Temperature-dependent Cu and Ag ion mobility and associated changes of transport properties in pavonite-type Cu1.4Ag0.4Bi5.4Se9

Abstract

Cu_1.4Ag_0.4Bi_5.4Se₉, the first quaternary compound in the system Cu/Ag/Bi/Se, was obtained from the elements by melt synthesis after pre-reaction and annealing steps. It exhibits a ⁴P pavonite-type crystal structure in the spacegroup C2/m, which consists of NaCl-type building blocks extending in two dimensions that are separated by rods of edge-sharing BiSe₇ polyhedra and “clusters” of several partially occupied Cu atom sites. Temperature-dependent single crystal X-ray diffraction up to 350 °C helped to explain thermoelectric properties. Cu atoms occupy more and different disordered sites at ≥200 °C. Ag atoms share Bi atom sites and prefer different sites as a function of temperature. Cu_1.4Ag_0.4Bi_5.4Se₉ is a metallic n-type thermoelectric material with Seebeck coefficients up to −150 μV K⁻¹. Lattice thermal conductivity significantly decreases from 0.55 W mK⁻¹ to 0.42 W mK⁻¹ with increasing structural disorder. Cu_1.4Ag_0.4Bi_5.4Se₉ may thus be described with the phonon-liquid electron-crystal (PLEC) concept and reaches a figure of merit of zT_max = 0.23 at 450 °C. B-factor analysis based on a single parabolic band model shows that the chemical potential of electrons approaches the one corresponding to optimal charge carrier concentration, especially at higher temperatures. With respect to most properties and the easily reproducible synthesis, the compound can well compete with other sulfosalt-like thermoelectric materials and is considerably less toxic than most of them.