High pressure structural stability of UO₂ by evolutionary algorithm

Abstract

The aim of this study is to investigate the crystal structure stability of UO₂ upon applying pressure using an evolutionary algorithm. UO₂ exhibits face-centred cubic crystal structure with space group $Fm\overline{3}m$ (S.G. No. 225) in the ambient pressure, and above 82 GPa transforms to an orthorhombic structure with space group Pnma (S.G. No. 62). The first-order structural phase transition is accompanied by a 5.4% volume collapse. The lattice parameters of both the cubic and orthorhombic phases decrease with the applied pressure. The bulk modulus (${B}_{\mathrm{o}}$) and the pressure derivative of bulk modulus (${B}_{\mathrm{o}}^{{\prime}}$) values of the cubic phase were found to be 210.41 ± 0.22 GPa and 3.60 ± 0.01, respectively. The ${B}_{\mathrm{o}}$ and ${B}_{\mathrm{o}}^{{{\prime}} }$ values of the orthorhombic phase were found to be 218.09 ± 3.29 GPa and 3.64 ± 0.03, respectively. The enthalpy of formation linearly increases with the applied pressure for both the cubic and orthorhombic phases, and at 82 GPa the enthalpy of the orthorhombic phase is found to be lower than the cubic phase indicating the structural phase transition.