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

Regardless 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, Mossbauer 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.