High-entropy (Nd_0.2Sm_0.2Eu_0.2Y_0.2Yb_0.2)₄Al₂O₉ with good high temperature stability, low thermal conductivity, and anisotropic thermal expansivity

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.