Configurational Entropy and Phase Stability in Lead-Free Mixed-Halide CsSn(Br_xI_1-x)₃

Metal halide perovskites have high compositional tunability, but halide mixing is often accompanied by phase segregation and instability in lead-free Sn-based systems. Here, we investigate the thermodynamics of Br/I alloying in CsSn(Br_xI_1-x)₃ by combining density functional theory calculations with partition functions over all symmetry-inequivalent configurations of the cubic, tetragonal, and orthorhombic phases. We find that the orthorhombic phase exhibits the lowest mixing free-energy curve and is closest to the thermodynamic miscibility boundary, whereas the cubic phase remains the least favorable for Br/I mixing. At 300 K, the free-energy difference ΔF_cub-orth = F_cub - F_orth is positive over the entire composition range ((1.27-3.42)k_BT), indicating a robust thermodynamic preference for the orthorhombic phase. The enhanced stability of the low-symmetry phase originates from more effective local structural relaxation. Our results further reveal a link between local octahedral distortions and thermodynamic stability, providing theoretical guidance for the compositional design of lead-free Sn-based mixed-halide perovskites.