Synthesis and equilibrium oxygen nonstoichiometry of PrBaFe2O5+
Pavel Karen
Abstract
A double-cell perovskite PrBaFe2O5+w is synthesized and its equilibrium nonstoichiometry investigated after quenching from flowing gas of varied oxygen partial pressures and temperatures. Whereas w = 0 is reached at 856 °C in wet H2 of pH2O = 0.02 bar yielding pO2 = 4×10−21 bar, w = 0.75 obtains at 400 °C in O2. Below ~1000 °C, compositions with w ≥ 0.5 order into a Cmmm superstructure that fully converts back upon heating to 1020 °C. PrBaFe2O5 of all equivalent Fe2.5+ sites orders into Fe2+ and Fe3+ upon cooling through a first-order transition at 204 K, with a 2 K hysteresis, as investigated by powder X-ray diffraction and differential scanning calorimetry. The valence-mixed and charge-ordered structures are refined from synchrotron X-ray powder diffraction patterns. The nonstoichiometry w is expressed as a function of T and pO2 of the flowing gas in a formal donor-dopant defect model via least-squares fit of thermodynamic parameters for point-defect equilibria in the solid.