Thermodynamics of surface and oxygen vacancy in rare earth stannates by first-principles calculations

The research of nanocrystalline pyrochlores highlights the importance of the surface structure, composition and segregated point defect in their thermal, electrical, optical, magnetic, and catalytic performances. In order to provide a basic view on the surface-related phenomena, thermodynamic stabilities of three low-index (100), (110), and (111) surfaces for A(2)Sn(2)O(7) (A = La, Ce, Pr, Nd, Pm, Sm, Eu, or Gd), together with their configurations, electronic structures and related oxygen vacancies are investigated using first-principles calculations. The (111) surfaces with A(3)SnO(8) and ASn(3)O(6) terminations are predicted to be stable due to their low surface energies. Meanwhile, the (110) surfaces with A(2)Sn(2)O(8) and A(2)Sn(2)O(6) terminations are found to may also form. For these surface structures, the amount of broken bonds play the main role in their structural stability, and the local coordination environment variation also has minor contribution to it. Moreover, oxygen vacancies are observed to segregate on the surface layer, owing to lower energy of breaking bonds accompanying with oxygen vacancy formation and the larger relaxation space comparing to the counterpart in bulk. These results are expected to provide guidance on optimizing the performances of these compounds through surface engineering.