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

Here we show that the low-temperature oxygen-storage capability of RBaCo 4 O 7+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 RBaCo 4 O 7+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 (R III ), the oxygen-release temperature ( T R ) decreases while the temperature where the phase decomposes ( T D ) increases such that the “safety window” between T R and T D widens. On the other hand, the maximum amount of excess oxygen taken by the RBaCo 4 O 7+δ lattice in 1 atm O 2 decreases from δ ≈ 1.4 (for R = Dy) to ∼1.0 (for R = Lu) with decreasing r (R III ). Hence the optimum oxygen-storage characteristics are found about R = Tm in terms of r (R III ). We also discuss the crystal structures of the oxygen-annealed RBaCo 4 O 7+δ samples.