Pressure-induced group-subgroup phase transitions and post-cotunnite phases in actinide dioxides

Taking uranium dioxide and plutonium dioxide as prototypes, the phase stabilities of actinide dioxides under high pressures are studied with density functional theory. The calculations are carried out within the framework of Perdew-Burke-Ernzerhof (PBE) and PBE+U formalism. Beyond the cotunnite Pnma phase, two novel orthorhombic phases with space groups Cmc2(1) and Cmcm are found as the possible ground state. Both of them are low temperature distortions of the hexagonal Ni2In phase with space group P6(3)/mmc, which is a common post-cotunnite phase adopted by many AX(2) and A(2)X binary compounds. According to the PBE+U calculations, the Cmc2(1) phase is stable above 120 GPa in UO2, and it transforms to the Cmcm phase at around 400 GPa. In PuO2, the Pnma phase directly transforms to the Cmcm phase at 123 GPa. Symmetry analysis shows that the transitions of Cmc2(1) to Cmcm in UO2 and Pnma to Cmcm in PuO2 are pressure induced group-subgroup transitions. The Cmcm phase is responsible for pressure-induced metallization both in UO2 and PuO2, but their electronic structures are quite different during metallization. The difference is originated from their insulating nature, where UO2 is a standard Mott-Hubbard insulator, while PuO2 belongs to an intermediate region between the Mott-Hubbard insulator and charge-transfer insulator. The equations of state are also analyzed, together with the pressure dependence of lattice parameters, fractional atomic coordinates, electronic band gap, and magnetic moment. The results indicate that these distorted-Ni2In phases will probably be adopted by other actinide dioxides.