Novel entropy-stabilized fluorite oxides with multifunctional properties

Development of new high-entropy oxides having configurational entropy dominating the phase stability has become a hot topic since the discovery of rock salt structure entropy-stabilized (ES)(MgCoNiCuZn)O in 2015. Herein, we report a set of novel entropy-stabilized fluorite oxides: Zr0.2Hf0.2Ce0.2Sn0.2Mn0.2O2-{\delta}, Zr0.2Hf0.2Ti0.2Mn0.2Ce0.2O2-{\delta}, Zr0.225Hf0.225Ti0.225Mn0.225Ce0.1O2-{\delta}, and Zr0.2Hf0.2Ti0.2Mn0.2Ce0.1Ta0.05Fe0.05O2-{\delta} synthesized using standard solid-state reaction. These compounds have been investigated using X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy techniques to discern their structural, microstructural, and chemical properties. The configurational-entropy dominated phase stability and hence the entropy stabilization of the compounds is confirmed by cyclic heat treatments. The mismatch in the ionic radii and oxidation state of the cations are the key factors in achieving a single-phase fluorite structure. Further, screening of physical properties including thermal conductivity, optical band gap, magnetic properties, and impedance spectroscopy is discussed. Thermal conductivity of 1.4-1.7 Wm-1K-1 is observed at 300 K and remains mostly invariant across a wide temperature range (300K-1073K), favorable for thermal barrier coating applications. These ES samples have an optical band gap of 1.6-1.8 eV, enabling light absorption across the visible spectrum and hence could be promising for photocatalytic applications. The impedance spectroscopy data of the entropy-stabilized samples reveal the presence of electronic contributions with small activation energy (0.3-0.4 eV) across a temperature range of 298K-423K. These observations in ES fluorite systems show potential for their multifunctional applications via further optimization and confirm the great chemical versatility of entropy-stabilized oxides.
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