Superior phase stability of high entropy oxide ceramic in a wide temperature range

A high entropy fluorite oxide (Hf_0.2Zr_0.2Ce_0.2Y_0.2Yb_0.2)O_2-δ (HEFO) was investigated to reveal its ultra-wide-temperature phase stability in air. The HEFO exhibited a single-phase fluorite structure, and its lattice constant, a, was evaluated to be 0.527 ± 0.02 nm from room temperature (RT) to > 2573 K. No precipitation of single-component oxides, such as HfO₂, ZrO₂, Y₂O₃ and CeO₂, was experimentally observed. Since both Gibbs free energy (G) change of possible precipitation reactions and the second derivative of G_HEFO with respect to all single-component oxides were positive, the HEFO exhibits no precipitation and decomposition at temperatures above 1700 K. The sluggish dynamics, which was verified by a long-term annealing experiment at 1473 K for 175 h showing a few Yb₂O₃ precipitation, contributed to the single-phase solution nature of HEFO below 1700 K. Such a superior phase stability in ultrawide temperature range help accelerate the engineering application of HEFO to be thermal structural components.