Atomic-Scale Structure and Stability of the Low-Index Surfaces of Pyrochlore Oxides

The multifunctional properties of complex ternary oxides such as pyrochlores are often influenced by surface structure. Optimizing the surface-driven attributes of these materials necessitates a detailed understanding of the structure and chemical composition of those surfaces. Here we report atomistic simulations elucidating the diverse atomic-scale structures of a set of low-index surfaces [(100), (110), (111), and (112)] in pyrochlore compounds as a function of both A and B cation chemistry. In pyrochlores, the low-index facets are all dipolar, requiring the introduction of surface defects to eliminate the surface dipole. We find that, due to the corresponding higher coordination of the surface cations, the (110) facet is the most energetically stable in all of the compounds considered, an interesting contrast to fluorite, in which the (111) surface is most stable. We also reveal a correlation between the surface energy and the energy to disorder the pyrochlore as a function of B cation chemistry, implying a similar physical origin for the two phenomena. Further, we find that surface rumpling is common across all pyrochlore compounds. An even more interesting feature emerging at these surfaces is the formation of extended structural defects such as steps and trenches, which are found to be stable after high-temperature annealing. As the formation of these features is a consequence of surface defects introduced to eliminate the surface dipole, we propose that the superior surface properties of materials of pyrochlores are due to these extended structural features, which are a direct consequence of the inherent dipole at the surfaces.