Reduced high-temperature thermal conductivity and infrared transmittance of rare-earth phosphates via high-entropy design

Reduced thermal conductivity and infrared transmittance at elevated temperatures have emerged as key design principles for thermal barrier coating materials. To achieve lower thermal conductivity, the high-entropy concept has been widely applied in ceramic systems. In this study, a series of high-entropy rare-earth phosphates with monazite structure are synthesized via a high-temperature solid-state method. At 1473 K, the thermal conductivity of high-entropy rare-earth phosphates decreases by over 20 % compared to single-component LaPO₄. Simultaneously, under the combined effects of the high-entropy composition, pores, and grain boundary, the infrared transmittance of the high-entropy compositions is significantly reduced. Notably, under the influence of pores and chemical disorder caused by high entropy, the average transmittance of high-entropy rare-earth phosphates at 200–2000 nm and 2.5–4.5 μm is lower than 14 % and 0.89 % respectively, which is much lower than that of LaPO₄ at 25.3 % and 20.9 %. The Young's modulus of high-entropy rare-earth phosphates is 187–234 GPa, and their hardness is 4.23–4.76 GPa. Therefore, monazite-type high-entropy rare-earth phosphates show great potential as next-generation thermal barrier coating materials.