Lone Pair Induced 1D Character and Weak Cation–Anion Interactions: Two Ingredients for Low Thermal Conductivity in Mixed-Anion Metal Chalcohalide CuBiSCl2

Mixed-anion compounds, which incorporate multiple types of anions into materials, display tailored crystal structures and physical/chemical properties, garnering immense interest in various applications such as batteries, catalysis, photovoltaics, and thermoelectrics. However, detailed studies regarding correlations among crystal structure, chemical bonding, and thermal/vibrational properties are rare for these compounds, which limits the exploration of mixed-anion compounds for associated thermal applications. In this work, we investigate the lattice dynamics and thermal transport properties of the metal chalcohalide, CuBiSCl2. A high-purity polycrystalline CuBiSCl2 sample exhibits a low lattice thermal conductivity (κL) of 0.9–0.6 W/(m·K) from 300 to 573 K. By combining various experimental techniques, including three-dimensional (3D) electron diffraction, with theoretical calculations, we elucidate the origin of low κL in CuBiSCl2. The stereochemical activity of the 6s2 lone pair of Bi3+ favors an asymmetric environment with neighboring anions involving both short and long bond lengths. This particularity often implies weak bonding, low structure dimensionality, and strong anharmonicity, leading to a low κL. In addition, the strong 2-fold linear S–Cu–S coordination with weak Cu···Cl interactions induces a large anisotropic vibration of Cu, which enables strong phonon–phonon scattering and decreases κL. The investigations into lattice dynamics and thermal transport properties of CuBiSCl2 broaden the scope of the existing mixed-anion compounds suitable for the associated thermal applications, offering a new avenue for the search for low thermal conductivity materials in low-cost mixed-anion compounds.