Phase Diagram and Bonding States of Pu–H Binary Compounds at High Pressures

The high-pressure processes of plutonium hydrides are usually involved in many practical situations, such as the volume expansion during the hydride formation, the pressure of helium bubbles due to the decay of plutonium, and the stress due to the formation of the oxide film. These cases could lead to the high-pressure phase transition of plutonium hydrides and thereby affect the properties of the material. The crystal structure and bonding properties of plutonium hydrides (PuH_1–10) under atmospheric pressure and high pressure are investigated by using the first-principle method in combination with a structure searching technique. Our results show that the predicted lattice structures of plutonium hydrides are stable over the given pressure range. A novel stable stoichiometry, PuH with space groupFm3̅m, is thermodynamically, dynamically, and mechanically stable in the pressure range of 62–188 GPa, which may be synthesized through the pressure-induced disproportionation of PuH₂. In particular, our theoretically predicted results indicate that PuH₃ undergoes pressure-induced phase transitions with the following sequence of phases, P6₃cm → Pnma → R3̅m → Cmcm, and the corresponding transition pressures are computed to be 4, 76, and 150 GPa, respectively. Interestingly, the most striking feature of PuH₃ is the pressure-induced transition from insulating to metallic forms. Analysis of the electric structures, charge density differences, and electronic localization functions indicates that all phases act mainly as metallic with ionic bonding at high pressure.