Uncovering complex phonon interactions in Mg₃Bi_2-xSb_x: topology and avoided crossings

Mg₃Bi_2-xSb_x compounds have emerged as promising mid-temperature thermoelectric materials, due to their excellent electrical performance and the ultralow thermal conductivity. In this study, we investigate the complex phonon interactions in Mg₃Bi_2-xSb_x compounds and reveal nontrivial symmetry-protected topological crossings and avoided-crossing phenomena in the phonon dispersion, arising from the strong coupling between the acoustic and low-energy optical phonons. By combining inelastic neutron scattering measurements with first-principles simulations, we identify robust band crossing protected by crystal symmetries and mode inversion along the Γ M direction in Mg₃Bi_2-xSb_x compounds, providing direct experimental evidence of topological phonons in a thermoelectric system. Furthermore, pronounced avoided crossings involving ultrasoft transverse acoustic modes are observed along the same direction, indicating significant hybridization with optical branches in the basal plane. The influence of Bi/Sb alloying on the phonon structure is also examined, revealing a broadened density of states (DOS) in the low- to mid-energy range for the Mg₃BiSb alloy. These findings establish Mg₃Bi_2-xSbₓ compounds as a model system for studying symmetry-protected phonon topology and strong phonon-phonon interactions, offering new insights for lattice engineering in quantum and energy materials.