Giant phonon anharmonicity driven by the asymmetric lone pairs in Mg₃Bi₂

Mg₃Bi₂-based materials exhibit high thermoelectric performance at ambient temperature benefitting from their low lattice thermal conductivity. However, the underlying physics of this low lattice thermal conductivity in a simple hexagonal crystal structure of Mg₃Bi₂ remain puzzling. Understanding the microscopic thermal transport behavior and its correlation with bonding, lattice dynamics are critical to improve the thermoelectric performance and design new promising functional materials. In this work, the giant anharmonic phonon modes are experimentally observed via measuring temperature-dependent inelastic x-ray scattering (IXS) for Mg₃Bi₂ single-crystal, which is also verified by the anomalously large Grüneisen parameters and frozen phonon potential calculations. Furthermore, we propose that the giant anharmonicty is associated with the asymmetric Bismuth 6s lone-pair electrons. The present work builds a microscopic connection between electronic structure and giant anharmonic phonon scattering, providing new insights on the low lattice thermal conductivity of Mg₃Bi₂, and paves the way to design novel high-efficient thermoelectrics for application in energy recycling and refrigeration.