3 crystal. The peaks at 320, 680 and 725 cm -a are assigned to /"2, /'3 and /'1 of TeO z- ion, respectively. The peak at 100 cm -1, is as- signed to the vibration of T10 n (n < 4) polyhe- dron [30].

4.2. Structure o f binary MO1/2-TeO 2 glasses

TeO 2 glass has a network structure in which TeO 4 tbp's are composed of tellurium atoms con- nected at vertices by Te-eqOax-Te linkages. The composition dependences of the Raman peak intensities are due to changes of basic structural units of coordination polyhedra of tellurium atoms.

The glasses having low alkali content have a continuous network structure composed of TeO 4 tbp's and TeO3+ 1 polyhedra sharing vertices. Since the peak at 270 cm-1, due to vibration of TeO 3 tp having NBOs, is absent in the spectra of the glasses, when the alkali content is less than 20 mol%, we assume that the fraction of TeO3+ 1 polyhedra having Te-O- bond increases with increasing alkali content in this compositional range. A part of these NBOs will interact weakly with adjacent tellurium atoms. The composition dependence of the intensity of peak E indicates a decrease in Te -O-Te linkages and a cleavage of the continuous network composed of TeO 4 tbp's and TeO 3 + 1 polyhedra. In glasses containing 20- 30 mol% alkali oxides, TeO 3 tp's having NBO's are formed in the continuous network. The de- crease in the intensity of peak E and the increase in the intensity of peak A' indicate a cleavage of the continuous network. The composition depen- dence of the intensity of peak C indicates a decrease in the fraction of TeO 4 tbp's and TeO3+ 1 polyhedra. The appearance of the peak at 270 cm -1 in the glasses containing 20 mol% alkali oxide indicates the formation of TeO 3 tp having NBO's. The increase of intensity of peak B' to 30 mol% alkali oxide indicates that the fraction of NBOs interacting weakly with adjacent tellurium atoms increases with increasing alkali content. It

seemg that the TeO 3 tp's are included in the continuous network until alkali content reaches 30 mol%. We think that the NBOs incorporated into the continuous network interact weakly with adjacent tellurium atoms and that the NBOs of isolated structural units have little interaction. When the alkali content is more than 30 mol%, the fraction of NBOs interacting weakly with adjacent tellurium atoms decreases. The compo- sition dependences of intensities of peaks A' and B' indicate that isolated structural unit, Te20 ~- ion, coexists with the continuous network. We assume that the fraction of TeO 4 tbp's decreases along with the decrease in the fraction of Te -O- Te linkages. The glasses containing nearly 50 mol% alkali component are composed of a con- tinuous network constructed by sharing vertices of TeO3+ 1 polyhedra and TeO 3 tp's, and of iso- lated structural units, such as isolated Te20 2- and TeO32- ions. Few TeO 4 tbp's will coexist in such glasses. The glass formation becomes diffi- cult with increasing fractions of isolated struc- tural units and NBOs.

Based on the similar shapes of their Raman spectra, the structures of xTIO1/2 (100-x)TeO 2 (0 < x < 30) glasses are similar to those of alkali tellurite glasses. In the glasses containing more than 40 mol% T101/2, Raman peaks, assigned to stretching vibration between tellurium and NBOs at about 750 cm-1 and to bending vibration of TeO 3 tp having NBOs at about 280 cm -1, have large intensities. We suggest that the fraction of TeO 3 tp's having NBOs in the T101/2-TeO 2 glasses is larger than that in binary alkali tellurite glasses. When T101/2 content is larger than 50 mol%, the glasses consist of isolated structural units, such as Te2 O2- and TeO32- ions. As men- tioned above, almost all tellurium atoms will form isolated TeO32- ions in 66T101/2"34TeO 2 glass. A wide glass formation range of this system may be due to a role of s2-type ion, such as T1 + and Pb 2+ [29,31]. A part of T1 + ions participates in the glass network and the other behaves as a modifier [30].

The present authors propose a new mecha- nism for the structural changes of the tellurite glasses on the basis of three structural assump- tions.

T. Sekiya et al. / Raman spectra of MO1/2-ZeO 2 (M = Li, Na, IV, Rb, Cs and Tl) glasses

(i) Te-O-Te linkages formed in the tellurite glasses are classified into two groups: one con- tains Teiv-eqOax-Teiv, Tem+i(m)-O---Teni+i, Te w- eq O- - -Te iii + i and Te:v- a~ O-TeIH + I(Im linkages formed by connecting long Te-O bond (> 0.20 nm) and short Te-O bond (< 0.20 nm), and the other contains Teiii+i(m)-O-Tem+Km) linkages formed by connecting two middle Te-O bonds (~ 0.20 nm).

(ii) NBO is formed in the shortest Te-O bond. (iii) The change of coordination state of tel-

lurium atom from 4 through 3 + 1 to 3 results from the large electron-donating ability of NBO. The production of TeO 3 tp is accelerated by an additional resonance energy between Te-O- and Te=O bonds. Our mechanism differs from the ones proposed by Yoko et al. [7] who take ac- count of only the effect of electron-donating abil- ity of NBO.

The structural change from TeO 4 tbp to TeO3+ 1 polyhedron having Te-O- bond is illus- trated in fig. 12. A small amount of MO1/2 added to TeO 2 glass breaks the Te-eqOax-Te linkage and, then, we imagine that two types of NBOs, Te-eqO- and Te-~xO- bonds are created as shown in the illustration (fig. 12(I), (II)). Since the TeO 4 tbp having Te-ax O- bond is unstable, it is distorted into TeO3+: polyhedron; the Te- ~,O- bond shortens and, as a result, the other Te-axO bond elongates, forming a Te---O bond (fig. 12(III)). The question remains whether the TeO 4 tbp having Te-eqO- bond is distorted into TeO3+ 1 polyhedron or remains in a non-dis- torted configuration. This type of TeO 4 tbp seems to be less stable than TeO3+ 1 polyhedron having Te-O- bond because the increase of the electron-donating ability in Te-eqO- bond will weaken the Te-axO bonds. We assume that this type of TeO 4 tbp remains in a less-distorted configuration in the glasses having low alkali con- tent and is highly distorted in the glasses having high alkali content. Since three vertices of this type of TeO 4 tbp will be shared with other TeO 4 tbp's in the glasses having low alkali content, we assume that the distortion attendant with a change of Te-O bond strengths of surrounding TeO 4 tbp's is forbidden. The increase of Raman inten- sity for the peak C, observed in the range 0-5

141

(I)

(ll)

2MOv2

4D-

M ~

"-c>

4Z>

O :Oxygen Atom :Telturium Atom

M :Modifier Atom

:Etectric Charge

(III)

T u M~C~

L

~:5hor t Bond (_2.0~) - - - : (> 2.2,~1

-*s~-:Shortening of Bond Length

L* :Elongating of Bond Length

Fig. 12. Mechanism for the structural change induced by the addition of MO1/2 component into the network composed of

TeO 4 tbp's.

mol% alkali content, may indicate the existence of TeO 4 tbp. The vibration of TeO 4 tbp corre- sponding to peak C is the stretching mode in reverse phase for two Te-eqO and two Te-~xO bonds [1]. The different polarizability changes for

142 T. Sekiya et al. / Raman spectra of mO1/2-TeO 2 (m = Li, Na, I~ Rb, Cs and Tl) glasses

the Te-eqO and Te-~xO bonds will contribute to the change of peak intensity. Since the polariz- ability change of Te-eqO- bond is larger than that of bridging Te-eqO bond, the peak intensity will increase along with the formation of TeO 4 tbp having Te-eqO- bond. In the glasses having higher alkali content, change of this type of TeO 4 tbp into TeO 3 + 1 polyhedron having Te-O - bond will be produced by a cleavage of the continuous network and an increase of the fraction of TeO 3 + 1 polyhedra formed in the continuous network.

The formation of TeO 3 tp having Te=O bond is shown in fig. 13. This mechanism is similar to

(m)

( I V ) ~ ~ "T" M

Cleave > I , - - - - ~

(v)

M L oA (V l )

. . . . . o . . ve

. . . . . _~. . - . (~

M ,4 Cleave

(VII) ~,_~__

% l ~ a v ~

M " ~ U

. - (3 . . . .

M

( vii; )

.%'"

e l :oxygen Atom ~ Short Bond (2.0~,) M :Modifier Atom -- (>2.2]k) :Electric Charge = Double Bond

Fig. 14. Mechanism for the structural change from the net- work composed of TeO3+ I polyhedra into isolated Te2 O2-

ion: the arrow -* indicates the transfer of electron.

O :Oxygen Atom ~:Shor t Bond (

T. Sekiya et al. / Raman spectra of MO1/e-TeO: (M = Li, Na, K~ Rb, Cs and Tl) glasses 143

found only in Cs2Te40 9 crystal. We assume that this structural unit is formed in a glass which contains a modifier ion having large ionic radius.

The structural change from TeO3+ 1 polyhe- dron to TeO 3 tp is illustrated in fig. 14. When the Te- - -O bond in TeO3+ 1 polyhedron is elongated and cleaved, a terminal TeO 3 tp is formed which has two NBOs (fig. 14 (VI), (VII)). When the Te-- -O bonds of neighboring TeO 3+ 1 polyhedra are cleaved, an isolated Te2 O2- ion is formed (fig. 14 (VIII)). Further addition of alkali oxide causes the TeI I I -O-Te m linkage of Te2 O2- ion to break and results in the formation of isolated TeO 2- ion (this is not shown in the figure). The TeO 3 tp which has two or three NBOs is stabi- lized by the resonance between Te=O and Te-O- bonds.

It is noted that the properties of these glasses, i.e., molar volume, glass transition temperature and thermal expansion coefficient, change almost linearly with increasing MO1/2 content (figs. 6-8). This change is explained by the coordination change of tellurium atom taking place succes- sively f rom TeO 4 tbp through TeO3+ 1 polyhe- dron to TeO 3 tp with increasing MO1/2 content.

5. Conclusions

The intensity-normalized Raman spectra of al- kali tellurite glasses containing equal amount of alkali oxide are similar to each other. With in- crease of alkali content, the coordination state of tellurium atom in binary glasses changes from TeO 4 tbp through TeO3+ I polyhedron to TeO 3 tp and NBOs increase. In the glasses containing nearly 50 mol% alkali oxide, isolated structural units, such as Te20 2- and TeO32- ions, coexist with a continuous network constituted by TeO 3__ 1 polyhedra and TeO 3 tp's. The thallium tellurite glasses have similar structure. In the 66T101/2. 34TEO 2 glass, most of tellurium atoms are in a form of isolated TeO~- ion. A new hypothesis is proposed for a mechanism for the basic structural changes in the tellurite glasses on the basis of three structural assumptions. First, two types of Te -O-Te linkages are present in glasses: one is formed by connecting long Te-O bond (> 0.20

nm) and short Te -O bond (< 0.20 nm) and the other is formed by the connection of two middle Te -O bonds (-- 0.20 nm). Second, NBO is formed in the shortest Te -O bond. Last, the change of coordination state of tellurium atom from 4 through 3 + 1 to 3 results from the large electron-donating ability of NBO. The production of TeO 3 tp is accelerated by an additional reso- nance energy between Te-O- and Te=O bonds.

References

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144 T. Sekiya et al. / Raman spectra of mOl /2 -TeO 2 (m = Li, Na, I~ Rb, Cs and Tl) glasses

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