Synthesis and thermal behavior of rare-earth-niobate ceramics with fluorite structure

High-entropy ceramics have potential as thermal barrier coatings (TBCs) and a new way to reduce thermal conductivity. (Gd 0.25 Ho 0.25 Er 0.25 Yb 0.25 ) 3 NbO 7 (4RE 3 NbO 7 ) and (Ho 0.33 Er 0.33 Yb 0.33 ) 3 NbO 7 (3RE 3 NbO 7 ) rare-earth niobates with disordered defective fluorite structures were synthesized using the solid-phase method. They show prominent phase stability at 1400 °C and excellent chemical compatibility with Al 2 O 3 at 1200 °C. The SEM and EDS results show that the samples have a dense and uniform grain structure, and the average grain size is less than 2 μm. The uniform distributions of rare earth cations further indicate the successful realization of equimolar ratio doping. Compared to yttrium oxide-stabilized zirconia (YSZ) materials, both 4RE 3 NbO 7 and 3RE 3 NbO 7 ceramics have glass-like thermal conductivity, and the thermal conductivity of 4RE 3 NbO 7 ceramics (1.032–1.177W/m·K, 25–800 °C) was lower than that of 3RE 3 NbO 7 ceramics (1.058–1.183W/m·K, 25–800 °C). In addition, 4RE 3 NbO 7 and 3RE 3 NbO 7 exhibited comparable coefficients of thermal expansion (9.78–9.61 × 10 −6  K −1 , 1200 °C) and enhanced mechanical properties ( Hv  = 7.14–9.07 GPa, K IC  = 2.02–1.62 MPa m 1/2 ). The results of this study motivate a follow-up study on niobate high entropy ceramics with desirable properties.