Point defect and electronic properties of uranium dioxide

Abstract

We present a comprehensive theoretical survey of defect structures and energies in uranium dioxide and related non-stoichiometric phases. Our work is based on accurate computational methods of simulating defects in insulators. Such simulations require reliable lattice models for the material investigated. Our potential for U02 is obtained by fitting to a wide range of crystal data; but we incorporate results of molecular orbital calculations of the interaction of oxide anions. Our model reproduces the essential bulk properties of the material. The results of the defect calculations establish several important points: divalent Frenkel defects are the predominant form of atomic disorder; electronic (hole-electron) disorder is, however, far more extensive than any such atomic process - a result with considerable importance for the interpretation of the thermodynamics of hyperstoichiometric UO2. We also suggest reinterpretations of transport data on UO2; and we are able to rationalize features of the structural data of UO2 + x which had previously been unexplained. Our defect models, we believe, form a reliable basis for the analysis of experimental data not only for UO2 but for other fluorite oxide phases; these include plutonium doped UO2, which is of considerable technological importance as a reactor fuel.