Orthorhombic to tetragonal polymorphic transformation of YTa3O9 and its inhibition through the design of high-entropy (Y0.2La0.2Ce0.2Nd0.2Gd0.2)Ta3O9

To explore the mechanism of phase transformation, YTa 3 O 9 was prepared by an integrated one-step synthesis and sintering method at 1500 °C using Y 2 O 3 and Ta 2 O 5 powders as starting materials. High-temperature XRD patterns and Raman spectra showed that a phase transformation from orthorhombic to tetragonal took place in YTa 3 O 9 through the bond length and angle changes at 300–400 °C, which caused a thermal conductivity rise. To inhibit the phase transformation, a high-entropy (Y 0.2 La 0.2 Ce 0.2 Nd 0.2 Gd 0.2 )Ta 3 O 9 (HE RETa 3 O 9 ) was designed and synthesized at 1550 °C using the integrated solid-state synthesis and sintering method. In tetragonal structured HE RETa 3 O 9 , phase transformation was inhibited by the high-entropy effect. Furthermore, HE RETa 3 O 9 exhibited low thermal conductivity, and its tendency to increase with temperature was alleviated (1.69 W/m·K, 1073 K). Good phase stability, low thermal conductivity and comparable fracture toughness to YSZ make HE RETa 3 O 9 promising as a new thermal barrier coating material.