The impact of chemical order on defect transport in mixed pyrochlores

Using temperature accelerated dynamics, an accelerated molecular dynamics method, we examine the relationship between composition and cation ordering and defect transport in the mixed pyrochlore Gd2(Ti1-xZrx)2O7, using the oxygen vacancy as a representative defect structure. We find that the nature of transport is very sensitive to the cation structure, transitioning, as a function of composition, from three-dimensional migration to two-dimensional to pseudo-one-dimensional to becoming essentially immobile before reverting back to three-dimensional as the Zr content is increased. The rates of migration are also affected by the cation structure in the various compositions. This behavior is driven by the connectivity of Ti polyhedra in the material, with more extensive networks of Ti ions leading to a greater ability of the vacancy to traverse the material. Our results indicate that the nature of transport is dictated by the cation structure of the material and that, conversely, the cation structure could be used to control transport and potentially other functionalities in mixed pyrochlores.