Microstructure and thermophysical properties of bixbyite-structured high-entropy oxides with nine-principal cations

A new nine-principal cation high-entropy oxide of (Y1/9La1/9Nd1/9Sm1/9Yb1/9Lu1/9Eu1/9Gd1/9Ce1/9)2O3 (9HEO) was synthesized by co-precipitation method combined with conventional sintering at 1600 °C for 10 h. Two samples of 9HEO-(Ce3+) and 9HEO-(Ce4+) were obtained by adjusting the valence state of Ce element in 9HEO. The ratio of Ce3+ to Ce4+ in 9HEO-(Ce3+) and 9HEO-(Ce4+) is approximately 2:1 and 14:15, respectively. These two samples have the same bixbyite structure and similar microscopic morphology. Their relative density, thermal expansion coefficient, hardness and Young’s modulus are close. The higher ratio of Ce3+/Ce4+ in 9HEO-(Ce3+) leads to the expansion of its lattice volume and the decrease of thermal conductivity (κ) compared with 9HEO-(Ce4+). In addition, the κ values of these two samples are lower than those of Y2O3 and (Eu0.2Er0.2Lu0.2Y0.2Yb0.2)2O3, which have a bixbyite structure and lower entropy. 9HEO-(Ce3+) and 9HEO-(Ce4+) show higher oxygen barrier property compared with Y2O3-stabilised ZrO2.