Novel high-entropy fluorite oxides with superior phase stability up to 1723 K: Promising candidates for advanced thermal barrier coatings

Novel high-entropy fluorite oxides Hf_0.2Zr_0.2Ce_0.2Y_0.2RE_0.2O_2-δ (RE = La, Nd, Gd, Er) ceramics were designed and synthesized via a solid-state reaction route, exhibiting exceptional phase stability up to 1450 °C, which surpasses the operational limit of conventional yttria-stabilized zirconia. Structural and microstructural characterizations confirmed the formation of a single-phase defective fluorite structure after sintering at 1550 °C for 6 h. Upon aging at 1450 °C for 150 h, a minor secondary phase emerged in the La-doped variant, whereas the Er-doped composition maintained a phase-pure fluorite structure with minimal lattice contraction, demonstrating superior phase stability up to 1450 °C. X-ray photoelectron spectroscopy confirmed the mixed cationic valence states and the presence of oxygen vacancies, with the derived δ value aligning with a charge-compensated model. The lattice strain, quantified by Williamson–Hall analysis, decreased systematically with the ionic radius of the RE3+ dopant. Thermodynamic stability was ensured by a strongly negative Gibbs free energy driven by high configurational entropy. Furthermore, the Er-doped ceramic exhibited the highest sintering resistance, as evidenced by its superior porosity retention during thermal exposure. Therefore, these results establish Hf_0.2Zr_0.2Ce_0.2Y_0.2Er_0.2O_2-δ as a breakthrough candidate for next-generation thermal barrier coatings capable of operating beyond 1450 °C.