Rare-earth tantalates and niobates (RE₃TaO₇ and RE₃NbO₇) have been considered as promising candidate thermal barrier coating (TBC) materials in next generation gas-turbine engines due to their ultra-low thermal conductivity and better thermal stability than yttria-stabilized zirconia (YSZ). However, the low Vickers hardness and toughness are the main shortcomings of RE₃TaO₇ and RE₃NbO₇ that limit their applications as TBC materials. To increase the hardness, high entropy (Y_1/3Yb_1/3Er_1/3)₃TaO₇, (Y_1/3Yb_1/3Er_1/3)₃NbO₇, and (Sm_1/6Eu_1/6Y_1/6Yb_1/6Lu_1/6Er_1/6)₃(Nb_1/2Ta_1/2)O₇ are designed and synthesized in this study. These high entropy ceramics exhibit high Vickers hardness (10.9–12.0 GPa), close thermal expansion coefficients to that of single-principal-component RE₃TaO₇ and RE₃NbO₇ (7.9×10⁻⁶-10.8×10⁻⁶C⁻¹ at room temperature), good phase stability, and good chemical compatibility with thermally grown Al₂O₃, which make them promising for applications as candidate TBC materials.

The critical requirements for the environmental barrier coating (EBC) materials of silicon-based ceramic matrix composites (CMCs) include good tolerance to harsh environments, thermal expansion matches with the interlayer mullite, good high-temperature phase stability, and low thermal conductivity. Cuspidine-structured rare-earth aluminates RE₄Al₂O₉ have been considered as candidates of EBCs for their superior mechanical and thermal properties, but the phase transition at high temperatures is a notable drawback of these materials. To suppress the phase transition and improve the phase stability, a novel cuspidine-structured rare-earth aluminate solid solution (Nd_0.2Sm_0.2Eu_0.2Y_0.2Yb_0.2)₄Al₂O₉ was designed and successfully synthesized inspired by entropy stabilization effect of high-entropy ceramics (HECs). The as-synthesized HE (Nd_0.2Sm_0.2Eu_0.2Y_0.2Yb_0.2)₄Al₂O₉ exhibits a close thermal expansion coefficient (6.96×10^-6 K^-1 at 300–1473 K) to that of mullite, good phase stability from 300 to 1473 K, and low thermal conductivity (1.50 W·^m–1·K^–1 at room temperature). In addition, strong anisotropic thermal expansion has been observed compared to Y₄Al₂O₉ and Yb₄Al₂O₉. The mechanism for low thermal conductivity is attributed to the lattice distortion and mass difference of the constituent atoms, and the anisotropic thermal expansion is due to the anisotropic chemical bonding enhanced by the large size rare-earth cations.

Synthesis of an exceptionally highly efficient NiS–CdS hybrid photocatalyst for H₂ evolution.