Potentiometric determination of the gibbs energies of formation of SrZrO and BaZrO

Abstract

The Gibbs free energies of formation of strontium and barium zirconates have been determined in the temperature range 960 to 1210 K using electrochemical cells incorporating the respective alkaline-earth fluoride single crystals as solid electrolytes. Pure strontium and barium monoxides were used in the reference electrodes. During measurements on barium zirconate, the oxygen partial pressure in the gas phase over the electrodes was maintained at a low value of 18.7 Pa to minimize the solubility of barium peroxide in the monoxide phase. Strontium zirconate was found to undergo a phase transition from orthorhombic perovskite (o) with space group Cmcm; D¹⁷_2h to tetragonal perovskite (t) having the space group 14/mcm;D ¹⁸_4h at 1123 (±10) K. Barium zirconate does not appear to undergo a phase transition in the temperature range of measurement. It has the cubic perovskite (c) structure. The standard free energies of formation of the zirconates from their component binary oxides AO (A = Sr, Ba) with rock salt (rs) and ZrO₂ with monoclinic (m) structures can be expressed by the following relations: SrO (rs) + ZrO₂ (m) → SrZrO₃ (o) ΔG± = -74,880 - 14.2T (±1200) J mol^-1 SrO (rs) + ZrO₂ (m) → SrZrO₃ (t) ΔG±= -73,645 - 15.37T (±200) J mol^-1 BaO (rs) + ZrO₂ (m) → BaZrO₄ (c) ΔG± = -127,760-1.79T (±1250) J mol^-1 The results of this study are in reasonable agreement with calorimetric measurements reported in the literature. Systematic trends in the stability of alkaline-earth zirconates having the stoichiometry AZrO₃ are discussed.