New class of high‐entropy rare‐earth niobates with high thermal expansion and oxygen insulation

Abstract Tailoring the structure and properties of materials using the high‐entropy (HE) effect is of significant interest in the fields of environmental and thermal barrier coatings (TBCs). In this work, a new class of dense HE rare‐earth niobates was successfully prepared by a solid‐phase reaction method, including (Sm 1/5 Dy 1/5 Ho 1/5 Er 1/5 Yb 1/5 )NbO 4 (5HERN), (Sm 1/6 Dy 1/6 Ho 1/6 Er 1/6 Yb 1/6 Lu 1/6 )NbO 4 (6HERN), (Sm 1/7 Dy 1/7 Ho 1/7 Er 1/7 Yb 1/7 Lu 1/7 Gd 1/7 )NbO 4 (7HERN), and (Sm 1/8 Dy 1/8 Ho 1/8 Er 1/8 Yb 1/8 Lu 1/8 Gd 1/8 Tm 1/8 )NbO 4 (8HERN), along with eight single rare‐earth niobates (RENbO 4 , RE = Sm, Dy, Ho, Er, Yb, Lu, Gd, and Tm). X‐ray diffraction analysis showed that 5–8HERN are single‐phase solid solutions with a monoclinic structure (space group C 12/ c 1). The thermal expansion coefficients of 7HERN and 8HERN exceed 11 × 10 −6 K −1 at 1200°C and are much higher than those of the RENbO 4 compositions (10.13–10.74 × 10 −6 K −1 ) and other some HE rare‐earth oxides (10.27–10.87 × 10 −6 K −1 ). Importantly, 5–8HERN have lower oxygen‐ion conductivity and higher activation energy than yttrium‐stabilized zirconia (YSZ) and the RENbO 4 compositions. The oxygen‐ion conductivity of 5HERN (7.52 × 10 −7 S cm −1 , 900°C) was 10 5 times lower than that of YSZ (0.01 S cm −1 , 750°C). The hardness of 5–8HERN is ∼7.81–8.46 GPa and these compositions have low intrinsic lattice thermal conductivity at high temperature (1.28–1.69 W m −1 K −1 at 900°C). The mechanism by which the HE effect improved the material properties was elucidated. Young's modulus, hardness, thermal expansion coefficient, and intrinsic lattice thermal conductivity are linearly related to the mass, size, and distortion degree of samples. In contrast, the oxygen‐ion conductivity depends on both the degrees of disorder and distortion and the oxygen‐ion vacancy concentration. Based on their overall performance, especially their high thermal expansion coefficients and excellent oxygen‐barrier performance, HE rare‐earth niobates show potential for further development as TBC materials.