Surface phase diagrams of La-based perovskites towards the O-rich limit from first principles

The exposed termination of transition-metal oxide surfaces plays a major role in determining the catalyst performance in redox reactions. In this contribution, the surface phase diagrams of LaMO₃(001) (M = Sc-Fe) and LaMO₃(110) (M = Co-Cu) are constructed by using the DFT+U method. The stabilities of six terminations derived from the stoichiometric MO₂ and LaO surfaces are determined over a wide range of temperatures and oxygen partial pressures. The surface phase diagrams are calculated towards the O-rich limit in which the chemical potential of oxygen anions of perovskites equals that of gas-phase oxygen while the chemical potential of M cations is limited by thermodynamic boundary conditions. It is found that the surface phase diagrams are closely related to the reducibility of M cations, which is reflected in the oxygen adsorption energy and oxygen vacancy formation energy on the MO₂- and LaO-terminated surfaces and can be measured by the third ionization energies of the M²⁺ cations. According to the surface phase diagrams, the most stable surface termination is predicted to be of MO₂ type for LaMO₃ (M = Sc-Fe) and LaO type for LaMO₃ (M = Co-Cu) under solid oxide fuel cell operating conditions. Because the M cations become more readily reduced on going from left to right across the period, LaCoO₃ may form an oxygen-deficient crystal structure at high temperatures and LaNiO₃ and LaCuO₃ would be decomposed into oxides containing the transition metals in a lower oxidation state.