Antibonding valence states induce low lattice thermal conductivity in metal halide semiconductors.

This article focuses on the intrinsic mechanisms underlying ultralow lattice thermal conductivity (κ_L) in metal halide semiconductors, including metal halide perovskites (MHPs), which are important for thermoelectric and optoelectronic applications. It highlights the role of filled antibonding valence states (AVS) near the Fermi level in weakening chemical bonds, reducing elastic moduli and sound velocities, and thereby lowering κ_L across a diverse set of binary, ternary, and quaternary compounds with varying crystal structures and dimensionalities. The review integrates first-principles density functional theory (DFT) calculations, chemical bonding analyses via crystal orbital Hamilton population (COHP), and experimental data to establish a universal correlation between AVS and thermal transport properties, emphasizing that strong p–d orbital hybridization and lattice anharmonicity contribute to phonon scattering and soft acoustic modes. Additionally, it discusses synthesis methods for MHPs, the impact of structural features such as organic cation dynamics in hybrid perovskites, and prospects for using chemical bonding descriptors in machine learning models to predict and design materials with tailored low thermal conductivity.