Contrasting Lone-Pair Stereochemistry in Sb ³⁺ and Bi ³⁺ Chalcogenides: Implications for Structure and Phonon Transport

The unique physicochemical properties of chalcogenides containing post-transition metals are often governed by the stereochemical activity of their ns2 lone pairs. A comparative structural analysis of bournonite (CuPbSbS3) and aikinite (CuPbBiS3) reveals that CuPbSbS3 has a larger unit cell volume (V = 552.58 Å3) than CuPbBiS3 (V = 534.42 Å3), despite antimony (Sb3+) being smaller than bismuth (Bi3+). This counterintuitive trend originates from the stronger lone pair effect of Sb3+, which induces a larger static distortion, leading to a tetrahedral SbS3L (l-lone pair) coordination, while Bi3+ exhibits a less asymmetric distortion corresponding to the more compact square-pyramidal BiS5 (or octahedral BiS5L) coordination. These structural differences are corroborated by temperature-dependent Raman spectroscopy, which captures distinct vibrational behaviors reflective of differing lattice dynamics. CuPbSbS3 exhibits higher anharmonicity, as quantified by anharmonicity coefficients extracted from Raman data. Electron localization function analysis and crystal orbital Hamilton population further confirm the higher activity of the Sb3+ lone pair compared to Bi3+. Interestingly, despite the lower atomic mass of Sb relative to Bi, both compounds exhibit nearly identical acoustic-phonon cutoff frequencies. Consequently, both CuPbSbS3 and CuPbBiS3 display partially suppressed Umklapp scattering and comparable low lattice thermal conductivities at high temperatures, despite their compositional and structural differences.