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 ns 2 lone pairs. A comparative structural analysis of bournonite (CuPbSbS 3 ) and aikinite (CuPbBiS 3 ) reveals that CuPbSbS 3 has a larger unit cell volume ( V = 552.58 Å 3 ) than CuPbBiS 3 ( V = 534.42 Å 3 ), despite antimony (Sb 3+ ) being smaller than bismuth (Bi 3+ ). This counterintuitive trend originates from the stronger lone pair effect of Sb 3+, which induces a larger static distortion, leading to a tetrahedral SbS 3 L ( l -lone pair) coordination, while Bi 3+ exhibits a less asymmetric distortion corresponding to the more compact square-pyramidal BiS 5 (or octahedral BiS 5 L) coordination. These structural differences are corroborated by temperature-dependent Raman spectroscopy, which captures distinct vibrational behaviors reflective of differing lattice dynamics. CuPbSbS 3 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 Sb 3+ lone pair compared to Bi 3+ . Interestingly, despite the lower atomic mass of Sb relative to Bi, both compounds exhibit nearly identical acoustic-phonon cutoff frequencies. Consequently, both CuPbSbS 3 and CuPbBiS 3 display partially suppressed Umklapp scattering and comparable low lattice thermal conductivities at high temperatures, despite their compositional and structural differences.