Oxygen nonstoichiometry in LnBaFe2O5+w across Ln = Nd, Sm, Gd

Abstract In double-cell perovskites LnBaFe2O5+w, only lanthanoids Ln ​= ​Pr to Gd support wide equilibrium nonstoichiometry w from 0 to >0.5. This study investigates point-defect equilibria behind the variation in w along Ln ​= ​Nd, Sm, Gd by quenching the oxides from flowing gas of controlled oxygen partial pressures and temperatures. The experimental data of w as a function of pO2 and T are least-squares fit via standard enthalpies and entropies of the defect-equilibrium reactions of formation for electronic-defect pair e′ and h• and for anion-Frenkel pair Oi′′ and vO••, and of the Fe2+ oxidation. NdBaFe2O5+w is closest to behaving as a pure undoped oxide at 1000 ​°C, where it has a single plateau in the w versus pO2 plot at the w ​= ​0.5 level. A second plateau, at w ​= ​0, emerges at lower temperatures. The smaller Sm and Gd increase the size difference versus Ba, and their defect equilibria show two plateaus even at 1000 ​°C. The temperature-dependent defect equilibria across this series are therefore approximated by a formal Ln3+ for Ba2+ donor-doping model, and the results are presented in Brouwer diagrams of defect concentrations. The decrease of the anion-Frenkel defect-equilibrium constant KaF due to decreasing ionic size from Nd3+ towards Gd3+ is magnified upon decrease in temperature that feeds the entropy of the anion-Frenkel disorder; at 400 ​°C, Ln ​= ​Nd has KaF ​= ​10−5.4, Sm 10−15.0 and Gd 10−24.3.