Unveiling surface stability and oxygen diffusion of rare-earth zirconate pyrochlores by density functional theory

The surface structures, bond variations, and segregation of oxygen vacancies play crucial roles in the structural stability and functionality of nanocrystalline rare-earth zirconate pyrochlores. In this work, the stabilities of (1 0 0), (1 1 0), and (1 1 1) surfaces of pyrochlore A₂Zr₂O₇ (A = La, Ce, Pr, Nd, Pm, Sm, Eu, or Gd) are investigated by first-principles calculations. Surface reconstruction occurs on (1 1 0) surface with a transition of ZrO₆ octahedron to ZrO₄ tetrahedron, leading to their large relaxation energies. In combination with the small amount of broken bonds during the surface formation process, the (1 1 0) surfaces are identified having the lowest surface formation energies than the (1 0 0) and (1 1 1) surfaces. Moreover, the reconstructed (1 1 0) surface has characteristics of the segregation of oxygen vacancies. The surface oxygen vacancies have the low migration barriers (<1.2 eV), which are comparable with those in bulk and ensure the long-distance diffusion of oxygen vacancies in A₂Zr₂O₇. These discoveries provide fundamental insight to the surface structure and related oxygen vacancy behavior, which are expected to guide the optimization of the surface related properties for nanocrystalline rare-earth zirconates.