Native point defects and oxygen migration of rare earth zirconate and stannate pyrochlores

Abstract

Various excellent properties of rare earth zirconate and stannate pyrochlores are close related with their native point defects. First-principles calculations are performed to systematically investigate the point defect mechanism and the oxygen diffusion behavior of A2B2O7 (A=La, Ce, Pr, Nd, Pm, Sm, Eu, Gd; B=Zr, Sn). The possible defect complexes and their associated reactions under stoichiometric and nonstoichiometric conditions are explored. The O Frenkel pairs are the most stable defect structure in stoichiometric zirconates, whereas the cation antisite defects are the predominant one in stoichiometric stannates. In the case of BO2 excess zirconates and stannates, the BA cation antisite defect with the A vacancy and/or the oxygen interstitial is energetically favorable, whereas the AB antisite defect together with the oxygen vacancy and/or the A interstitial is preferable under the A2O3 excess condition. Meanwhile, the maximum point defect concentrations of zirconates are much higher than those of stannates. Furthermore, the oxygen migration barriers are similar in these compounds, ranging in 0.68 eV∼ 0.80 eV. The predicted point defects and oxygen diffusion mechanisms play the critical role in their engineering applications and are expected to guide the future property improvement of pyrochlores through the control of point defects and/or composition.