A new series of rare‐earth‐cerate high‐entropy ceramics with compositions of (La0.2Nd0.2Sm0.2Gd0.2Dy0.2)2Ce2O7 (HEC1), (La0.2Nd0.2Sm0.2Gd0.2Yb0.2)2Ce2O7 (HEC2), (La0.2Nd0.2Sm0.2Yb0.2Dy0.2)2Ce2O7 (HEC3), (La0.2Nd0.2Yb0.2Gd0.2Dy0.2)2Ce2O7 (HEC4), (La0.2Yb0.2Sm0.2Gd0.2Dy0.2)2Ce2O7 (HEC5) as well as a single component of Nd2Ce2O7 are fabricated via sintering the corresponding sol–gel‐derived powders at 1600°C for 10 h. HEC1–5 samples exhibit a single‐cerate phase with fluorite structure and high configurational entropy. Compared with Nd2Ce2O7, HEC1–5 samples have a lower grain growth rate owing to the sluggish diffusion effect. The chemical compositional uniformity of HEC1–5 as well as Nd2Ce2O7 does not apparently change after annealing at 1500°C for different time intervals (1, 6, 12, and 18 h). Compared with 8YSZ, HEC1–5 samples display the decreased thermal conductivity and increased thermal expansion coefficient. The lattice size disorder parameter of HEC1–5 is negatively related to the thermal conductivity in 26–450°C. Furthermore, HEC1–5 and Nd2Ce2O7 exhibit lower oxygen‐ion conductivity, meaning an increased resistance to oxygen diffusion.

This work reported the grain growth kinetics of high‐entropy diboride (HEB) and HEB‐SiC ceramics containing 10, 20, and 30 vol% SiC during heat treatment at 1800°C. The coarsening of HEB phase occurred in the four kinds of ceramics during heat treatment, especially in HEB ceramics. The kinetic analysis showed that the grain growth of HEB phase in HEB and HEB‐SiC ceramics is controlled by interface‐controlled kinetics and grain‐boundary pinning, respectively. The growth rate constant of HEB grains is lower than ZrB2, which is related to the low grain‐boundary energy and the sluggish diffusion effect in dynamics of high‐entropy materials. The growth rate of matrix phase in HEB‐SiC ceramics is similar to that in ZrB2–SiC ceramics, indicating that the pinning effect of the SiC second‐phase played the dominant role in inhibiting the grain growth of the high‐entropy matrix phase and disguised the sluggish diffusion effect. This study reveals that the grain growth inhibition through sluggish diffusion effect in a high‐entropy ceramic system may be magnified by the possible existence of segregated second‐phase particles located at the grain boundaries.