First-principles study of point defects in cerium dioxide and comparison to uranium dioxide
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First-principles study of point defects in cerium dioxide and comparison to uranium dioxide
Lei Shi1, Emerson Vathonne1, Michel Freyss1, Marjorie Bertolus1, Vincent Oison2, Roland Hayn2 1. CEA, DEN, DEC, Centre de Cadarache, 13108 Saint-Paul lez Durance, France. 2. IM2NP, Aix-Marseille University, Marseille, France.
ABSTRACT Uranium dioxide, as the standard nuclear fuel in pressurized water reactors, motivates intensive research to get further insight into the link between radiation damage and microstructure evolution of the material. Cerium dioxide is often considered as a non-radioactive model material for uranium dioxide, for which the experimental study of radiation damage could be performed more easily. Using first-principles calculations based on the density functional theory (DFT) and its DFT+U variant, we compare these two oxides in terms of point defect formation.
INTRODUCTION Uranium dioxide and cerium dioxide share several common bulk properties that foster the use of cerium dioxide as a model material for uranium dioxide in radiation damage studies. UO2 and CeO2 are both ionic crystals with the same (4+) valence of the cations and in the same fluorite crystal structure at ambient pressure. They are both insulators with band gaps of 2.1 eV for UO2 [1] and around 3.0 eV for CeO2 [2,3].They share similar lattice parameters (5.47 Å for UO2 [4] and 5.41 Å for CeO2 [5]), similar bulk moduli (207 GPa for UO2 [4] and 220 GPa CeO2 [5]) and similar elastic constants [6,7]. The objective of the present study is to determine to what extent CeO2 and UO2 also share a similar behavior in terms of radiation damage. To this end and in order to get better insight into the stability of defects in CeO2, we calculate the formation energies of various point defects in CeO2, considering the various possible charge state of the defects. The results are compared to those calculated in UO2 [8,9]. CeO2 has already been extensively studied using first-principles calculations. On the one hand, DFT in its standard LDA or GGA approximations and DFT+U have been used to study bulk properties of the CeO2 crystal [10–14]. On the other hand, the DFT+U method has been used by several authors in the study of non-stoichiometric CeO2 and its point defects [15–24]. Here, using DFT+U combined to occupation matrix control, we consider more exhaustively all possible charge states of the point defects in CeO2. We use the same calculation method and equivalent calculation parameters as in the study of UO2 [8] to allow for a precise comparison of CeO2 and UO2. The outline of this article is as follows. We first present briefly the calculation method and parameters used. Second, we report the bulk properties of CeO2 calculated. Then, the results on the formation energies and charge states of point defects in CeO2 are presented and finally compared to those obtained in UO2 by Wiktor et al. [8] and Vathonne et al. [9].
METHOD OF CALCULATION Unlike bulk UO2 which counts two localized 5f electrons, bulk CeO2 does not count any localized 4f electrons
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