Thermochemistry of the Water Splitter SrTi0.5Mn0.5O3−δ Studied Using Density Functional Theory (DFT+ U )

Abstract SrTi0.5Mn0.5O3 (STM55) perovskite has been proven experimentally to be a promising material for hydrogen production using thermochemical water splitting. However, optimizing the performance of STM55 in this application requires a synergistic study of its thermochemical properties using both experiments and computations. In particular, there are several questions with regard to its crystal structure, magnetic ordering, electronic structure, and, more importantly, its off-stoichiometry variation as a function of temperature and oxygen chemical potential. Herein, we tackle these questions employing density functional theory calculations equipped with onsite Hubbard (U) terms applied to Mn and Ti 3d states. This study revealed that STM55 behaves electronically as a metal and is best modeled as a ferromagnet in the pm-3m crystal structure. Oxygen vacancy creation is controlled by the Mn4+/Mn3+ redox couple with minor intervention from Ti cations. Upon a thorough analysis, we selected a set of U parameters (UMn = 3 eV and UTi = 5 eV) that reasonably predict the oxide thermodynamic stability, the oxide off-stoichiometry variations with temperature and oxygen partial pressure, and the temperature dependence of the enthalpy of reduction in the dilute defect regime. Our work furnishes grounds for future extension of our predictive thermodynamic modeling to the nondilute regime by including defect–defect interactions and has general implications for the thermochemistry of the class of metallic oxides.