First-principles investigation of trends in high-temperature thermochemistry of perovskite oxides

ABO₃ perovskite oxides containing cations of mixed and identical oxidation states at A- and B-sites are attractive candidates for high-temperature thermochemical energy conversion processes. The structural and thermochemical properties, and hence defect thermochemistry of such perovskites strongly depend on the identity of their A-site and B-site cations. Analyzing their temperature-dependent thermochemical and reduction properties can reveal their applicability for high-temperature energy applications, including solar thermal water splitting, solid oxide fuel cells, mixed ionic and electronic conduction processes etc. Since the typical A-site cations are comprised mainly of alkaline, alkaline-earth or rare-earth elements, and B-site cations mostly 3d, 4d, and 5d transition metals from the periodic table can form ABO₃ perovskite structure, thus identifying the generalized trends in the physical and chemical properties of ABO₃ perovskite could reveal a way to estimate the similar properties for other perovskite oxides that have not yet been investigated. Towards understanding the generalized trends in the structural, thermochemical and reduction properties of ABO₃ perovskite with cations of mixed oxidation states at A- and B-sites, cubic BaMO₃ (M = Ti - Cu) perovskites have been investigated by first-principles calculations. Similar analysis has been accomplished for cations of identical oxidation states at A- and B-sites with cubic RCoO₃ (R = La, Ce, Nd, Sm). The structural and thermochemical properties of BaMO₃ and RCoO₃ perovskites have been characterized by first-principles calculations to reveal the influence of A-/B-site cations among the respective group of perovskites. In addition, the effect of oxygen vacancies and temperatures on reduction properties of BaMO₃ and RCoO₃ have been studied. The following conclusions have been drawn from the identified generalized trends in BaMO3 and RCoO₃. The formation energies of ABO₃ cations have a linear correlation with their atomic number of A-/B-site cations, whereas the formation energies have a significant inclination with the increasing atomic number of B-site cation for a given period in BaMO₃ series, but have minor increases with the increasing atomic number of A-site cation in RCoO₃ series. Similar to the formation energies, the oxygen vacancy formation energies increase to the greater extent with the increasing atomic number at B-site in defect-BaMO₃₋_δ, but to a lesser extent with the increasing atomic number at A-site in defect-RCoO₃₋_δ. However, the incorporation of thermal corrections into reduction free energies of defect-BaMO₃ and defect-RCoO₃₋_δ, led to a significant deviation from their reduction properties without thermal contributions. A comparison of calculated entropic contributions into oxygen-deficient- and defect-free-ABO₃ perovskites demonstrate that the oxygen vacancies have substantial effects on the entropic contributions, which ultimately influence the temperature-dependent reduction properties of BaMO₃ and RCoO₃. On the other hand, at an equivalent temperature the thermochemical properties such as entropy, specific heat and relative molar enthalpy, show a consistent increase from Ti to Cu at B-site in defect-free BaMO₃, while reverse trends are observed from La to Sm at A-site in RCoO₃. The physical origins of all these generalized trends are elucidated via the electronic structure analysis and phonon mode analysis. Rationalizing the role of A-site and B-site cations in BaMO₃ and RCoO₃ perovskite series reveal that, the B-site cation has dominant effects on the structural, thermochemical and reduction properties of ABO₃ perovskites.