Simulation of uranium dioxide polymorphs and their phase transitions

Abstract

In this article first-principles DFT calculations and molecular dynamics simulations using empirical potentials have been used to study four different polymorphs of uranium dioxide that appear under high compressive and tensile deformations. It has been found, as expected, that the ground-state structure is the fluorite-type structure (space group $Fm\overline{3}m$). Under high compressive deformation urania transforms into cotunnite-type structure (space group $Pnma$), as already known experimentally. The calculated transition pressure is 28 GPa in agreement with the experimental data. Under tensile deformation urania transforms into either scrutinyite-type structure (space group $Pbcn$) or rutile-type (space group $P{4}_{2}/mnm$) structure. These two phases are almost energetically degenerate; hence it is impossible to distinguish which phase is the most favorable. The transition pressure for both phases is found to be equal to $\ensuremath{-}10$ GPa. Subsequently, assessment of four of the most used empirical potentials for ${\mathrm{UO}}_{2}$---Morelon, Arima, Basak, and Yakub---have been carried out comparing the equations of state with those found with DFT calculations. The Morelon potential has been found to be the most accurate to describe the different urania polymorphs. Using this empirical potential and a dedicated minimization procedure, complete transition pathways between the ground state ($Fm\overline{3}m$) and both tensile structures ($Pbcn$ or $P{4}_{2}/mnm$) are described. Finally, uniaxial tensile load molecular dynamics simulations have been performed. It has been found that for load in the $\ensuremath{\langle}100\ensuremath{\rangle}$ direction urania transforms into the $Pbcn$ structure while for load in the $\ensuremath{\langle}110\ensuremath{\rangle}$ direction it transits towards the $P{4}_{2}/mnm$ structure.