Novel entropy-stabilized fluorite oxides with multifunctional properties

Abstract

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 (MgCoNiCuZn)O in 2015. Herein, we report a set of novel entropy-stabilized fluorite oxides: Zr_0.2Hf_0.2Ce_0.2Sn_0.2Mn_0.2O_2−δ, Zr_0.2Hf_0.2Ti_0.2Mn_0.2Ce_0.2O_2−δ, Zr_0.225Hf_0.225Ti_0.225Mn_0.225Ce_0.1O_2−δ, and Zr_0.2Hf_0.2Ti_0.2Mn_0.2Ce_0.1Ta_0.05Fe_0.05O_2−δ synthesized using standard solid-state reactions. 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 are confirmed by cyclic heat treatments. The mismatch between the ionic radii and oxidation states of the cations is the key factor in achieving a single-phase fluorite structure. Furthermore, screening of physical properties, including thermal conductivity and optical band gap, and magnetic properties and impedance spectroscopy studies are discussed. A thermal conductivity of 1.4–1.7 W m⁻¹ K⁻¹ is observed at 300 K and remains invariant across a wide temperature range (300–1073 K), favorable for thermal barrier coating applications. These entropy-stabilized 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 a small activation energy (0.3–0.4 eV) across a temperature range of 298–423 K. These observations in entropy-stabilized fluorite systems show potential for their multifunctional applications via further optimization and confirm the great chemical versatility of entropy-stabilized oxides.