Lanthanide-substituted YMnO3+δ nanoparticles with the hexagonal phase, denoted as R0.25Y0.75MnO3+δ (R = Er, Dy, Tb, Gd, and Sm), have been successfully synthesized by the polymerized complex method. The substitutions did not largely affect the morphologies and specific surface area of the obtained R0.25Y0.75MnO3+δ nanoparticles. From the evaluation for the oxygen storage/release properties, the oxygen storage capacity (OSC) increased significantly by the Tb substitution, and the oxygen absorption/release rate strongly depended on the ion size of the substituted lanthanides. It was found that Tb4+ existed in Tb0.25Y0.75MnO3+δ after oxygen absorption, demonstrating that the remarkable increase in the OSC of the Tb-substituted sample was due to the oxidation of not only Mn3+ to Mn4+ but also Tb3+ to Tb4+. In addition, the unit cell volume increasing with the R ion size, which can lead to the promotion of the oxygen diffusion in the crystal structure, was the factor leading to the increase of the oxygen absorption rate. Especially, Sm0.25Y0.75MnO3+δ showed an excellent OSC of 3 + δ= 3.34 (the weight increase rate was 2.64 wt %) even under a rapid temperature swing rate of 20 °C/min.
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