Lone Pair Induced 1D Character and Weak Cation–Anion\nInteractions: Two Ingredients for Low Thermal Conductivity in Mixed-Anion\nMetal Chalcohalide CuBiSCl<sub>2</sub>

Mixed-anion compounds,\nwhich incorporate multiple types of anions\ninto materials, display tailored crystal structures and physical/chemical\nproperties, garnering immense interest in various applications such\nas batteries, catalysis, photovoltaics, and thermoelectrics. However,\ndetailed studies regarding correlations among crystal structure, chemical\nbonding, and thermal/vibrational properties are rare for these compounds,\nwhich limits the exploration of mixed-anion compounds for associated\nthermal applications. In this work, we investigate the lattice dynamics\nand thermal transport properties of the metal chalcohalide, CuBiSCl<sub>2</sub>. A high-purity polycrystalline CuBiSCl<sub>2</sub> sample\nexhibits a low lattice thermal conductivity (κ<sub>L</sub>)\nof 0.9–0.6 W/(m·K) from 300 to 573 K. By combining various\nexperimental techniques, including three-dimensional (3D) electron\ndiffraction, with theoretical calculations, we elucidate the origin\nof low κ<sub>L</sub> in CuBiSCl<sub>2</sub>. The stereochemical\nactivity of the 6s<sup>2</sup> lone pair of Bi<sup>3+</sup> favors\nan asymmetric environment with neighboring anions involving both short\nand long bond lengths. This particularity often implies weak bonding,\nlow structure dimensionality, and strong anharmonicity, leading to\na low κ<sub>L</sub>. In addition, the strong 2-fold linear S–Cu–S\ncoordination with weak Cu···Cl interactions induces\na large anisotropic vibration of Cu, which enables strong phonon–phonon\nscattering and decreases κ<sub>L</sub>. The investigations into\nlattice dynamics and thermal transport properties of CuBiSCl<sub>2</sub> broaden the scope of the existing mixed-anion compounds suitable\nfor the associated thermal applications, offering a new avenue for\nthe search for low thermal conductivity materials in low-cost mixed-anion\ncompounds.