Pressure-induced structural and electronic phase transitions of uranium trioxide

The crystal structures, phonon spectra, and electronic properties of uranium trioxide (UO₃) under high pressure have been systematically explored using a particle swarm optimization structure prediction method in conjunction with first-principles calculations. Our calculated lattice constants and the transition pressure of the two experimentally reported phases of γ- and η-UO₃ are consistent with previous experiments. At pressures of 13, 62, 220 GPa, three new structures of P6₃/$mmc$, Pm3̄n, and Fm3̄m are predicted in sequence to be thermodynamically stable. Based on our calculated elastic constants and phonon spectra, we indicate that these three phases are mechanically and dynamically stable. Interestingly, upon phase transition from P6₃/$mmc$ to Pm3̄n, UO₃ undergoes a semiconductor-to-metal electronic transition. In addition, we report results of specific heat, entropy, bulk modulus, shear modulus, Young's modulus, Poisson's ratio, and Debye temperature. Our results provide key insights into understanding the structural as well as the electronic behaviors of UO₃ under the condition of external pressure.