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

Abstract

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 $Cmc{2}_{1}$ and Cmcm are found as the possible ground state. Both of them are low temperature distortions of the hexagonal Ni${}_{2}$In phase with space group $P{6}_{3}/mmc$, 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 $Cmc{2}_{1}$ phase is stable above $120\phantom{\rule{0.28em}{0ex}}\text{GPa}$ in UO${}_{2}$, and it transforms to the Cmcm phase at around $400\phantom{\rule{0.28em}{0ex}}\text{GPa}$. In PuO${}_{2}$, the Pnma phase directly transforms to the Cmcm phase at $123\phantom{\rule{0.28em}{0ex}}\text{GPa}$. Symmetry analysis shows that the transitions of $Cmc{2}_{1}$ to Cmcm in UO${}_{2}$ and Pnma to Cmcm in PuO${}_{2}$ are pressure induced group-subgroup transitions. The Cmcm phase is responsible for pressure-induced metallization both in UO${}_{2}$ and PuO${}_{2}$, but their electronic structures are quite different during metallization. The difference is originated from their insulating nature, where UO${}_{2}$ is a standard Mott-Hubbard insulator, while PuO${}_{2}$ 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-Ni${}_{2}$In phases will probably be adopted by other actinide dioxides.