Cooperative Oxide-Ion Transport in Pyrochlore Y₂Ti₂O₇: A First-Principles Molecular Dynamics Study

We developed a mapping approach for analyzing time-series data of local structural units based on the Voronoi–Dirichlet partition to quantify the defect states and oxide-ion migration mechanism in pyrochlore Y₂Ti₂O₇ (space group: Fd3̅m) observed during first-principles molecular dynamics simulations. Defect states are classified into three types: a Frenkel pair, a single vacancy, and a split vacancy. Although the energetically most favorable defect type is a split vacancy at low temperatures, split and single vacancies are found to be equally favorable at high temperatures. Oxide-ion migration occurs by a two-step cooperative mechanism via a split vacancy. Climbing-image nudged elastic band calculations show the energy barrier for the cooperative diffusion mechanism (0.65 eV) to be substantially less than that of a simple stepwise mechanism (0.96 eV). This lower-energy barrier is comparable to experimental activation energies of oxide-ion diffusion in Y₂Ti₂O₇ reported in the literature, making this previously unrecognized cooperative mechanism the most likely candidate for enabling oxide-ion transport in the titanate pyrochlore.