Single-Crystal Structure of Ta₃S₂. Structure and Bonding in Ta₆S_n (n = 1, 3, 4, 5?) Pentagonal-Antiprismatic Chain Compounds

The single-crystal structure of the new binary sulfide Ta₃S₂ is reported and discussed in light of the known structures of Ta₆S, Ta₂S, and the as yet unrealized one-dimensional Ta₆S₅ chain. Ta₂S₂ crystallizes in the orthorhombic space group Abm2 (No. 39) with lattice parameters a = 7.4783 (6) Å, b = 17.222 (1) Å, c = 5.6051 (4) Å, V = 721.88 (9) ų, and Z = 8. While the structure is in accord with that reported for Ta₃S_2-x (0.2 ≤ x ≤ 0.28) by Wada and Onoda using X-ray Rietveld analysis, we find Ta₃S₂ to be a stoichiometric compound with no appreciable phase width up to 1300 °C. Band structure calculations reveal that as sulfur content in the Ta₆S_n (n = 1, 3, 4, 5) series increases, the Ta-Ta bonding remains optimal, as judged by the behavior of COOP curves for the metal-metal bonding in these compounds. While Ta₂S should be an electrical conductor, Ta₃S₂ should be a poor conductor or semimetallic and Ta₆S₅ chains are predicted to be semiconducting.