Phase stability and structural comparison of phases in the Cu-Zn-Sn-S system using solid-state NMR

Phases in the Cu-Zn-Sn-S system are of interest to the solar and thermoelectrics communities because all elements are earth-abundant and non-toxic. A better understanding of the structures of these phases and the equilibria between them is necessary to guide device manufacturers. This work reviews all the phases reported in this system and reports which phases form and are stable at 200 °C and 400 °C. In this work, the stable phases in the Cu-Zn-Sn-S system (Cu₄Sn₇S₁₆, Cu₄SnS₄, Cu₂ZnSnS₄, and Cu₂SnS₃) are synthesized and their structures are investigated using solid-state NMR, X-ray diffraction, and Raman spectroscopy. The existing Cu-Sn-S and Cu-Zn-Sn-S phase diagrams are revised based on our synthesis results. Cu₃SnS₄ and Cu₂ZnSn₃S₈ are removed from the quaternary and ternary phase diagrams because we did not observe either phase. Prior reports (in thin films and nanocrystals) of Cu₃SnS₄ may be related to trace amounts of In or large surface areas. Monoclinic Cu₂SnS₃ is not stable at 400 °C and, at this temperature, a disordered tetragonal phase is stable. At lower temperatures (room-temperature and 200 °C), this same (disordered) tetragonal phase is stable at compositions that are Cu-rich and Sn-poor from Cu₂SnS₃. Cu₄SnS₆ was added to the phase diagram at 400 °C. Significant differences in reaction rate when forming Cu₄SnS₄ from either elemental or binary sulfide precursors are noted and explained. The NMR results are generalized to aid in distinguishing octahedral versus tetrahedral Sn⁴⁺ coordinations.