Mechanical property enhancements and amorphous thermal transports of ordered weberite-type RE₃Nb/TaO₇ high-entropy oxides

A₃BO₇-type (A = rare earth (RE), B = Nb or Ta) oxides have been studied as protective coating materials because of their low thermal conductivity; however, their hardness, toughness, and stiffness are insufficient, particularly for members with webeirte-type structures. In this work, we have synthesized two high-entropy oxides (HEOs) of weberite-type RE niobates/tantalates (RE₃Nb/TaO₇), i.e., (Nd_1/7Sm_1/7Eu_1/7Gd_1/7Dy_1/7Ho_1/7Er_1/7)₃NbO₇ (7HEOs-Nb) and (Nd_1/7Sm_1/7Eu_1/7Gd_1/7Dy_1/7Ho_1/7Er_1/7)₃(Nb_1/2Ta_1/2)O₇ (7HEOs-NbTa), to overcome the mechanical deficiencies. The short- and long-range ordered arrangements of RE cations in the A-site and Nb/Ta cations in the B-site were identified by the X-ray diffraction (XRD), scanning electron microscopy equipped with energy-dispersive spectrometry (EDS), and transmission electron microscopy. The enhancements in hardness (H = 9.4 GPa) and fracture toughness (K_IC = 2.0 MPa·m^1/2) were realized by grain refinement, solid solution strengthening, and high stiffness (K). The exceptional phase stability at 25−1500 ℃, amorphous thermal conductivity (k = 1.5−1.7 W·m⁻¹·K⁻¹ at 25−900 ℃), and high thermal expansion coefficients (TEC > 11.0×10⁻⁶ K⁻¹ at 1500 ℃) further supported their potential application as protective coating materials.