Extended Occupational and Positional Disorder in Pavonite Homologous Copper Bismuth Chalcogenide Halogenides

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

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.

Two new quaternary copper bismuth sulfide halides: CuBi2S3Cl and CuBi2S3Br as candidates for copper ion conductivity

Two new copper bismuth sulfide halides, CuBi2S3Cl and CuBi2S3Br, were synthesized by a two-step process of ball milling followed by annealing. Both compounds are obtained as dark grey powders and crystallize in the monoclinic space group C2/m with lattice parameters a = 12.9458(11) Å, b = 3.9845(3) Å, c = 9.1024(8) Å and β = 91.150(3)° for the sulfide chloride and a = 13.3498(8) Å, b = 4.1092(2) Å, c = 9.4173(6) Å and β = 90.322(4)° for the sulfide bromide. Also known for related compounds, the copper atoms are strongly disordered. Quantum-chemical calculations suggest that modelling the structure with fixed copper positions does not satisfactorily describe all structural features, which insinuates copper ion mobility at elevated temperatures.