| Perovskite-type oxides (ABO3) find a great variety of applications in solid oxide fuel cells (SOFCs), including solid electrolytes, cathodes and interconnects, which are closely related to the defect chemistry involved. Thermodynamic studies are needed to systematically understand the nature of the structure-property relations and provide guidance to predict and/or select proper materials. High temperature solution calorimetry in molten oxide solvents is a powerful tool and has been applied for several perovskite systems that have simple (undoped) and complex (doped) compositions.; LaBO3 perovskites (B = Al, Ga, Sc, In, Cr, Fe, Co, Ni) represent a group of excellent parent materials for electrolytes, cathodes, and interconnects in SOFCs. Their enthalpies of formation from oxides generally exhibit a relationship between stability and the major structural parameter for perovskites, the tolerance factor. As the tolerance factor deviates more from unity, the enthalpy of formation from oxides becomes less exothermic. This work verifies this general trend for A3+B3+O3 type perovskites, joining other two types, i.e., A1+B5+O 3 and A2+B4+O3.; In alkaline earth doped perovskites, though structural parameters are likely to continue affecting stability, defects, which are introduced upon doping, actually play a more profound role in defining energetic trends. In the newly developed electrolyte materials, Mg, Sr, and Ba-doped LaGaO 3 perovskites, oxygen vacancies are created to compensate the charge imbalance between dopant and host ions. Oxygen vacancies have a destabilization effect on the structure due to the partial disconnection of the corner-shared BO6 octahedral framework. On the other hand, they tend to order at the short-range scale, forming vacancy-dopant clusters, as evidenced by neutron diffraction.; In alkaline earth doped perovskites that contain transition metals, two charge compensation scenarios are possible: oxidation of the transition metal or creation of oxygen vacancies, which strongly depend on the oxygen partial pressure and temperature. Major defects are dopants, oxygen vacancies and electron holes. La1-xSrxFeO3-delta (cathode materials) and La1-x-AxCrO 3-delta (A = Ca or Sr) (interconnect materials) were chosen for study. The overall energetic trends in these complex perovskites are modeled by several competing factors: (1) the chemical nature of dopant ions (acidity/basicity), (2) coupled substitution involving oxygen vacancy formation, (3) redox of the transition metal in each sample series with a given dopant composition, and (4) short-range oxygen vacancy ordering in oxygen-deficient samples. |
