  Mixed-anion compounds, which incorporate multiple types of anions into
materials, displays tailored crystal structures and physical/chemical
properties, garnering immense interests in various applications such as
batteries, catalysis, photovoltaics, and thermoelectrics. However, detailed
studies regarding correlations between 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 chalcohalides, CuBiSCl2. A high-purity polycrystalline CuBiSCl2
sample, successfully synthesized via modified solid-state synthetic method,
exhibits a low lattice thermal conductivity of 0.9-0.6 W m-1 K-1 from 300 to
573 K. By combining various experimental techniques including 3D electron
diffraction with theoretical calculations, we elucidate the origin of low
lattice thermal conductivity in CuBiSCl2. The stereo-chemical 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 low lattice thermal conductivity. In addition, the strong two-fold linear
S-Cu-S coordination with weak Cu -- Cl interactions induces large anisotropic
vibration of Cu or structural disorder, which enables strong phonon-phonon
scattering and decreases lattice thermal conductivity. The investigations into
lattice dynamics and thermal transport properties of CuBiSCl2 broadens the
scope of the existing mixed-anion compounds suitable for the associated thermal
applications, offering a new avenue for the search of low thermal conductivity
materials in low-cost mixed-anion compounds.
