Syntheses, modulated crystal structures of Ba6−2xU2+xAg4Se12 (x = 0 and 0.5), and crystal structure and spectroscopy of Sr4Th2.78Cu4S12

Abstract

Single crystals of Ba6−2xU2+xAg4Se12 (x = 0 and 0.5) and Sr4Th2.78Cu4S12 were obtained by standard solid-state chemistry methods at 1173 K. The Ba6−2xU2+xAg4Se12 structures are modulated. They were solved and refined in the monoclinic C2(α0γ)0 super space group. The structure consists of layers formed by the connection of USe6 and AgSe4 polyhedra. The incommensurate nature of the structure arises from the disorder among Ba atoms and U atoms in the layers. The crystal structure of the Sr4Th2.78Cu4S12 was solved in space group Pmn21 of the orthorhombic system. The refinement of the structure reveals three partially occupied Th sites. The connection of ThS7 and CuS4 polyhedra form a channel structure filled by Sr atoms. From optical measurements Sr4Th2.78Cu4S12 is a semiconductor with an indirect band gap of 2.27(2) eV.