Structural behavior of uranium dioxide under pressure by LSDA+U calculations

The structural behavior of U O2 under high pressure up to 300 GPa has been studied by first-principles calculations with LSDA+U approximation. The results show that a pressure-induced structural transition to the cotunnite-type (orthorhombic Pnma) phase occurs at 38 GPa. It agrees well with the experimentally observed ∼42 GPa. An isostructural transition following that is also predicted to take place from 80 to 130 GPa, which has not yet been observed in experiments. Further high compression beyond 226 GPa will result in a metallic and paramagnetic transition. It corresponds to a volume of 90 Å3 per cell, in good agreement with a previous theoretical analysis in the reduction of volume required to delocalize 5f states.