Electronic Structure and Physical Properties of the Semiconducting Polytelluride Ba₂SnTe₅ with a Unique Te₅^4- Unit

A new modification of Ba 2 SnTe 5 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 SnTe 4 tetrahedra, SnTe 5 trigonal bipyramids, and nonlinear Te 5 4- units, space group P 2 1 / c, with lattice dimensions of a = 9.6791(5) Å, b = 13.1696(7) Å, c = 27.5957(15) Å, V = 3516.3(3) Å 3, β = 91.585(1)° ( Z = 12), while the earlier reported bronze-colored Ba 2 SnTe 5 (orthorhombic, space group Pnma ) is comprised of SnTe 4 tetrahedra and dimeric Te 2 2- units. The new, black Ba 2 SnTe 5 is a small band gap semiconductor, as predicted via electronic structure calculations and as verified by Seebeck coefficient and electrical conductivity measurements.