Crystal structure of α-Tl2Te2O5

CRYSTAL STRUCTURE OF a-Tl2Te2O5 B. Jeansannetas, P. Thomas, J.C. Champarnaud-Mesjard, and B. Frit* Laboratoire de Mate´riaux Ce´ramiques et Traitements de Surface, ESA CNRS 6015, 123 avenue Albert Thomas, 87060 Limoges Cedex, France (Communicated by B. Raveau) (Received March 6, 1998; Accepted March 6, 1998) ABSTRACT a-Tl2Te2O5 crystallizes with the monoclinic symmetry (space group P21/n) and unit-cell parameters a 5 7.119(1), b 5 12.138 (2), c 5 8.439 (2) Å, b 5 114.28 (3)°, Z 5 4. Its crystal structure was solved from single crystal X-ray diffraction data to a final R1 5 0.0476. The strong stereochemical activity of the lone pairs E of Te and Tl atoms divides the structure into independent Tl2Te2O5 sheets parallel to (101) and constituted of MO4E trigonal bipyra- mids sharing edges and corners. © 1998 Elsevier Science Ltd KEYWORDS: A. oxides, C. X-ray diffraction, D. crystal structure INTRODUCTION TeO2-based glasses are promising materials due to their high values of linear and nonlinear refractive indices and to their good visible and infrared light transmittance [1–4]. Among them, thallium(I) tellurite glasses are of special interest since they exhibit very high x(3) values [5]. The stereochemical activity of the electron lone pair ns2 of Tl(I) and Te(IV) atoms is surely at the origin of such properties. The short-range order in these glasses is still a matter of speculation [6,7]. Considering that knowledge of the crystal structure of the stable or metastable phases which appear during the crystallization of these glasses [8] should provide precious information about their structure, we have undertaken a systematic structural study of them by single crystal X-ray diffraction techniques. The crystal structures of the Tl2TeO3 [9] and Tl2Te3O7 [10] compounds were previously determined and analyzed; this paper reports on the determination and description of the stable form of the Tl2Te2O5 compound. *To whom correspondence should be addressed. Materials Research Bulletin, Vol. 33, No. 11, pp. 1709–1716, 1998 Copyright © 1998 Elsevier Science Ltd Printed in the USA. All rights reserved 0025-5408/98 $19.00 1 .00 PII S0025-5408(98)00171-8 1709 EXPERIMENTAL Single crystals suitable for X-ray diffraction experiments were obtained by heating an intimate mixture of Tl2CO3 and 2TeO2 in a gold crucible at 330°C for 18 h, under pure flowing nitrogen, and then slowly cooling (0.6°C/h) it down to 250°C. The crystal selected was prismatic and apparently untwinned under polarized light. Intensity data were collected with a P4 Siemens four-circle automatic diffractometer using monochromatized Mo Ka1 radiation (Table 1). They were corrected for absorption effects by using a psi-scan method [11]. The monoclinic symmetry (P21/n) and the unit-cell parameters previously proposed [12] were confirmed. STRUCTURE DETERMINATION The thallium and tellurium atoms were first located on four general 4e positions of the P21/n space group, by direct methods using the SHELXTL-PC program package [11]. A Fourier difference analysis allowed us then to locate the oxygen atoms. Successive full-matrix least-squares refinements of atomic coordinates and anisotropic thermal parameters for all atoms (SHELXL-93 program [13]) led to a final reliability factor R15 0.048 for 1736 TABLE 1 Crystal Data and Structure Refinement Conditions for a-Tl2Te2O5 Formula weight 743.94 g Wavelength 0.71073 Å Crystal system Monoclinic Space group P21/n Unit-cell dimensions a 5 7.119(1), b 5 12.138(2), c 5 8.439(2) Å, b 5 114.28(3)8 Volume 665.6(2) Å3 Z 4 Density (calculated) 7.424 Mg/m3 (observed) 7.40(5) Mg/m3 Absorption coefficient 56.897 mm21 F(000) 1224 Crystal size 0.2 3 0.1 3 0.04 mm Theta range for data collection 3.13 to 30.00° Index ranges 210 # h # 10, 21 # k # 17, 211 # l # 11 Reflections collected 3242 Independent reflections 1736 [R(int) 5 0.0913] Refinement method Full-matrix least-squares on F2 Data/restraints/parameters 1736/0/83 Goodness-of-fit on F2a 1.067 Final R [I . 2s(I)]b R1 5 0.0476, wR2 5 0.1090 R (all data) R1 5 0.0613, wR2 5 0.1163 Extinction coefficient 0.0010(2) Largest diff. peak and hole 2.862 and 22.840 ezÅ23 aG.O.F. 5 $(w(Fo2 2 Fc2)/(Nobs 2 Nparm)%1/2 bR1(F) 5 (iFou 2 uFci/uFou; wR2(F2) 5 $([w(Fo2 2 Fc2)2]/([w(Fo2)]%1/2 1710 B. JEANSANNETAS et al. Vol. 33, No. 11 independent reflections. The refined parameters are reported in Table 2. The more significant interatomic distances, angles, and bond valences calculated by using Brown’s method [14,15] are given in Table 3. DESCRIPTION OF THE STRUCTURE A projection of the unit-cell content onto the xOz plane is shown in Figure 1 and the various coordination polyhedra of cations are represented in Figure 2. The distribution of anions around each cation is highly anisotropic and characteristic of a strong stereochemical activity of the electron lone pair E. It corresponds, if only the shortest distances are taken into account, to more or less distorted disphenoids MO4, with the lone pair E so directed as to constitute the fifth equatorial corner of a MO4E trigonal bipyramid (see Fig. 2). Cations are distributed at the corners of a highly distorted face-centered cubic network, visualized by dotted lines in Figure 1. Anions approximately occupy 5/8 of the tetrahedral interstices of this network: among the four [010] columns of tetrahedral sites that surround TABLE 2 Atomic Coordinates (3 104) and Thermal Parameters (Å2 3 103) for a-Tl2Te2O5 Atom Site x y z Ueq Tl(1) 4e 2227(1) 7850(1) 7210(1) 30(1) Tl(2) 4e 7384(1) 9469(1) 5118(1) 32(1) Te(1) 4e 2247(1) 276(1) 110(1) 18(1) Te(2) 4e 2248(1) 7925(1) 2957(1) 20(1) O(1) 4e 3545(16) 9532(7) 2283(13) 28(2) O(2) 4e 3828(14) 1705(7) 975(14) 27(2) O(3) 4e 176(15) 945(7) 708(13) 25(2) O(4) 4e 9871(14) 8709(7) 2687(14) 25(2) O(5) 4e 3935(14) 8441(7) 5174(11) 23(2) Atom U11 U22 U33 U23 U13 U12 Tl(1) 28(1) 29(1) 38(1) 22(1) 18(1) 4(1) Tl(2) 29(1) 24(1) 48(1) 22(1) 19(1) 3(1) Te(1) 18(1) 14(1) 24(1) 0(1) 10(1) 0(1) Te(2) 19(1) 19(1) 23(1) 22(1) 7(1) 2(1) O(1) 33(5) 21(4) 21(5) 1(4) 4(4) 23(4) O(2) 13(4) 21(4) 42(6) 25(4) 7(4) 0(3) O(3) 26(5) 17(4) 40(6) 211(4) 22(4) 27(4) O(4) 12(4) 18(4) 46(6) 2(4) 14(4) 2(3) O(5) 31(5) 16(4) 18(4) 3(3) 7(4) 23(4) esd’s in the last digit are given in parentheses. Ueq is defined as one-third of the trace of the orthogonalized Uij tensor. The anisotropic displacement factor exponent takes the form 22p2[h2a*2U11 1 . . . 1 2hka*b*U12] 1711THALLIUM TELLURITEVol. 33, No. 11 each Tl(2)–Te(1) row, one is empty, two are three-quarters occupied, and the fourth one is full (see Fig. 1). In fact, because of the strong stereochemical activity of the lone pairs E, they are set off-center towards one triangular face or one edge of their tetrahedron and are therefore in dissymetric [3] or [2 1 1] coordination. Such a distribution of anions divides the structure into independent Tl2Te2O5 sheets parallel to (101). This orientation logically corresponds to the preferential cleaving planes observed for the crystals. One of these sheets is shown in Figure 3a. TABLE 3 Main Interatomic Distances (Å), Angles (°) and Bond Valences in a-Tl2Te2O5 Tl(1) O(2) O(2)1 O(3)2 O(4) O(5) Svij O(2) 2.840(9) 4.677(20) 5.686(15) 3.171(15) 2.774(14) O(2)1 117.0(3) 2.646(9) 4.712(15) 2.683(14) 2.978(14) O(3)2 137.4(3) 105.3(3) 3.260(9) 5.481(14) 5.437(14) O(4) 71.5(3) 61.7(3) 139.1(3) 2.584(9) 3.254(14) O(5) 61.4(3) 69.5(3) 136.8(3) 78.1(3) 2.580(8) vij 0.16 0.28 0.05 0.33 0.33 1.15 5 2.663 Å Tl(2) O(1) O(1)2 O(4)1 O(5)1 O(5)2 Svij O(1) 2.813(9) 4.337(20) 5.313(14) 3.274(14) 2.689(14) O(1)2 101.2(3) 2.799(9) 2.949(14) 2.689(14) 3.274(14) O(4)1 60.5(3) 131.5(3) 3.028(9) 2.812(14) 5.307(14) O(5)1 58.6(3) 73.3(3) 58.7(3) 2.681(9) 4.134(19) O(5)2 71.8(2) 57.7(3) 132.2(3) 98.5(3) 2.775(9) vij 0.18 0.18 0.10 0.25 0.20 0.91 5 2.819 Å Te(1) O(1) O(2) O(3) O(3)1 O(4) Svij O(1) 1.908(9) 2.899(15) 2.802(15) 2.870(15) 4.437(14) O(2) 94.6(4) 2.034(9) 2.682(14) 4.145(14) 3.171(14) O(3) 93.9(4) 85.2(4) 1.925(9) 2.551(20) 2.882(14) O(3)1 89.5(4) 161.9(5) 77.0(4) 2.162(9) 3.240(14) O(4) 172.7(5) 87.1(4) 79.1(4) 86.7(3) 2.538(9) vij 1.20 0.86 1.15 0.61 0.22 4.04 5 2.113 Å Te(2) O(1)1 O(2) O(4)2 O(5) Svij O(1)1 2.327(9) 4.341(14) 2.949(14) 2.689(14) O(2) 167.4(4) 2.041(9) 2.683(14) 2.774(14) O(4)2 88.5(4) 86.4(4) 1.873(9) 2.812(14) O(5) 78.9(4) 90.3(4) 97.4(4) 1.870(9) vij 0.39 0.84 1.32 1.34 3.89 5 2.028 Å esd’s are given in parentheses. 1712 B. JEANSANNETAS et al. Vol. 33, No. 11 Two Te(1)O4 disphenoids form Te(1)2O6 units by sharing their O(3)–O(3) edges. These units are linked to four similar units, via Te(2)O311 polyhedra with which they share O(1) and O(2) corners, so constituting the bidimensional network Te2O5 shown in Figure 3b. It is worth pointing out that 1. Te(1)2O6 units are at the crossing of identical interconnected chains developing respec- tively along the [111] and [111] directions. FIG. 1 Projection onto (010) of the unit-cell content for the a-Tl2Te2O5 crystal structure. Dotted lines vizualize the face-centered cationic network, and arrows indicate the stereochemically active lone pairs E. 1713THALLIUM TELLURITEVol. 33, No. 11 2. If we only take into account the shortest three Te(2)–O distances (i.e., Te(2)–O(2), Te(2)–O(4), and Te(2)–O(5)), isolated Te(1)2Te(2)2O10 groups can be observed. Such groups, more or less distorted, could constitute the basic structural units in the most stable glasses observed in the Tl2O–TeO2 system. The diamond-shaped holes of this bidimensional network are capped on both sides by Tl(1)O4 and Tl(2)O4 pyramids. Each Tl(2)O4 pyramid shares two opposite O(1)–O(5) edges with two Te(2)O4 disphenoids and two corners O(1) with two different Te(1)O4 polyhedra. Each Tl(1)O4 pyramid shares two edges (O(2)–O(5) and O(4)–O(2)) with two Te(2)O311 polyhedra and two O(2) corners with two Te(1)O4 polyhedra. By sharing alternately square O(1)–O(5)–O(1)–O(5) faces and O(5) and O(2) corners the Tl(1)O4 and Tl(2)O4 pyramids constitute infinite zigzag chains parallel to the [111] direction. ACKNOWLEDGMENTS One of us, B. Jeansannetas, wants to thank the Conseil Regional du Limousin for financial support. FIG. 2 The various cation coordination polyhedra. Notations for atoms are those of Figure 1. 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