Bismuth exhibits a Peierls distortion away from a simple cubic structure, reflecting both an anharmonic potential energy surface and electron-phonon coupling in a semimetal. It has attracted considerable interest in studies of low-temperature thermal transport, thermoelectrics, and optical excitation of coherent phonon. Yet, the temperature-dependence of phonons in bismuth has remained relatively unexplored. We report extensive inelastic neutron scattering (INS) and first-principles simulations of the lattice dynamics in bismuth, including phonon energy shifts and linewidths as a function of temperature. Comprehensive four-dimensional data across the momentum ($\mathbf{Q}$) - energy ($E$) space were collected at temperatures from 2 to 300 K. Moreover, the intrinsic linewidth of TA modes was resolved using inelastic neutron spin echo with a Wollaston prism apparatus, surpassing the limitations of conventional neutron spectrometry techniques. Our results quantify the softening of phonon frequencies and suppression of phonon lifetimes at elevated temperatures, with experiments and simulations showing excellent agreement. Further, we analyzed the scattering phase space through simulations, assessing the scattering channels between acoustic and optic phonon modes that dominate the thermalization of photo-excited coherent optic phonons. These findings enhance our understanding of phonon-phonon interactions in bismuth and provide insights into the influence of lattice anharmonicity on its thermal properties.

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• Anharmonic lattice dynamics
• Lattice thermal conductivity
• Phonons
• First-principles calculations
• Inelastic neutron scattering