Thermodynamic analysis and crystal-chemical modeling of the defect structure of calcium iron garnets

This paper presents detailed analysis of the phase equilibria and thermodynamic stability of CaO, Fe2O3, Gd2O3, Nd2O3, Pr2O3, ZrO2, NiO, and ThO2, and defect formation during high-temperature synthesis of gadolinium (yttrium) calcium and zirconium calcium iron garnets doped with praseodymium, neodymium, nickel, cerium, thorium, and other oxides. The origin of point defects in each garnet is discussed. Crystal-chemical modeling indicates that all calcium-containing garnets are nonstoichiometric, but differ in the origin of non-stoichiometry, delta. The values of delta are shown to correlate with the activation energy for oxygen-ion conduction determined by emf measurements on solid electrolytes. The mechanisms of formation of different garnets from binary oxides are compared.