Phase evolution and thermophysical properties of high-entropy RE2(Y0.2Yb0.2Nb0.2Ta0.2Ce0.2)2O7 oxides

Pursuing novel thermal barrier–coating materials with lower thermal conductivity and high-temperature stability can simultaneously improve the working efficiency and service temperature of a gas turbine. In this study, a series of high-entropy RE 2 (Y 0.2 Yb 0.2 Nb 0.2 Ta 0.2 Ce 0.2 ) 2 O 7 (RE = La, Nd, Sm, Gd, Dy, and Er) oxides were prepared though solid-state reaction. Through tuning the rare-earth cations, an order–disorder transition occurs from certain partially ordered weberite structure ( C 222 1 ) to disordered defective fluorite structure ( Fm 3 ¯ $\bar{3}$ m ). All the high-entropy RE 2 (Y 0.2 Yb 0.2 Nb 0.2 Ta 0.2 Ce 0.2 ) 2 O 7 oxides possess low thermal conductivity in the range of 0.91–1.34 W m −1 K −1 at room temperature, which can be attributed to increased lattice anharmonicity and disorder, resulting in additional phonon scattering. Herein, we proved that the incorporation of heterovalent cations at B-sites in high-entropy A 2 B 2 O 7 crystals is an effective strategy to reduce the thermal conductivity without compromising the decrease of oxygen vacancy. Moreover, the high-entropy RE 2 (Y 0.2 Yb 0.2 Nb 0.2 Ta 0.2 Ce 0.2 ) 2 O 7 oxides show the relatively higher thermal expansion coefficients of 10.3–10.7 × 10 −6 °C −1 and excellent phase stability at elevated temperatures.