Energetics of compounds (A²⁺B⁴⁺O₃) with the perovskite structure

The heats of formation from oxides for perovskite compounds, ABO₃ (A = Ca, Sr, Ba, Pb, Pb, Cd, B = Ti, Zr), were determined by high-temperature solution calorimetry using an alkali borate solvent. The values obtained agree with the results of earlier HF solution calorimetry, when available. The heat of formation from the oxides generally becomes more negative the closer the tolerance factor for that perovskite is to unity. The heat of formation can be divided into two contributions, that from changes in electrostatic (Madelung) energy, which becomes more negative as the tolerance factor decreases from 1 to ∼0.8, reflecting a smaller volume for the perovskite, and that from other energy terms, mainly repulsive interactions. The change in these terms is generally positive, reflecting greater repulsion in the perovskite structure than in the binary oxides, and increases with increasing deviation of the tolerance factor from unity. These energy terms also correlate with the apparent contraction of the octahedral sublattice relative to that calculated for ideal geometry. From the heats of solution of the polymorphs of CdTiO₃ and BaTiO₃, the heat of transition between ilmenite- and perovskite-type CdTiO₃ and that between perovskite- and high-temperature hexagonal-type BaTiO₃ were calculated to be 8.2 ± 2.3 and 5.6 ± 3.4 kJ/mole.