Investigations of Some Disordered Quaternary Compounds in the Systems Ag/Pb/Sb/Se and Ag/Pb/Sb/Te

Electrical and thermal transport measurements on quenched NaCl-type Ag1/3Pb1/3Sb1/3Se reveal an n-type semiconductor with a Seebeck coefficient up to −140 μVK−1 and a thermal conductivity as low as 0.52 WmK−1. Short-range order is indicated by disorder diffuse scattering in electron diffraction patterns. In contrast, 4L-Ag0.61Pb1.79Sb2.61Se6 (space group Cmcm with a = 4.2129(1) Å, b = 13.852(1) Å, and c = 20.866(1) Å, Z = 4) features the first lillianite-type structure in the system Ag/Pb/Sb/Se. It consists of slab-like NaCl-type building blocks that are interconnected via trigonal [PbSe6] prisms. As such structures typically do not form with Te as an anion, the first “sulfosalt-like” compound, Ag0.38Pb0.25Sb2.38Te4, in the system Ag/Pb/Sb/Te forms a layered tetradymite-like structure (space group R3-m with a = 4.2887(1) Å, c = 41.544(1) Å, Z = 3). Its slabs, which are separated by van der Waals gaps, are built up from three layers of distorted [MTe6] octahedra. Crystals of Ag0.38Pb0.25Sb2.38Te4 were grown by chemical transport.