Synthesis, structure refinement and characterisation of a new oxyphosphate Mg0.50TiO(PO4)

Synthesis, structure refinement and characterization of

tetrahydrated acid gadolinium diphosphate HGdP2O7�4H2O

Sana Hraiech a, Fathia Chehimi-Moumen a,*, Mokhtar Ferid b,D. Ben Hassen-Chehimi a, Malika Trabelsi-Ayadi a

a Laboratoire de Physico-Chimie Minerale, Faculte des Sciences de Bizerte, 7021 Zarzouna-Bizerte, Tunisiab Laboratoire des Procedes Chimiques, Institut National de Recherche Scientifique et Technique,

B.P. 95, Hammam-Lif, Tunisia

Received 29 April 2005; received in revised form 6 June 2005; accepted 5 July 2005

Available online 26 July 2005

Abstract

Synthesis and single crystal structure are reported for a new gadolinium acid diphosphate tetrahydrate

HGdP2O7�4H2O. This salt crystallizes in the monoclinic system, space group P21/n, with the following unit-

cell parameters: a = 6.6137(2) A, b = 11.4954(4) A, c = 11.377(4) A, b = 87.53(2)8 and Z = 4. Its crystal structure

was refined to R = 0.0333 using 1783 reflections. The corresponding atomic arrangement can be described as an

alternation of corrugated layers of monohydrogendiphosphate groups and GdO8 polyhedra parallel to the (1 0 1)

plane. The cohesion between the different diphosphoric groups is provided by strong hydrogen bonding involving

the W4 water molecule.

IR and Raman spectra of HGdP2O7�4H2O confirm the existence of the characteristic bands of diphosphate group

in 980–700 cm�1 area. The IR spectrum reveals also the characteristic bands of water molecules vibration (3600–

3230 cm�1) and acidic hydrogen ones (2340 cm�1). TG and DTA investigations show that the dehydration of this

salt occurs between 79 and 900 8C. It decomposes after dehydration into an amorphous phase. Gadolinium

diphosphate Gd4(P2O7)3 was obtained by heating HGdP2O7�4H2O in a static air furnace at 850 8C for 48 h.

# 2005 Elsevier Ltd. All rights reserved.

Keywords: A. Inorganic compound; B. Chemical synthesis; C. X-ray diffraction; D. Crystal structure

www.elsevier.com/locate/matresbu

Materials Research Bulletin 40 (2005) 2170–2179

* Corresponding author. Tel.: +216 72 491 526; fax: +216 72 491 526.

E-mail address: [email protected] (F. Chehimi-Moumen).

0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2005.07.008

1. Introduction

The synthesis of acid diphosphates of rare earth trivalent cations of general formula HLnP2O7�xH2O (Ln = La–Lu, x = 3; 3, 5; 3–4) has been reported by several authors [1–7]. It has been repor-

ted that they form three isostructural groups; from La to Sm, diphosphates display the orthorhombic

symmetry [4,6] and from Sm to Yb, diphosphates display the triclinic symmetry [4,5]. Structures

of HLnP2O7�3H2O (Ln = Gd [5], and La [6]) have been reported, showing that the gadolinium

compound crystallizes in the P1 space group [5], while lanthanum diphosphate displays the Aba2

space group [6]. The samarium acid diphosphate tetrahydrate HSmP2O7�4H2O is a unique isolated

compound found in a third crystalline form [7]. It crystallizes in the monoclinic system, space group

P21/n. It can be noted that the samarium diphosphate is found to be stable in the three crystalline

forms.

The present paper reports the chemical preparation and the characterization of a new gadolinium

compound HGdP2O7�4H2O, which exhibits monoclinic symmetry and is isostructural to the samarium

diphosphate tetrahydrate previously reported [7].

2. Experimental

Single crystals of HGdP2O7�4H2O were obtained by adding 10 ml of GdCl3�6H2O aqueous solution

(10�2 M) to 20 ml of diphosphoric acid solution (10�2 M) and evaporation of the solvent at room

temperature. The crystallization of prismatic crystals of HGdP2O7�4H2O started from the solution after a

few days. The single crystals are then isolated and washed with distilled water. The obtained compound is

found to be stable for months in normal room conditions.

The aqueous solution of H4P2O7 was obtained from the sodium salt Na4P2O7�10H2O by using an ion-

exchange resin (Amberlite IR 120).

The XRD data were collected on a Nonius Kappa CCD diffractometer equipped with molybdenum

radiation (l = 0.71073 A).

The IR absorption spectrum of a KBr pressed pellets of the powdered sample was studied in the range

4000–400 cm�1 using a Perkin-Elmer FTIR 1000 spectrophotometer.

The Raman spectrum was recorded using a Renishaw RM 1000 spectrometer associated to a

microscope (Leica) allowing the selection of a region of good optical quality in the crystalline sample.

The wavelength available is 514 nm provided by an argon ion laser.

TG-DTA thermograms were performed using the multimodule 92 Setaram analyser operating from

room temperature up to 900 8C in a platinium crucible, at 5 8C/mn heating rate.

3. Results and discussion

3.1. Structure analysis

The crystal structure determination was performed using the Patterson heavy atom method for the

location of the heavier atom (Gd) and successive difference-Fourier syntheses for phosphorous and

oxygen atoms which were refined with anisotropic thermal parameters. The hydrogen atoms were

S. Hraiech et al. / Materials Research Bulletin 40 (2005) 2170–2179 2171

localized by geometry. The final cycle of least-squares refinement included 127 parameters. The final

refinements were run down to R = 0.0333 and Rw = 0.0948.

Some refinement details are given in Table 1. The final atomic positions and anisotropic thermal

parameters for the non-hydrogen atoms are given in Tables 2 and 3, respectively.

As shown in Fig. 1, which is a projection on the (1 0 1) plane, the HGdP2O7�4H2O atomic arrangement

can be described as an alternation of corrugated layers of diphosphoric groups and GdO8 polyhedra

parallel to the ð1 0 1Þ plane.W1, W2 and W3 water molecules are located between these layers, whereas W4 is located inside the

monohydrogendiphosphate layer.

S. Hraiech et al. / Materials Research Bulletin 40 (2005) 2170–21792172

Table 1

Crystal data and experimental parameters for the X-ray intensity data collection

Crystal data

Chemical formula HGdP2O7�4H2O

Formula weight (g mol�1) 404.262

Crystal system Monoclinic

Space group P21/n

a (A) 6.613(2)

b (A) 11.495(4)

c (A) 11.737(4)

b (8) 87.953(2)

V (A3) 891.81(1)

Z 4

rcal (g cm�3) 3.026

F(0 0 0) 382

Absorption coefficient, m (mm�1) 3.921

Crystal size (mm3) 0.09 � 0.07 � 0.07

Intensity measurement

Temperature (K) 293

Wavelength Mo Ka (0.7107 A)

Diffractometer Nonius Kappa CCD

Monochromator Graphite

Scan mode w/2u

Theta range (8) 1.00–27.48

Measurement area �8 � h � 7

�13 � k � 14

�15 � l � 15

Total number of scanned reflections 2045

Total number of independent reflections 1783

Structure determination

Program used WINGX [8]

Structure resolution SHELXS ’97 [9]

Structure refinement SHELXL ’97 [10]

Unique reflection include

Weighting scheme WGHT = 1/[s2(F20) + (0.0261P)2 + 08564P], where P = (F2

0 + 2F2c )/3

Goodness of fit 1.135

Unweighted agreement factor, R 0.0333

Weighted agreement factor, Rw 0.0948


Table 2

Final atomic coordinates for HGdP2O7�4H2O, Ueq(A2) for non-hydrogen atoms and Uiso(A

2) for hydrogen atoms

Atoms x(s) y(s) z(s) Ueq(s) (A3)

Gd 0.25539(3) 0.58352(2) 0.16385(2) 0.0119(1)

P1 0.75534(16) 0.45187(12) 0.14283(11) 0.0116(3)

P2 0.97948(17) 0.36793(10) 0.33771(10) 0.0124(3)

O(E11) 0.5644(5) 0.4998(3) 0.1979(3) 0.0187(10)

O(E12) 0.9208(5) 0.5413(3) 0.1249(3) 0.0168(10)

O(E13) 0.7155(5) 0.3840(3) 0.0353(3) 0.0193(10)

O(E21) 0.8419(6) 0.4242(3) 0.4324(4) 0.0214(12)

O(E22) 1.1547(5) 0.4440(3) 0.3056(3) 0.0172(10)

O(E23) 1.0252(5) 0.2466(3) 0.3723(3) 0.0159(10)

O(L) 0.8378(5) 0.3551(3) 0.2295(3) 0.0155(9)

O(W1) 0.0521(6) 0.7598(3) 0.1753(5) 0.0358(16)

O(W2) 0.3211(7) 0.6603(3) 0.3544(3) 0.0293(11)

O(W3) 0.2681(6) 0.3796(3) 0.0665(3) 0.0227(11)

O(W4) 0.7380(8) 0.1373(4) 0.0990(5) 0.0440(16)

Atoms x (s) y (s) z (s) Uiso

H(a) 0.77910 0.50230 0.43920 0.0500

H(11) �0.10060 0.77630 0.18420 0.0500

H(12) 0.08630 0.82890 0.19950 0.0500

H(21) 0.31330 0.72700 0.40010 0.0500

H(22) 0.24590 0.60189 0.39560 0.0500

H(31) 0.24750 0.37667 �0.01600 0.0500

H(32) 0.29250 0.30390 0.07910 0.0500

H(41) 0.77150 0.16169 0.17680 0.0500

H(42) 0.71740 0.20300 0.05900 0.0500

Table 3

Anisotropic thermal parameters (A2) for HGdP2O7�4H2O

Atoms U11 U22 U33 U23 U13 U12

Gd 0.0101(2) 0.0119(2) 0.0137(2) 0.0004(1) �0.0005(1) �0.0006(1)

P(1) 0.0089(6) 0.0127(6) 0.0131(6) 0.0015(5) �0.0007(5) �0.0012(4)

P(2) 0.0112(6) 0.0122(6) 0.0138(6) 0.0012(4) 0.0000(4) 0.0008(4)

O(E11) 0.0123(16) 0.0219(17) 0.022(2) 0.0022(14) �0.0023(14) 0.0033(13)

O(E12) 0.0148(16) 0.0192(17) 0.0166(18) 0.0016(14) �0.0021(14) �0.0036(14)

O(E13) 0.0242(19) 0.0181(16) 0.0159(19) 0.0005(15) �0.0046(15) �0.0016(15)

O(E21) 0.026(2) 0.021(2) 0.017(2) �0.0021(13) 0.0038(18) 0.0030(13)

O(E22) 0.0101(16) 0.0224(17) 0.019(2) 0.0053(14) �0.0008(14) �0.0051(13)

O(E23) 0.0173(16) 0.0116(16) 0.0189(18) 0.0026(13) �0.0018(13) 0.0028(13)

O(L) 0.0201(17) 0.0126(15) 0.0141(17) 0.0021(13) �0.0050(13) 0.0011(12)

O(W1) 0.0168(18) 0.0178(19) 0.073(4) �0.0150(19) �0.003(2) 0.0023(14)

O(W2) 0.045(2) 0.0215(19) 0.021(2) �0.0048(16) 0.0064(17) �0.0065(18)

O(W3) 0.029(2) 0.0201(17) 0.019(2) 0.0038(16) �0.0005(16) 0.0010(16)

O(W4) 0.060(3) 0.020(2) 0.054(3) 0.009(2) �0.029(2) �0.009(2)

As found in HSmP2O7�4H2O arrangement, the monohydrogendiphosphate group observed in the

HGdP2O7�4H2O has no internal symmetry and has a slightly staggered configuration.

The corresponding distortion angles are: O(E11)–P1–P2–O(E21) = 10.658; O(E12)–P1–P2–

O(E22) = 8.828; O(E13)–P1–P2–O(E23) = 20.088.The external P–O distances are comprised between 1.487(4) and 1.552(4) A, the two P–O distances in

P–O–P bridges are 1.612(4) A. The P–P distance is found equal to 2.935(2) A and the P–O–P angle is

130.7(2)8. All these distances and angles are in excellent accordance with all the values commonly

observed for the geometry of anions in condensed phosphate chemistry [11–15].

The oxygen–acidic hydrogen (O–H) distance (0.9912 A) and the H–O–P angle (131.948) are higherthan those observed in HSmP2O7�4H2O [7]. This suggests that the acidic hydrogen is engaged in an H-

bond stronger than the one observed in HSmP2O7�4H2O.

Fig. 2 shows that the gadolinium atom is eight-fold coordinated, sharing three oxygen atoms with

water molecules (W1–W3) and five oxygen atoms with four adjacent P2O7 groups. Gd–O distances are

comprised between 2.307(3) and 2.399(3) A, and Gd–(OH2) between 2.433(4) and 2.608(3) A (Table 4).

The shortest Gd–Gd distance in this arrangement is found to be 6.093(0) A, due to the fact that GdO8

dodecahedra in this arrangement are isolated from each other (Table 5).

Examination of the hydrogen bond network shows that the diphosphate anions are interconnected to

each other by means of the W4 water molecule so as to form chains parallel to the b-axis (Fig. 3). In

fact the acidic hydrogen of the HP2O73� group is engaged in a strong H-bond with the oxygen of the

W4 water molecule (donor–acceptor distance is 2.536 A). In addition its corresponding H–O

(acceptor) distance is 1.620 A, implying that the bond is slightly stronger than that in the

HSmP2O7�4H2O arrangement and explain the O–H and O–H� � �O values found in this investigation.

The hydrogen atoms of the W4 water molecule, H(41) and H(42), are engaged in weak hydrogen

bonds with two oxygen atoms of the same HP2O73� group and not with two oxygen atoms of two


Fig. 1. Projection in the (1 0 1) plane of the atomics arrangement in HGdP2O7�4H2O.

adjacent HP2O73� groups as it was found in HSmP2O7�4H2O arrangement. H(41) is connected to O(L)

and H(42) to an external oxygen atom of the diphosphoric group with donor–acceptor distances of

3.020(6) and 2.938(6) A, respectively. It can be noticed that oxygen atom O(L) is two times involved in

H-bond as an acceptor, a fact which is not frequently observed in diphosphate arrangement. All the

details of the hydrogen bond network are reported in Table 6.

3.2. Infra-red and Raman spectra

The IR spectrum of HGdP2O7�4H2O is shown in Fig. 4a. It is identical to that given for

HSmP2O7�4H2O. Bands in the region 3600–1600 cm�1 can be attributed to OH/H2O vibrations. The


Fig. 2. The coordination of the gadolinium atom.

Table 4

Bond lengths (A) and angles (8) for HGdP2O7�4H2O

P1 O(E11) O(E12) O(E13) O(L)

O(E11) 1.503(4) 2.525(7) 2.502(7) 2.492(6)

O(E12) 113.8(2) 1.511(4) 2.515(7) 2.521(6)

O(E13) 112.26(19) 112.4(2) 1.515(4) 2.469(6)

O(L) 106.20(19) 107.29(19) 104.0(2) 1.612(4)

P2 O(E21) O(E22) O(E23) O(L)a

O(E21) 1.552(4) 2.466(6) 2.525(8) 2.515(7)

O(E22) 111.6(2) 1.489(4) 2.540(6) 2.530(6)

O(E23) 108.4(2) 117.1(2) 1.487(4) 2.459(6)

O(L)a 105.1(2) 105.01(19) 108.86(19) 1.612(4)

P1–P2 2.935(2) A P1–O(L)–P2 130.7(2)

O(E21)–H(a) 0.9912 A P2–O(E21)–H(a) 131.948a Symmetry operations: x + 1, y, z.

three bands at 3600, 3446 and 3232 cm�1, corresponding to H2O stretching vibration, confirm the

existence of hydrogen bonds of different strengths in the HGdP2O7�4H2O arrangement. The bands of

weak intensity between 2920 and 2330 cm�1 corresponds to the P–O–H vibration modes. Around

1600 cm�1 appear two bands, which can be attributed to H2O bending modes.

Bands in the range 1300–400 cm�1 show the different diphosphate vibration modes in both IR and

Raman spectra (Fig. 5). IR absorption bands in the 1286–1198 and 1036 cm�1 are attributed respectively

to the nas(PO3) and ns(PO3) vibration modes. The corresponding Raman bands are observed between

1265 and 1036 cm�1(Fig. 5). The characteristics bands of the P2O7 group, nas(POP) and ns(POP), are

noticed in the IR spectrum in the 974–926 and 790–732 cm�1 areas, respectively. They are situated at

970, 924 and 722 cm�1 in the Raman one. The P2O7 deformation vibration modes appear as weak


Table 5

Main interatomic distances (A) and bond angles (8) in the GdO8 dodecahedra

Gd O(E11) O(E12)a O(E13) O(E22)b O(E23)b O(W1) O(W2) O(W3)

O(E11) 2.307(3) 2.525(7) 2.502(7) 3.012(6) 3.022(7) 3.208(7) 3.027(7) 2.884(7)

O(E12)a 143.30(12) 2.327(3) 2.515(7) 2.899(7) 3.774(7) 2.736(7) 4.09(7) 2.76(7)

O(E13) 101.29(12) 83.30(12) 2.368(4) 4.437(7) 4.576(7) 3.307(8) 4.534(8) 2.967(7)

O(E22)a 79.97(12) 75.72(12) 143.56(12) 2.388(3) 2.540(6) 2.877(7) 2.791(7) 2.972(8)

O(E23)b 79.78(12) 134.56(12) 71.01(12) 143.10(12) 2.399(3) 2.837(8) 2.985(8) 4.172(8)

O(W1) 146.02(13) 69.94(13) 87.02(16) 112.40(15) 71.85(12) 2.433(4) 3.042(9) 3.575(8)

O(W2) 78.79(13) 117.01(14) 146.17(12) 70.19(12) 75.81(12) 76.46(16) 2.458(4) 2.743(8)

O(W3) 71.70(12) 75.07(12) 73.12(11) 72.87(11) 127.95(12) 141.44(14) 135.86(11) 2.608(3)a Symmetry operations: x � 1, y, z.b Symmetry operations: �x + 3/2, y � 1/2, �z + 3/2.

Fig. 3. Projection of an isolated layer of diphosphoric anions.


Table 6

Principal characteristics of the hydrogen bonding in the atomic arrangement of HGdP2O7�4H2O

O–H� � �O O–H (A) O� � �H (A) O–H� � �O (8) O� � �O (A)

O(E21)–H(a)� � �O(W4) 0.9912 1.6204 151.44 2.536(6)

O(W1)–H(11)� � �O(L)a 1.0292 2.0480 149.68 2.982(5)

O(W1)–H(12)� � �O(E11)a 0.8760 2.4935 140.14 3.215(5)

O(W1)–H(12)� � �O(E22)b 0.8760 2.1636 139.37 2.885(5)

O(W2)–H(22)� � �O(E21)c 0.9565 2.1022 134.95 2.859(6)

O(W2)–H(21)� � �O(W3)a 0.9361 1.8723 153.76 2.743(5)

O(W3)–H(31)� � �O(E12)d 0.9833 1.9662 136.50 2.764(5)

O(W3)–H(32)� � �O(W2)e 0.8983 1.9639 144.22 2.743(5)

O(W4)–H(41)� � �O(L)b 0.9880 2.3532 124.14 3.020(6)

O(W4)–H(42)� � �O(E13)b 0.9022 2.0993 154.21 2.938(6)

H(11)–O(W1)–H(12) 93.66

H(21)–O(W2)–H(22) 105.56

H(31)–O(W3)–H(32) 99.26

H(41)–O(W4)–H(42) 106.65a Symmetry operations: 1/2 � x, 1/2 + y, 1/2 � z.b Symmetry operations: 3/2 � x, 1/2 + y, 1/2 � z.c Symmetry operations: 1 � x, 1 � y, 1 � z.d Symmetry operations: 1 � x, 1 � y, �z.e Symmetry operations: 1/2 � x, �1/2 + y, 1/2 � z.

Fig. 4. IR spectrum of HGdP2O7�4H2O dried at room tempreture (a) and calcined at 900 8C in static air furnace (b).

intensity bands between 675 and 429 cm�1 in the IR spectrum and between 606 and 411 cm�1 in the

Raman one. Raman bands in the 350–200 cm�1 area are assigned to the external modes.

3.3. Thermal behaviour

Fig. 6 shows both the TG and DTA thermograms of HGdP2O7�4H2O carried out in an argon

atmosphere from room temperature to 900 8C. The TG weight loss can be divided into two areas

79–176 8C (A–B) and 176–900 8C (B–C). The total weight loss is found to be 20.40%, close to the

calculated value (20.03%) for the elimination of 4.5 water molecules. The weight loss in the first stage


Fig. 5. Raman spectrum of HGdP2O7�4H2O.

Fig. 6. TG and DTA thermograms of HGdP2O7�4H2O.

(14.02%) due to the elimination of three water molecules is close to the calculated value (13.35%). The

second weight loss can be attributed to the elimination of the last water molecule in the structure followed

by the onset of decomposition of the anhydrous salt and the removal of the acidic hydrogen.

Tested by X-ray diffraction and IR spectroscopy, the products isolated for samples heated at 176 and

900 8C are amorphous and exhibit large IR bands. But when heating HGdP2O7�4H2O in a static air

furnace at 850 8C for 48 h a crystallized product is obtained. Its IR spectrum (Fig. 4b) exhibit the

characteristic vibration bands of diphosphate group (nas(POP): 948 cm�1 and ns(POP): 792–758 cm

�1).

This spectrum is found identical to that reported for Gd4(P2O7)3 [16].

4. Conclusion

Chemical preparation and crystal structure are described for HGdP2O7�4H2O. From X-ray diffraction

data, it is shown that the compound crystallizes in the monoclinic space group P21/n. The structure is

built by an alternation of corrugated layers of diphosphate groups and GdO8 polyhedra. The part played

by the water molecules in this structure is quite important. Three of them participate in the gadolinium

cation environment and the fourth is responsible for the cohesion between the anionic groups by means of

strong hydrogen bonding involving the acidic hydrogen. For this reason the removal of crystallization

water alters completely the crystalline structure and leads to amorphous phases as shown by the thermal

study.

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Synthesis, structure refinement and characterisation of a new oxyphosphate Mg0.50TiO(PO4)

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