Structural behavior of uranium dioxide under pressure by<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi>LSDA</mml:mi><mml:mo>+</mml:mo><mml:mi mathvariant="normal">U</mml:mi></mml:mrow></mml:math>calculations

Abstract

The structural behavior of $\mathrm{U}{\mathrm{O}}_{2}$ under high pressure up to $300\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ 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 $38\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. It agrees well with the experimentally observed $\ensuremath{\sim}42\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. An isostructural transition following that is also predicted to take place from $80\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}130\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, which has not yet been observed in experiments. Further high compression beyond $226\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ will result in a metallic and paramagnetic transition. It corresponds to a volume of $90\phantom{\rule{0.3em}{0ex}}{\mathrm{\AA{}}}^{3}$ per cell, in good agreement with a previous theoretical analysis in the reduction of volume required to delocalize $5f$ states.