Oxygen Storage Capacity and Phase Stability of Variously Substituted YBaCo₄O_7+δ

Here, we investigate various approaches of chemical substitution to enhance the phase stability of the oxygen storage material candidate, YBaCo 4 O 7+δ, under oxygen-containing conditions. From the results of Sr-for-Ba and R-for-Y (where R is a rare-earth element) substitutions, it can be concluded that the smaller the unit-cell volume, the less the phase absorbs oxygen but the higher the phase-decomposition temperature. Studies on the YBa(Co 0.95 M 0.05 ) 4 O 7+δ samples with M = Mn, Fe, Ni, Cu, Zn, Al, and Ga, then reveal that partial substitution of Co with Al, Ga, or Zn efficiently suppresses the phase decomposition. Most importantly, through cosubstitution with Al and Ga, the phase decomposition can be completely avoided for the YBa(Co 0.85 Al 0.075 Ga 0.075 ) 4 O 7+δ sample without markedly sacrificing the oxygen storage capability. Finally, it is shown that, among the RBa(Co 0.85 Al 0.075 Ga 0.075 ) 4 O 7+δ samples with R = Y, Dy, Ho, and Lu, only those with R = Y and Lu remain stable under oxidizing conditions at high temperatures. Possible factors affecting the phase stability of YBaCo 4 O 7+δ and its derivatives are discussed.