Structural analysis of a solid solution of tin in antimony (γ) and a hypothesis of short-range clustering in the γ-phase

A comprehensive structural and thermodynamic investigation of the Sb-rich solid solution (γ-phase) in the Sn–Sb binary system was conducted using electron probe microanalysis (EPMA), high-resolution X-ray diffraction (XRD), and CALPHAD-based thermodynamic modeling. Precision lattice parameter determinations reveal a pronounced, non-monotonic deviation from Vegard's law localized near x Sb ≈ 0.9444. To elucidate the origin of this compression, a phenomenological hypothesis of short-range clustering based on a Sn 1 Sb 17 local motif is proposed. While direct atomic-scale imaging is beyond the scope of this study, alternative physical contributions, specifically equilibrium thermal vacancies and lattice microstrain, were quantitatively evaluated and determined to be insufficient to account for the observed unit-cell contraction. Incorporating a subregular solution framework with temperature-dependent Redlich–Kister interaction parameters ((L 0 , L 1 ) into a CALPHAD assessment constrained by fundamental low-temperature boundary conditions successfully reproduces experimental phase boundaries without generating non-physical artifacts. Notably, enforcing a valid low-temperature thermodynamic boundary condition (solvus extrapolation to 0 K) naturally yields a critical point that coincides with the hypothesized Sn 1 Sb 17 stoichiometry at T c ≈ 450K, providing a compelling convergence of structural and thermodynamic evidence.