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Ta5+ Displacements in CsTaQ3 (Q = S, Se, and Te):  New One-Dimensional Materials with the BaVS3 Structure

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journal contribution
posted on 04.06.1997, 00:00 by Michael A. Pell, Grigori V. M. Vajenine, James A. Ibers
The new compounds CsTaQ3 (Q = S, Se, and Te) have been synthesized through stoichiometric reactions of the elements with Cs2Qn reactive fluxes at 923 K. Their crystal structures have been determined by single-crystal X-ray methods. CsTaQ3 crystallize in the hexagonal space group −P63/mmc with two formula units per cell. Crystal data:  CsTaS3, a = 7.266(2) Å, c = 5.961(2) Å, V = 272.55(10) Å3 (T = 115 K), Rw(F2) = 0.072 (NO = 114, NV = 11), R1 = 0.034; CsTaSe3, a = 7.500(1) Å, c = 6.182(1) Å, V = 301.15(7) Å3 (T = 115 K), Rw(F2) = 0.106 (NO = 112, NV = 11), R1 = 0.036; CsTaTe3, a = 7.992(4) Å, c = 6.496(4) Å, V = 359.3(4) Å3 (T = 115 K), Rw(F2) = 0.131 (NO = 111, NV = 10), R1 = 0.038. The compounds adopt the BaVS3 structure type and feature chains of face-sharing octahedra. The Ta5+ centers are displaced from the centers of the Q6 octahedra along the direction of the chains in a centrosymmetric fashion. Geometrical considerations and band structure calculations at the extended Hückel and density functional (DFT) levels of theory suggest that the most likely distortion pattern is the one with all Ta5+ centers displaced in the same direction within a given chain. A second-order Jahn−Teller effect is responsible for this distortion. Very weak interchain communication would result in a centrosymmetric structure. The chains are separated by 12-coordinate Cs+ cations. CsTaSe3 is an insulator. CsTaTe3 shows semiconducting behavior and is diamagnetic or weakly paramagnetic. Differential scanning calorimetry (DSC) and high-temperature powder diffraction data indicate that CsTaTe3 undergoes a phase transition at ≈493 K.