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A series of high entropy niobates with cation at A-site varying in size disorder and mass disorder is designed. High entropy niobates are all monoclinic structures with uniform rare-earth element distribution, but different space groups, which are closely related to the average radius at A-site. High entropy niobates possess excellent sintering resistance and ultra-low thermal conductivity due to stronger phonon-point defect scattering and shorter phonon lifetime. The effect of high entropy on average grain size and thermal conductivity is inversely proportional to size disorder. Moreover, the radiative heat transfer of (Dy 0.2 Ho 0.2 Er 0.2 Y 0.2 Yb 0.2)NbO 4 is substantially reduced due to the co-doping of active multi-component Dy3+, Er3+, and Yb3+, which broadens the range of intrinsic absorption band and improves lattice absorption derived from lattice distortion. Besides, the (La 0.2 Nd 0.2 Sm 0.2 Eu 0.2 Y 0.2)NbO 4 exhibits an ultra-high thermal expansion coefficient (12.34 ×10−6 K−1, 673 °C). The excellent thermophysical properties imply that high entropy niobates are more promising thermal barrier coating materials. [ABSTRACT FROM AUTHOR]