High-temperature and high-pressure phase transitions in uranium

The phase diagram of uranium has been explored up to 100 GPa and 2000 K by means of ab initio molecular dynamics (AIMD) simulations. The lattice dynamics and energetics of the stable phases observed experimentally in this range of pressure and temperature are studied in this work. The phonon spectra of the α and γ phases are shown to evolve strongly as a function of temperature, unveiling the huge anharmonic effects present in this material. If the elastics constants and the bulk and shear moduli of the γ phase do not disclose any temperature effects, the shear modulus of the α phase decreases strongly as a function of temperature. Using the pressure- and temperature-dependent vibrational density of states and the Gibbs free energy of these two structures, we found a line transition between the α and γ phases which slightly underestimates the experimental one. Coherently with experiments, the bct structure is never found stable between 0 and 100 GPa. (authors)