Effects of cation disorder on oxygen vacancy migration in Gd2Ti2O7

Atomistic simulations were used to calculate defect formation and migration energies for oxygen vacancies in the pyrochlore Gd2Ti2O7, with particular attention to the role of cation antisite disorder. Oxygen occupies two crystallographically distinct sites (48f and 8a) in the ordered material, but the 8b sites become partially occupied with disorder. Because cation and anion disorder are coupled, oxygen vacancy formation and migration energetics are sensitive to the configuration of the cation disorder. The V-O48f vacancy and V-O48f + O8b(i) Frenkel defects are energetically favored in the ordered material, but V-O8a is favored at higher disorder. The V-O8a + O8b (i) Frenkel is favored for some disorder configurations. Eight possible oxygen vacancy migration paths converge toward a unique migration energy as cation disorder increases, reflecting a reversion towards the fluorite structure. Oxygen vacancy migration is determined by O-48f --> O-48f transitions along the shortest edges of the TiO6 octahedra. The transition V-O8a --> V-O48f is also possible for low disorder, and can activate the V-O48f --> V-O48f migration network by depositing vacancies there. The reverse transition may occur at very high disorder to retard ionic conduction, and is consistent with Frenkel defect stabilities. Local regions of ordered and disordered material both appear necessary to explain the observed trends in ionic conductivity.