Thermodynamic stability of stoichiometric LaFeO₃ and BiFeO₃: a hybrid DFT study

BiFeO₃ perovskite attracts great attention due to its multiferroic properties and potential use as a parent material for Bi_1-xSr_xFeO_3-δ and Bi_1-xSr_xFe_1-yCo_yO_3-δ solid solutions in intermediate temperature cathodes of oxide fuel cells. Another iron-based LaFeO₃ perovskite is the end member for well-known solid solutions (La_1-xSr_xFe_1-yCo_yO_3-δ) used for oxide fuel cells and other electrochemical devices. In this study an ab initio hybrid functional approach was used for the study of the thermodynamic stability of both LaFeO₃ and BiFeO₃ with respect to decompositions to binary oxides and to elements, as a function of temperature and oxygen pressure. The localized (LCAO) basis sets describing the crystalline electron wave functions were carefully re-optimized within the CRYSTAL09 computer code. The results obtained by considering Fe as an all-electron atom and within the effective core potential technique are compared in detail. Based on our calculations, the phase diagrams were constructed allowing us to predict the stability region of stoichiometric materials in terms of atomic chemical potentials. This permits determining the environmental conditions for the existence of stable BiFeO₃ and LaFeO₃. These conditions were presented as contour maps of oxygen atoms' chemical potential as a function of temperature and partial pressure of oxygen gas. A similar analysis was also performed using the experimental Gibbs energies of formation. The obtained phase diagrams and contour maps are compared with the calculated ones.