A low-temperature face-centered-cubic polymorph of Cu6GeWSe8 with a semiconductor-like conductivity

Abstract

Hexagonal Cu6GeWSe8 (H-phase) with an intermediate band gap has been reported lately. In this paper, we show a low-temperature face-centered-cubic modification of Cu6GeWSe8 (C-phase), which crystallizes in the noncentrosymmetric space group F-43m with four formula in the cell of dimension a = 11.1372(2) Å. In the C-phase, double layers organized by MeSe4 (Me = Cu, Ge and W) tetrahedra alternately stack along the <111> direction. A weakly first-order or nearly second-order displacive phase transition from cubic to hexagonal polytypes undergoes initially at ∼ 440 °C upon increasing temperature. Electrical resistivity and Seebeck coefficient reveal a semiconductor-like conduction with holes as the predominant charge carriers. The semiconductivity is also supported by the almost zero electronic specific-heat coefficient γ. The mechanism of thermal conduction was analyzed by abstracting electronic and lattice thermal conductivities through thermal transport measurement and meanwhile the thermoelectric properties were characterized. The differences in band structures and formation energy between the two variants were studied by ab-initio calculations, which indicate that the H-phase is a semiconductor and the C-phase is a semimetal with low density of states, and the hexagonal modification is comparatively stable. The cubic polytype of Cu6GeWSe8 expands the Cu-Ge-W-Se system. A stereo phase diagram making reviews on the rich materials, and multiple crystallographic and electronic states in this system was established, which would be helpful to guide for seeking new materials and to probe the interplay between polymorphism and property in chalcogenide family.