Xenotime-type high-entropy (Dy_1/7Ho_1/7Er_1/7Tm_1/7Yb _1/7Lu_1/7Y_1/7)PO₄: A promising thermal/environmental barrier coating material for SiC_f/SiC ceramic matrix composites

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, a relatively high thermal conductivity (kappa) of the REPO4 becomes the bottleneck for their practical applications. In this work, novel xenotime-type high-entropy (Dy1/7Ho1/7Er1/7Tm1/7Yb1/7Lu1/7Y1/7)PO4 (HE (7RE(1/7))PO4) has been designed and synthesized for the first time to solve this issue. HE (7RE(1/7))PO4 with a homogeneous rare-earth element distribution exhibits high thermal stability up to 1750 degrees C and good chemical compatibility with SiO2 up to 1400 degrees C. In addition, the thermal expansion coefficient (TEC) of HE (7RE(1/7))PO4 (5.96x10(-6) degrees C-1 from room temperature (RT) to 900 degrees C) is close to that of the SiC-CMCs. What is more, the thermal conductivities of HE (7RE(1/7))PO4 (from 4.38 W.m(-1).K-1 at 100 degrees C to 2.25 W.m(-1).K-1 at 1300.) are 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 (7RE(1/7))PO4 has the potential to be applied as the T/EBC materials for the SiC-CMCs in the future.