Ordered sphalerite derivative Cu5Sn2S7: a degenerate semiconductor with high carrier mobility in the Cu–Sn–S diagram

Abstract

Regardless of the complexity of the phase diagram of the Cu–Sn–S system, several compositions near the prototypical mohite Cu₂SnS₃ have arisen as potential non-toxic, earth-abundant and cost-efficient photovoltaic and thermoelectric materials. In this work, we revisited the Cu_2+xSn_1−xS₃ system and discovered a monoclinic (C2) ordered sphalerite derivative member, Cu₅Sn₂S₇. Using a combination of synchrotron diffraction and spectroscopy, transmission electron microscopy, precession-assisted electron diffraction tomography, Mössbauer spectroscopy, first principles calculations and transport properties measurements, we discuss the structure–thermoelectric properties relationships and clarify the interesting crystal chemistry in this system. The ternary sulfide Cu₅Sn₂S₇ exhibits a degenerate semiconducting behavior with exceptionally high hole mobility originating from the interplay between atomic ordering and charge delocalization.