R-Site Substitution Effect on the Oxygen-Storage Capability of RBaCo₄O_7+δ

Abstract

Here we show that the low-temperature oxygen-storage capability of RBaCo4O7+δ (R = Y, Dy, Ho, Er, Tm, Yb, and Lu) depends on the size of the rare earth constituent R. For all these R constituents the RBaCo4O7+δ phase can be charged and discharged with large amounts of oxygen in a narrow temperature range below 400 °C. With decreasing ionic radius, r(RIII), the oxygen-release temperature (TR) decreases while the temperature where the phase decomposes (TD) increases such that the “safety window” between TR and TD widens. On the other hand, the maximum amount of excess oxygen taken by the RBaCo4O7+δ lattice in 1 atm O2 decreases from δ ≈ 1.4 (for R = Dy) to ∼1.0 (for R = Lu) with decreasing r(RIII). Hence the optimum oxygen-storage characteristics are found about R = Tm in terms of r(RIII). We also discuss the crystal structures of the oxygen-annealed RBaCo4O7+δ samples.