Solution combustion synthesis of Ce-free high entropy fluorite oxides: Formation, oxygen vacancy and long-term thermal stability

High-entropy oxides have gained more attention in recent years due to their excellent performance. In this work, five new Ce-free high entropy fluorite oxides ((Zr0.2Ti0.2Y0.2Nd0.2Sc0.2)O2-δ, (Zr0.2Ti0.2Y0.2La0.2Sc0.2)O2-δ, (Zr0.2Ti0.2La0.2Nd0.2Sc0.2)O2-δ, (Zr0.2Ti0.2Y0.2Nd0.2La0.2)O2-δ and (Zr0.2Ti0.2Dy0.2Nd0.2La0.2)O2-δ) were successfully synthesized by solution combustion synthesis with glycine as fuel. Their formation, oxygen vacancy, and long-term thermal stability were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) techniques. XRD results show that single fluorite phase was observed in (Zr0.2Ti0.2Y0.2La0.2Sc0.2)O2-δ, (Zr0.2Ti0.2Y0.2Nd0.2La0.2)O2-δ and (Zr0.2Ti0.2Dy0.2Nd0.2La0.2)O2-δ samples when sintering at 300, 600 and 900 °C, respectively, but (Zr0.2Ti0.2Y0.2Nd0.2Sc0.2)O2-δ sample with single-phase fluorite structure can only obtain by sintering at 900 °C and 1200 °C. For (Zr0.2Ti0.2La0.2Nd0.2Sc0.2)O2-δ sample, single fluorite phase can be obtained at 300 °C using calculated fuel and at 300, 600, and 900 °C with using excess fuel. In addition, all the samples may have lower crystallinity due to excess glycine. XPS results indicate that factors of sintering temperature, fuel ratio and cationic radii strongly affect the generation of oxygen vacancy, especially for the sintering temperature. Morphology of the high entropy fluorite oxides presents in sponge-like microstructure. Superior long-term stability at high temperature was found in samples (Zr0.2Ti0.2Y0.2Nd0.2Sc0.2)O2-δ and (Zr0.2Ti0.2Y0.2Nd0.2La0.2)O2-δ. The findings in this work may have great potential applications in oxygen-deficient catalyst and high temperature catalyst.