Synthesis and Crystal Structure of Tl2[(S0.6Se0.4)O4]Te(OH)6

Abstract

Alkali-sulfate and selenate tellurate compounds having the formula M2XO4Te(OH)6 with (M= K, NH4, Tl and X= S, Se) have been crystallographically studied in order to examine the effect and the influence of cationic and/or anionic substitution over the crystalline symmetry and physical properties. The ammonium sulfate tellurate, (NH4)2SO4Te(OH)6, is monoclinic Cc, whereas the (NH4)2SeO4Te(OH)6 material crystallizes in the P21/c space group.1,2 On the other hand, the thallium sulfate tellurate Tl2SO4Te(OH)6 crystallizes in the P21/a space group. 3 We have been continuing our study of sulfate-selenate solid solutions of tellurate salts, among which we have reported Rb2[( S0.5Se0.5)O4]Te(OH)6 and Cs2 [(S0.57Se0.43)O4]Te(OH)6. 4,5 We have now prepared Tl2[(S0.6Se0.4)O4]Te(OH)6. In this study we clearly show the crystallographic differences and the effect of extensions of the anionic and cationic volume groups, which can help us to understand which, if any, geometrical features can be responsible for the electrical properties.Crystals of TlSSeTe were prepared at room temperature by the slow evaporation of an aqueous solution of a mixture of telluric acid, H6TeO6 (Aldrich, 99%), thallium carbonate Tl2C O3 (Aldrich 99.9%), selenic acid H2SeO4 (Aldrich 94%) and thallium sulfate Tl2SO4 (Aldrich 99.999%) at room temperature (The stoichiometric ratio is 1:0.5:0.5:0.5).Schematically the reactions are:Several recrystallizations were necessary to obtain single crystals suitable for an X-ray diffraction study. The composition of the obtained crystals was determined by chemical analysis, and confirmed by a crystal-structure refinement. A schematic diagram of the compound is shown in Fig. 1. The unit-cell dimensions were measured and refined using indexation of diffraction markings collected with a BrukerNonius X8-APEX2 CCD area-detector diffractometer using the APEX2 program. The structure of Tl2[(S0.6Se0.4)O4]Te(OH)6 was analyzed with the crystallographic CRYSTALS program. Density was measured, at room temperature, using the pycnometric method, by flotation in CCl4. The chemical crystal data, the parameters used for the X-ray diffraction data collection and the strategy used for the crystal structure determination and their results are listed in Table 1. The final positions and equivalent isotropic thermal parameters are given in Table 2. Hydrogen atoms were not determined. X-ray Structure Analysis Online Synthesis and Crystal

New mixed compound Cs2.8Tl1.2(SO4)1.48(SeO4)0.52.[Te(OH)6]2: Synthesis, structural characterization, vibrational studies and phase transitions

“…S3). In fact, the intense peak around 650 cm -1 corresponds to the symmetric stretching ν1(TeO6) [2,12,15]. This vibration is detected in the IR spectra at 965 cm -1 .…”

“…However, the S/Se-O distances vary from 1.512( 9) to 1.562(9) Å for (TlSSeTe) structure [12]. In fact, the distances and angles related to the two tetrahedrons are listed in Table 1.…”

“…Furthermore, these peaks refer to three-phase transitions. In fact, the first peak corresponds to a structural phase transition [1,12,15] and the second one corresponds to the ferroparaelectric phase transition [1,15,24,29]. Moreover, the third one is intense and exhibits the characteristics of a protonic conduction phase transition [1,15,24,30].…”

“…So far, the most important structural property of these crystals is the presence of some anionic groups (TeO  6 6 , SO  2 4 and SeO  2 4 ) in the unit cell. Overall, another key feature of these compounds pertains to the hydrogen bonds, formed in their crystal structure, which represents further evidence for the significance of such family [11][12][13][14][15][16].…”

“…In fact, tellurates, caesium sulfate selenate tellurate, Cs2(SO4)0.57(SeO4)0.43.Te(OH)6 (CsSSeTe) [11], thallium sulfate selenate tellurate, Tl2(SO4)0.61(SeO4)0.39.Te(OH)6 (TlSSeTe) [12] represent some of these compounds that are recognized by centrosymmetry and crystallization in the monoclinic system with P21/c space group. Yet, in the original compound, caesium sulfate tellurate (CsSTe) undergoes a ferroelectric-paraelectric phase transition at T = 490 K and a superprotonic one at 500 K [10].…”