Lattice evolution and point defect chemistry in Ta-doped ceria

Abstract

Point defects in donor-doped ceria impact the physical, chemical and electronic properties. This work reports on the point defects in Ta-doped ceria, relating the changes in the lattice parameters with the type and concentration of point defects. The accurate Ce3+ quantification using magnetization method is a key factor in solving the defect equilibria. Combining the experimental data and lattice expansion models it is demonstrated that the extra positive charge of Ta5+ donors is compensated by both the Ce3+ and the oxygen interstitials. An accurate quantitative analysis of the lattice expansion indicates that the theoretical calculations significantly overestimate the effect of the lattice response to oxygen interstitials. Furthermore, it is demonstrated that at high temperatures the anion Frenkel defects, whose concentration increases with the Ta substitution level, bring additional lattice expansion. The calculated enthalpy of formation of the anion Frenkel defects (HAF = 3.27–4.24 eV) for Ta-doped ceria is in good agreement with the ab-initio calculations and with the experimental values for the U-doped ceria. Furthermore, the accurate data of the lattice expansion caused by the interstitial oxygen reported here can be used to better understand and to predict the ‘swelling’ of the isomorphous UO2, PuO2 and NpO2 nuclear waste materials.