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

International audienceRegardless 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 Cu2+xSn1−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