Preparation, crystal structure, properties, and electronic band structure of TlTaSe3

TlTaSe3 was prepared from a fused mixture of Tl4Ta2Se11, Ta and Se in the molar ratio 1:2:1. The compound shows dimorphism with H-TlTaSe3, hexagonal, space group P6(3)/mmc, a = 7.2436(6), c = 5.9736(6) angstrom, c/a = 1.213 and O-TITaSe3, orthorhombic, space group Pnma, a = 9.554(13), b = 3.6244(6), c = 14.7271(17) angstrom. The crystal structure of H-TlTaSe3 is isotypic to BaVSe3 whereas that of O-TlTaSe3 is closely related to the NH4CdCl3-type. Characteristic features of the structures are: 1(infinity) [TaSe32-] chains of regular octahedra sharing faces along [001] for the hexagonal form and columns of double edge-sharing octahedra _1 [Ta2Se62-] running along [010] for O-TlTaSe3. The columns are each separated by Tl+ ions with the coordination number CN = 12 and CN = 8 respectively. The structures are compared and discussed in context with other isotypic structures of chalcogenides. The orthorhombic modification O-TlTaSe3 is a semiconductor while H-TlTaSe3 shows conventional metallic behaviour. The electronic structures of both modifications are discussed on the base of band structure calculations performed within the framework of density functional theory.