Ti‐Rich Nonstoichiometric BaTiO ₃ : III, High‐Temperature Thermodynamic and X‐Ray Diffraction Measurements *

A thermodynamic and high‐temperature X‐ray diffraction study was performed on polycrystalline nonstoichiometric BaTiO 3 . The oxygen composition was measured from 940° to 1215°C at O 2 partial pressures between 10 −10 and 10 −21 atm by a thermogravimetric technique. The isothermal weight change data exhibit a two‐slope behavior. The data at the lower pressures indicate that the maximum observable weight change is influenced by impurities, whereas the data at the higher pressures appear to exhibit native nonstoichiometric behavior. Values of ΔH̄ o2 and ΔS̄ o2 were calculated for the higher O 2 partial pressure region; ΔH̄ o2 is consistent with the O vacancy formation energies obtained by Long and Blumenthal from electrical conductivity measurements. High‐temperature X‐ray measurements indicate that the cubic structure is stable in the range investigated. Lattice parameters and coefficients of expansion were obtained from 200° to 1100°C in pure O 2 ; lattice parameters of reduced BaTiO 3 at 1100°C are also presented.