Rare-earth phosphates (REPO4) are regarded as one of the promising thermal/environmental barrier coating (T/EBC) materials for SiCf/SiC ceramic matrix composites (SiC-CMCs) owing to their excellent resistance to water vapor and CaO-MgO-Al2O3-SiO2 (CMAS). Nevertheless, relatively high thermal conductivity of REPO4 becomes the bottleneck for their practical application. In this work, novel xenotime-type high entropy (Dy1/7Ho1/7 Er1/7Tm1/7Yb1/7Lu1/7Y1/7)PO4 (HE (7RE1/7)PO4) has been designed and synthesized for the first time to solve this issue. HE (7RE1/7)PO4 with a homogeneous rare-earth element distribution exhibits high thermal stability up to 1750 °C and good chemical compatibility with SiO2 up to 1400 °C. In addition, the thermal expansion coefficient (TEC) of HE (7RE1/7)PO4(5.96 × 10 -6 /°C at room temperature to 900 °C) is close to that of SiC-CMCs. What's more, the thermal conductivity of HE (7RE1/7)PO4 (from 4.38 W•m -1 K -1 at 100 °C to 2.25 W•m -1 K -1 at 1300 °C) is significantly decreased compared to those of single-component REPO4 with the minimum value ranging from 9.90 to 4.76 W•m -1 •K -1 . These results suggest that HE (7RE1/7)PO4 has the potential to be applied as T/EBC material for SiC-CMCs in the future.

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