Anion-Controlled Crystal Symmetry and Transport Properties in Diamond-Like Cu2FeSnS4- x Se x

Abstract

Diamond-like chalcogenides attract increasing interest owing to their structural flexibility and intrinsically low lattice thermal conductivity. However, the influence of anion chemistry on crystal symmetry, band structure and transport behavior remains insufficiently understood. Here we investigate the Cu2FeSnS4-xSex system and show that sulfur-selenium substitution governs crystal symmetry and lattice evolution. S/Se substitution induces a change in crystal symmetry from P4̅ to I4̅2m and is accompanied by a temperature-driven convergence toward a pseudocubic lattice (c/2a → 1 near 773 K), which modifies band degeneracy and dispersion near the band edges. First-principles calculations reveal semiconducting electronic behavior with unusually large density-of-states effective masses (m* ≈ 3.7–4.6 me for S-rich and mixed compositions), which account for the coexistence of high Seebeck coefficients (220–240 μV K–1) and high hole concentrations (∼1020 cm–3). Despite the presence of a structural instability in Cu2FeSnSe4, the temperature-dependent electronic transport properties remain stable across the investigated chemical composition range. The lattice thermal conductivity is intrinsically low (1.5–1.8 W m–1 K–1 at 300 K) and decreases to a very low value of ∼0.7 W m–1 K–1 at 773 K. Debye–Callaway analysis indicates that anion-induced point-defect scattering as the dominant heat-transport-limiting mechanism in intermediate compositions at elevated temperatures. These results highlight the interdependence of crystal structure, lattice dynamics, and transport properties as a function of anion substitution in diamond-like chalcogenides.