Stoichiometry of the LaFeO₃ (010) surface determined from first-principles and thermodynamic calculations

The phase diagram of LaFeO₃ (010) surfaces is developed by ab initio thermodynamics. The stabilities of LaO- and FeO₂-terminated surfaces are investigated at temperatures representative of solid oxide fuel cell (SOFC) operating conditions [773, 1073, and 1223 K at p(O₂) 0.21 atm]. For LaO-type surfaces, it is predicted that the most stable surface structure is oxidized at all temperatures considered. For FeO₂-type surfaces, the most stable surface structure is predicted to change from oxidized (at 773 K) to stoichiometric (at 1073 and 1223 K). Even though both LaO and FeO₂ surfaces can be oxidized under SOFC operating conditions, the degree of oxidation is much greater for the LaO surface. In addition, as reduced surfaces are predicted to be significantly more unstable than stoichiometric and oxidized terminations at these temperatures and oxygen partial pressures, surface oxygen vacancies are not predicted to form on either the LaO or the FeO₂ terminations. Moreover, at high temperatures [above ∼1500 K at p(O₂) = 0.21 atm], only FeO₂-type surfaces are predicted to be stable. Importantly, the calculated transition temperatures where surface oxygen stoichiometries are predicted to change are in good agreement with the results of temperature-programmed desorption experiments.