Thermal Expansion Behavior of Ultrahigh-Entropy A2B2O7 Ceramics

Samples of ultrahigh-entropy ceramics with compositions represented by the formulas RE2Zr2O7, RE2Hf2O7, RE2Sn2O7, RE2Ti2O7, RE2Ce2O7, and RE2(Zr,Hf,Sn,Ti,Ce)2O7, where RE = [La, Sm, Nd, Gd, Dy, Yb, Y, Er, Eu, Tb, Ho, Tm, Lu], were synthesized and investigated. The feasibility of solid-state synthesis of the ultrahigh-entropy RE2(Zr,Hf,Sn,Ti,Ce)2O7 phase was demonstrated for the first time. RE2Ce2O7 with a fluorite-derived structure exhibited the highest coefficient of linear thermal expansion (CLTE). Its average CLTE over 20–1400°C was 12.88 × 10–6 K–1, and its CLTE at 1400°C was approximately 16.5 × 10–6 K–1. At 1000°C, the CLTE was approximately 14.5 × 10–6 K–1, which is close to that of nickel-based superalloys at this temperature. The study provides new information on the effect of the composition of ultrahigh-entropy A2B2O7 phases on their CLTE. The data do not provide sufficient evidence to conclude that an increase in the configurational entropy of mixing in the B sublattice has a direct effect on the CLTE. At the same time, they demonstrate the potential for deliberately selecting the compositions of high-entropy A2B2O7 phases to obtain ceramic materials with CLTE–temperature dependences required for specific applications.