The structural behavior of $\mathrm{U}{\mathrm{O}}_2$ under high pressure up to $300\mathrm{GPa}$ has been studied by first-principles calculations with $\mathrm{LSDA}+\mathrm{U}$ approximation. The results show that a pressure-induced structural transition to the cotunnite-type (orthorhombic $Pnma$) phase occurs at $38\mathrm{GPa}$. It agrees well with the experimentally observed $\sim 42\mathrm{GPa}$. An isostructural transition following that is also predicted to take place from $80\text{to}130\mathrm{GPa}$, which has not yet been observed in experiments. Further high compression beyond $226\mathrm{GPa}$ will result in a metallic and paramagnetic transition. It corresponds to a volume of $90{\mathrm{\AA }}^3$ per cell, in good agreement with a previous theoretical analysis in the reduction of volume required to delocalize $5f$ states.

• Received 24 August 2006

DOI:https://doi.org/10.1103/PhysRevB.75.054111