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

A high entropy fluorite oxide (Hf0.2Zr0.2Ce0.2Y0.2Yb0.2)O2-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 GHEFO 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.