Stability of oxygen point defects inUO2by first-principlesDFT+Ucalculations: Occupation matrix control and Jahn-Teller distortion

Abstract

Point-defect formation energies in uranium dioxide UO 2 are still a matter of debate due to the significant discrepancies between the various studies published in the literature. These discrepancies stem from the density functional theory ͑DFT͒ + U approximation that creates multiple energy minima and complexifies the search for the ground state. We report here DFT+ U values of the formation energies for the single oxygen interstitial and vacancy in UO 2 , both in the fluorite and the Jahn-Teller distorted structures, using a scheme developed on bulk UO 2 ͓B. Dorado, B. Amadon, M. Freyss, and M. Bertolus, Phys. Rev. B 79, 235125 ͑2009͔͒ and based on occupation matrix control. We first investigate the Jahn-Teller distortion in UO 2 in the noncollinear antiferromagnetic order and we show that the distortion stabilizes the system by 50 meV/ UO 2 compared to the fluorite structure. Moreover, it is found that the oxygen atoms are displaced in the ͗111͘ directions, in agreement with experiments. For the bulk fluorite structure, we show that the use of the Dudarev approach of the DFT+ U without occupation matrix control systematically yields the first metastable state, located 45 meV/ UO 2 above the ground state. As a result, all previously published point-defect formation energies are largely underestimated. We then use the occupation matrix control scheme to calculate the formation energies of the single oxygen interstitial and vacancy in UO 2 . We confirm that this scheme always allows one to reach the lowest energy states and therefore yields reliable formation energies. Finally, we compare our values with those obtained in previous studies and show that the discrepancies observed stem from the calculations of defective supercells which have reached different metastable states.

Occupation matrix control of d- and f-electron localisations using DFT + U

“…The approach implemented is similar to that utilised by Dorado and co-workers 16,17,19 (VASP and ABINIT) and Torrent and co-workers 52,53 (ABINIT) but goes beyond it in a number of ways. It is not solely restricted to the definition of f orbitals and allows d orbital occupations (as well as s and p) to be defined.…”

Impact of homogeneous strain on uranium vacancy diffusion in uranium dioxide

“…However, the symmetry of a cation lattice in UO 2 may actually deviate from ideal cubic symmetry due to Jan-Teller distortion of the oxygen sub-lattice at low temperature, by lowering the symmetry as well as the energy of the system. 20,73,74 This Jahn-Teller distortion is directly linked to the orientation of the magnetic moment of uranium ions. 73,75,76 In our calculations, 1k antiferromagnetic (AFM) ordering is employed, as it has been argued that 1k AFM order is closer to the actual paramagnetic state at high temperature, and is shown to be a good approximation for defect energies.…”

“…20,73,74 This Jahn-Teller distortion is directly linked to the orientation of the magnetic moment of uranium ions. 73,75,76 In our calculations, 1k antiferromagnetic (AFM) ordering is employed, as it has been argued that 1k AFM order is closer to the actual paramagnetic state at high temperature, and is shown to be a good approximation for defect energies. 20,77 Consistent with this, DFT+U calculations of UO 2 show deviations from cubic symmetry for uranium vacancy and, therefore we observe off-diagonal elements for dipole tensor compared to relatively large diagonal components.…”

First Principles Simulations on Surface Properties and Oxidation of Nitride Nuclear Fuels

“…Nevertheless, even though the AFM state of UN bulk is reproduced, the ground state is hardly obtainable when using the DFT+U method [33]. The value of U-parameter must be carefully suggested otherwise large errors may appear when calculating defect formation energies [34][35]. We avoid application of this method in the present study due to ferromagnetic nature of UN surface [36] reproducible by standart DFT functionals.…”