Investigating the Cubic-to-Tetragonal Phase Transition of Cu2+yZn1–ySnSxSe4–x Solid Solutions

The Cu2+y Zn1-y SnS x Se4-x (0 <= x <= 4; y = 0, 0.125) system is an earth-abundant, nontoxic chalcogenide with tunable polymorphism and chemical disorder, making it a promising candidate for sustainable thermoelectric and photovoltaic applications. Recent stabilization of cubic sphalerite Cu2ZnSnS4 and Cu2ZnSnSe4 via mechanochemical synthesis has demonstrated enhanced thermoelectric performance attributed to low-energy optical phonon modes and topological conduction pathways for charge carriers. In this study, we explore the role of anion substitution and Cu-induced CuZn antisite disorder in stabilizing the cubic phase and driving its transformation to the partially disordered tetragonal kesterite structure at high temperature. A combination of X-ray diffraction, Raman spectroscopy, and first-principles simulations (DFT, DFPT, AIMD) reveals that Cu-rich compositions deviate from Vegard's law, show increased stacking fault density, and exhibit pronounced distortion in the tetrahedral motifs. AIMD results indicate that the higher symmetry of the cubic phase permits a broad distribution of tetrahedral configurations, stabilizing disorder and stacking faults. Upon thermal activation, entropy favors the emergence of more stable S/Se-Cu3Sn, S/Se-Cu2ZnSn, and S/Se-CuZn2Sn motifs, stabilizing the kesterite phase with a reduced quantity of microstructural defects. Notably, in compositions close to Cu2+y Zn1-y SnS2Se2, classified as high-entropy alloys, Baur bond and angle distortions peak, suggesting structural robustness despite high defect concentrations. This work provides a fundamental understanding of microstructural disorder from the atomic motif level, offering valuable guidelines for tuning phase stability and properties in Cu2+y Zn1-y SnS x Se4-x kesterite and sphalerite materials.