Lone Pair Induced 1D Character and Weak Cation-anion Interactions: Two Ingredients for Low Thermal Conductivity in Mixed-anion Metal Chalcohalides

TL;DR

This study explores how lone pair electrons and weak cation-anion interactions in mixed-anion metal chalcohalides lead to low thermal conductivity, combining experimental and theoretical methods to understand the underlying mechanisms.

Contribution

It provides a detailed analysis of the structural and bonding features responsible for low thermal conductivity in CuBiSCl2, a mixed-anion chalcohalide, using combined experimental and computational approaches.

Findings

CuBiSCl2 exhibits low thermal conductivity of 0.9-0.6 W/m·K from 300 to 573 K.

Stereo-chemical activity of Bi3+ lone pairs causes asymmetric environments and weak bonding.

Weak Cu--Cl interactions induce anisotropic vibrations, enhancing phonon scattering.

Abstract

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.