First principles calculations of lattice dynamics and thermal transport properties of alpha uranium under high pressure

Abstract

Through first-principles calculations based on density functional theory (DFT) and the Boltzmann transport equation (BTE), the thermal transport properties of α-uranium under high pressure are investigated. In order to investigate the effects of pressure on the phonon dispersion relations and thermal conductivity of <i>α</i>-U, the phonon dispersion relations and lattice thermal conductivity at different pressures are obtained using a 4×4×4 supercell. First, for the calculation of electronic thermal conductivity, the ratio of thermal conductivity to relaxation time is calculated from the Boltzmann transport equation. Then, the relaxation time is calculated using deformation potential energy theory, relaxation time approximation, and effective mass approximation method derived from DFT band structure. Finally, the electronic thermal conductivity is obtained through the Wiedemann-Franz law. The calculation results indicate that <i>α</i>-U remains dynamically stable under a pressure of 80 GPa. The thermal conductivity of <i>α</i>-U exhibits a typical “V”-shaped temperature dependence: at low temperatures, phonon thermal conductivity dominates and decreases with the increase of temperature; at high temperatures, the electronic thermal conductivity becomes more significant and increases with temperature increasing. The combined effect of phonon thermal conductivity and electron thermal conductivity results in the total thermal conductivity reaching its minimum value at a temperature of approximately 160 K. When the temperature is 300 K, the thermal conductivity of <i>α</i>-U at 0 GPa is 25.11 W/(m·K), and increases to 250.75 W/(m·K) at 80 GPa as pressure increases. This result clearly indicates that an increase in pressure significantly enhances thermal conductivity. The calculation results also highlight the influences of pressure on phonon group velocity, phonon lifetime, and electron phonon interactions, all of which promote an increase in thermal conductivity. These findings provide a comprehensive understanding of the thermal conductivity of <i>α</i>-U depending on temperature and pressure and offer valuable insights into potential applications in extreme environments.