Abstract
We have investigated the crystal structure, phase transition and electronic properties of ThO2 and PuO2 using density functional theory (DFT). The lattice parameters of cubic ThO2 and PuO2 are found to be 5.62 Å and 5.48 Å, respectively. At higher pressure, AnO2 (An = Th, Pu) shows a transition from fluorite (cubic) to cotunnite (orthorhombic) structure at 26.6 GPa with 6.08% collapse in volume for ThO2 and 43.5 GPa with 7.22% collapse in volume for PuO2. Cubic ThO2 and PuO2 show indirect band-gap of 4.32 eV and 2.46 eV respectively. Orthorhombic ThO2 shows a direct band-gap of 3.57 eV while cotunnite PuO2 also has an indirect band-gap of 1.21 eV. The band-gap decreases in cotunnite structure as compared to the cubic structure for both oxides. With an increase in pressure, the band-gap and covalent characteristic increase in cubic AnO2.
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