Trends in stability of perovskite oxides.

Perovskite oxides with general formula AMO3 have a large variety of applications as dielectrics and piezoelectrics, ferroelectrics and/or ferromagnetic materials, among others. Rare earth and alkaline earth metal perovskites are useful as catalysts for hydrogen generation, as oxidation catalysts for hydrocarbons, and as effective and inexpensive electrocatalysts for state-of-the-art fuel cells, mainly due to the possibility of tuning their mixed ionic–electronic conductivity through substitution of A and M and subsequent formation of oxygen vacancies. Despite the general interest in perovskites, so far there have been no ab initio studies devoted to their formation energies, and the trends in stability are unknown. Among the available theoretical techniques to investigate perovskites, DFT is an appealing candidate, since it has proved useful for understanding metals and alloys at the atomic scale. Nevertheless, the well-known shortcoming of DFT in describing strongly correlated systems has prevented its use for the estimation of properties such as band gaps and electron localization–delocalization of oxides, and there are numerous corrections. Despite these limitations, Figure 1a shows the experimental formation energies from elements and O2 of 20 perovskites at 298 K and the corresponding standard DFT energies using the RPBE-GGA exchange-correlation functional. The simulations are able to reproduce trends in the formation energies, and the calculated energies are shifted by about 0.75 eV compared to experiments. The A component is Y, La, Ca, Sr, or Ba, while M is a 3d metal from Ti to Cu. However, it is possible to combine the formation energies of these compounds with those of their sesquioxides (A2O3 and M2O3), rutile dioxides (MO2), monoxides (AO and MO), and O2 to reproduce the energetics of several reactions (Figure 1b–d). The reactions are shown in the Supporting Information. The excellent correspondence between experiments and theory shows that DFT very accurately captures the mixing energies between oxides. The chemical reaction depicted in Figure 1a and the way of representing its Gibbs energy, are given by Equations (1) and (2).