Electronic Structure and Physical Properties of the Semiconducting Polytelluride Ba2SnTe5 with a Unique Te54- Unit

A new modification of Ba2SnTe5 was prepared by directly reacting the elements in stoichiometric ratios at 750 C, while the first reported modification was formed in a potassium polytelluride flux at 450 C. The former crystallizes in a unique structure type with SnTe4 tetrahedra, SnTe5 trigonal bipyramids, and nonlinear Te54- units, space group P21/c, with lattice dimensions of a = 9.6791(5) A, b = 13.1696(7) A, c = 27.5957(15) A, V = 3516.3(3) A3,(beta)= 91.585(1) (Z = 12), while the earlier reported bronze-colored Ba2SnTe5 (orthorhombic, space group Pnma) is comprised of SnTe4 tetrahedra and dimeric Te22- units. The new, black Ba2SnTe5 is a small band gap semiconductor, as predicted via electronic structure calculations and as verified by Seebeck coefficient and electrical conductivity measurements