Ta⁵⁺ displacements in CsTaQ₃ (Q = S, SE, and Te): New one- dimensional materials with the bavs₃ structure

The new compounds CsTaQ₃ (Q = S, Se, and Te) have been synthesized through stoichiometric reactions of the elements with Cs₂Q(n) reactive fluxes at 923 K. Their crystal structures have been determined by single- crystal X-ray methods. CsTaQ₃ crystallize in the hexagonal space group D(6h)/₄-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) ų (T = 115 K), R(w)(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) ų (T = 115 K), R(w)(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) ų (T = 115 K), R(w)(F²) = 0.131 (NO = 111, NV = 10), R₁ = 0.038. The compounds adopt the BaVS₃ structure type and feature ←[TaQ₃^-] 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 hand structure calculations at the extended Huckel 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.