Thermodynamics of nanoscale calcium and strontium titanate perovskites

The surface enthalpies of nanocrystalline CaTiO₃ and SrTiO ₃ perovskites were determined using high-temperature oxide melt solution calorimetry in conjunction with water adsorption calorimetry. The nanocrystalline samples were synthesized by a hydrothermal method and characterized using powder X-ray diffraction, FTIR spectroscopy, and Brunauer-Emmett-Teller surface area measurements. The integral heats of water vapor adsorption on the surfaces of nanocrystalline CaTiO₃ and SrTiO₃ are -78.63 ± 4.71 kJ/mol and -69.97 ± 4.43 kJ/mol, respectively. The energies of the hydrous and anhydrous surfaces are 2.49 ± 0.12 J/m² and 2.79 ± 0.13 J/m² for CaTiO₃ and 2.55 ± 0.15 J/m² and 2.85 ± 0.15 J/m² for SrTiO₃, respectively. The stability of the perovskite compounds in this study is discussed according to the lattice energy and tolerance factor approach. The energetics of different perovskites suggest that the formation enthalpy becomes more exothermic and surface energy increases with an increase in ionic radius of the "A" site cation (Ca, Sr, and Ba), or with the tolerance factor. PbTiO₃ shows a lower surface energy, weaker water binding, and a less exothermic enthalpy of formation than the alkaline-earth perovskites.