New Quaternary Barium Copper/Silver Selenostannates: Different Coordination Spheres, Metal−Metal Interactions, and Physical Properties

Abstract

BaAg2SnSe4, BaCu2SnSe4, and Ba3Cu2Sn3Se10 were prepared by directly reacting the elements in stoichiometric ratios at 800 °C, followed by long heating between 600 and 650 °C. BaAg2SnSe4 crystallizes in the BaAg2SnS4 type, space group I222, BaCu2SnSe4 in the SrCu2GeSe4 type, space group Ama2, and Ba3Cu2Sn3Se10 in a new structure type, space group P21/n. All three structures comprise almost undistorted SnSe4 tetrahedra and pairs of the group 11 elements (M = Cu and Ag). While both the M atoms are bonded to four Se atoms in each case, the deviations from an MSe4 tetrahedron are severe, and distinctly different within this series. BaAg2SnSe4, BaCu2SnSe4, and Ba3Cu2Sn3Se10 are black semiconductors, with computed band gaps of 0.2, 0.7, and 1.2 eV, respectively. The experimental gaps for the former two are 0.24 and 0.48 eV, respectively. The electrical conductivities consistently decrease with increasing band gap.