Defect configuration and phase stability of cubic versus tetragonal yttria-stabilized zirconia

a r t i c l e i n f o Using first-principles calculations, we have carried out a systematic comparative study of the microscopic atomic defect configurations in cubic and tetragonal yttria-stabilized zirconia (YSZ) and their correlation with the macroscopic lattice parameters and relative phase stability, as a function of Y concentration. We found that Y atoms sit at the second-nearest-neighbor cation sites to oxygen vacancies and repel each other; oxygen vacancies form pairs and these pairs repel each other. Using the optimized defect configurations as inputs, we correctly identify the experimentally observed tetragonal to cubic transition point and predict the changes of lattice parameters with the increasing Y concentration, in excellent agreement with experiment. Our studies reveal an interesting correlation between the microscopic atomic defect configuration and macroscopic lattice properties. YSZ is the most commonly used electrolytes in solid oxide fuel cells (SOFC) for its high ionic and low electronic conductivity. Pure zirconia displays a monoclinic structure (P2 1 /c) up to a temperature of around 1400 K [1], where monoclinic to tetragonal (P4 2 /nmc) transition occurs. The cubic (Fm3m) phase exists at temperatures higher than 2600 K [2]. The addition of yttria introduces oxygen vacancies for charge compensation, and stabilizes the tetragonal and cubic phases at low temperatures [3,4]. Although the ionic conductivity of YSZ is attributed to the presence of oxygen vacancies introduced by Y doping, it does not simply increase monotonically with the increasing Y concentration but instead displays a maximum at the 15%–18% Y concentration [5]. This interesting behavior of ionic conductivity is ascribed to the change of atomic configurations of YSZ as a function of Y concentration. In particular, two possible reasons are identified: (1) too high an oxygen vacancy concentration leads to the formation of high activation energy pathways for oxygen diffusion [6,7], and (2) the formation of ordered oxygen vacancy complexes at high Y concentrations hinders the vacancy mobility [8]. It has become evident that the atomic defect configuration of Y and oxygen vacancy complexes have a profound effect on the oxygen diffusion and transport in YSZ, and hence on the efficiency of SOFC. Furthermore, the defect configuration is expected to be correlated with the phase stability of different YSZ phases, especially the cubic and tetragonal phases stabilized upon Y doping. Therefore, a detailed study of the defect configuration and phase stability of cubic versus tetragonal YSZ, as a function …