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

A new nine-principal cation high-entropy oxide of (Y 1/9 La 1/9 Nd 1/9 Sm 1/9 Yb 1/9 Lu 1/9 Eu 1/9 Gd 1/9 Ce 1/9 ) 2 O 3 (9HEO) was synthesized by co-precipitation method combined with conventional sintering at 1600 °C for 10 h. Two samples of 9HEO-(Ce 3+ ) and 9HEO-(Ce 4+ ) were obtained by adjusting the valence state of Ce element in 9HEO. The ratio of Ce 3+ to Ce 4+ in 9HEO-(Ce 3+ ) and 9HEO-(Ce 4+ ) 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 Ce 3+ /Ce 4+ in 9HEO-(Ce 3+ ) leads to the expansion of its lattice volume and the decrease of thermal conductivity (κ) compared with 9HEO-(Ce 4+ ). In addition, the κ values of these two samples are lower than those of Y 2 O 3 and (Eu 0.2 Er 0.2 Lu 0.2 Y 0.2 Yb 0.2 ) 2 O 3 , which have a bixbyite structure and lower entropy. 9HEO-(Ce 3+ ) and 9HEO-(Ce 4+ ) show higher oxygen barrier property compared with Y 2 O 3 -stabilised ZrO 2 .