Evolution behaviour of the lattice and thermal expansion of a high-entropy fluorite oxide (Zr0.2Ce0.2Hf0.2Y0.2Al0.2)O2-8 during heating and cooling in an inert atmosphere

High-entropy fluorite oxides (HEFOs) exhibit improved performance as promising thermal barrier coatings (TBCs). A proper thermal expansion coefficient (TEC) is required owing to the direct contact of TBCs with metal substrates. Herein, we study the evolution behaviour of the lattice and thermal expansion of a HEFO (Zr0.2Ce0.2Hf0.2Y0.2Al0.2)O2-8 during heating and cooling in an inert atmosphere. The HEFO was characterised as follows: (1) X-ray photoelectron spectroscopy to determine the valence states of the chemical elements, (2) X-ray diffraction to examine the crystal phase and calculate the TECs, and (3) thermogravimetry-differential scanning calorimetry to probe the temperature ranges of the reduction reactions. Results show that as the temperature increased from 300 to 800 degrees C, the nominal composition changed from(Zr0.2Hf0.2Ce0.2Y0.2Al0.2)O1.765 to (Zr0.2Hf0.2Ce0.2Y0.2Al0.2)O1.711, because of the partial valence reduction of Ce4+, Zr4+, and Hf 4+. Furthermore, 44.59% of Ce, 20.28% of Zr, and 23.93% of Hf presented +3 valence at 800 degrees C. Valence reduction occurred only during heating. Although suboxide cations were present, (Zr0.2Ce0.2Hf0.2Y0.2Al0.2)O2-8 maintained a high-entropy fluorite crystal structure. The TEC increased from 9.38 to 13.12 x 10-6 K-1 during heating and decreased from 10.71 to 9.15 x 10-6 K-1 during cooling. These results reveal the evolutionary trends of defective fluorite-type high-entropy oxide compositions with reduction reactions at elevated temperatures. This provides a new perspective on the relationship between the lattice, TEC, and chemical composition.