Thermophysical Properties and CMAS Corrosion of Xenotime‐Type High‐Entropy Rare‐Earth Phosphates

Xenotime‐type high‐entropy rare‐earth phosphates (HE‐REPO 4 ) are promising ceramics for environmental barrier coatings (EBCs), owing to their remarkable tunability of thermophysical properties and corrosion resistance. In this study, three HE‐REPO 4 (Dy 0.2 Ho 0.2 Er 0.2 Tm 0.2 Yb 0.2 )PO 4 (HE‐REPO 4 ‐1), (Dy 0.2 Ho 0.2 Er 0.2 Yb 0.2 Lu 0.2 )PO 4 (HE‐REPO 4 ‐2), and (Dy 0.2 Er 0.2 Tm 0.2 Yb 0.2 Lu 0.2 )PO 4 (HE‐REPO 4 ‐3) are synthesized via coprecipitation followed by fast hot‐pressing sintering. All samples have a xenotime‐type structure and uniform elemental distribution. With lattice contraction and increased bond strength of the samples, the elastic modulus progressively increases, the thermal expansion coefficient (TEC) decreases, and the thermal stability enhances. Among the three samples, HE‐REPO 4 ‐3 displays the best thermal stability, the highest elastic modulus (171 GPa), and the lowest TEC (7.01 × 10 −6 K −1 from 25 to 1200°C). It also exhibits low thermal conductivity and favorable oxygen barrier property. Furthermore, HE‐REPO 4 ‐3 forms a reaction layer primarily composed of Ca 8 MgRE(PO 4 ) 7 , which effectively suppresses Si diffusion during calcium–magnesium–aluminosilicate corrosion at 1300°C. This research provides a new insight into property of xenotime‐type HE‐REPO 4 for EBC applications.