Thermal/environmental barrier coatings (T/EBCs) are used to protect hot-section superalloys and/or ceramic matrix composite components from hot corrosion and oxidation; however, the majority of T/EBCs exhibit extremely high thermal and ionic conductivities. Here, we obtain a novel rare-earth tantalate with excellent oxygen and thermal insulation via a high-entropy strategy. The high-entropy component (8RE_1/8)TaO₄ (RE = rare earth), which is designed by large size disorder and mass disorder, has been reassembled into a stabilized monoclinic structure. (8RE_1/8)TaO₄ had 30.0%–31.1% and 59.2%–67.5% lower intrinsic thermal conductivity than single-RE RETaO₄ and 8(Y₂O₃–ZrO₂) 8YSZ at 1200 °C, respectively, and exhibited lower intrinsic thermal conductivity across the entire temperature range of 100–1200 °C. This is the result of strong scattering by the phonon–phonon, grain boundary, domain boundary, dislocation, and vacancy defects. The ionic conductivity of (8RE_1/8)TaO₄ is 3712–29,667 times lower than that of 8YSZ at 900 °C, benefiting from the strong Ta–O bonding strength, low concentration of mobile oxygen vacancies and severe lattice distortions that impede carrier transport. Moreover, (8RE_1/8)TaO₄ had superior high-temperature stability and excellent mechanical properties. Analysis of above results demonstrates that (8RE_1/8)TaO₄ is a promising candidate for T/EBCs.

Thermal barrier coating (TBC) materials can improve energy conversion efficiency and reduce fossil fuel use. Herein, novel rare earth tantalates RETaO₄, as promising candidates for TBCs, were reassembled into multi-component solid solutions with a monoclinic structure to further depress thermal conductivity via an entropy strategy. The formation mechanisms of oxygen vacancy defects, dislocations, and ferroelastic domains associated with the thermal conductivity are demonstrated by aberration-corrected scanning transmission electron microscopy. Compared to single-RE RETaO₄ and 8YSZ, the intrinsic thermal conductivity of (5RE_1/5)TaO₄ was decreased by 35%–47% and 57%–69% at 1200 ℃, respectively, which is likely attributed to multi-scale phonon scattering from Umklapp phonon–phonon, point defects, domain structures, and dislocations. ${\overline{r}}_{\text{RE}}^{3+}/r_{\text{Ta}}^{5+}$ and low-temperature thermal conductivity are negatively correlated, as are the ratio of elastic modulus to thermal conductivity (E/κ) and high-temperature thermal conductivity. Meanwhile, the high defects’ concentration and lattice distortion in high-entropy ceramics enhance the scattering of transverse-wave phonons and reduce the transverse-wave sound velocity, leading to a decrease in the thermal conductivity and Young’s modulus. In addition, 5HEC-1 has ultra-low thermal conductivity, moderate thermal expansion coefficients, and high hardness among three five-component high-entropy samples. Thus, 5HEC-1 with superior thermal barrier and mechanical properties can be used as promising thermal insulating materials.

Effective manipulations of thermal expansion and conductivity are significant for improving operational performances of protective coatings, thermoelectric, and radiators. This work uncovers determinant mechanisms of the thermal expansion and conductivity of symbiotic ScTaO₄/SmTaO₄ composites as thermal/environmental barrier coatings (T/EBCs), and we consider the effects of interface stress and thermal resistance. The weak bonding and interface stress among composite grains manipulate coefficient of thermal expansion (CTE) stretching from 6.4×10⁻⁶ to 10.7×10⁻⁶ K⁻¹ at 1300 ℃, which gets close to that of substrates in T/EBC systems. The multiscale effects, including phonon scattering at the interface, mitigation of the phonon speed (v_p), and lattice point defects, synergistically depress phonon thermal transports, and we estimate the proportions of different parts. The interface thermal resistance (R) reduces the thermal conductivity (k) by depressing phonon speed and scattering phonons because of different acoustic properties and weak bonding between symbiotic ScTaO₄ and SmTaO₄ ceramics in the composites. This study proves that CTE of tantalates can be artificially regulated to match those of different substrates to expand their applications, and the uncovered multiscale effects can be used to manipulate thermal transports of various materials.

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.

In this paper, (Gd_1−xY_x)TaO₄ ceramics have been fabricated by solid-phase synthesis reaction. Each sample was found to crystallize in a monoclinic phase by X-ray diffraction (XRD). The properties of (Gd_1−xY_x)TaO₄ were optimized by adjusting the ratio of Gd/Y. (Gd_1−xY_x)TaO₄ had a low high-temperature thermal conductivity (1.37–2.05 W∙m⁻¹∙K⁻¹), which was regulated by lattice imperfections. The phase transition temperature of the (Gd_1−xY_x)TaO₄ ceramics was higher than 1500 ℃. Moreover, the linear thermal expansion coefficients (TECs) were 10.5×10⁻⁶ K⁻¹ (1200 ℃), which was not inferior to yttria-stabilized zirconia (YSZ) (11×10⁻⁶ K⁻¹, 1200 ℃). (Gd_1−xY_x)TaO₄ had anisotropic thermal expansion. Therefore, controlling preferred orientation could minimize the TEC mismatch when (Gd_1−xY_x)TaO₄ coatings were deposited on different substrates as thermal barrier coatings (TBCs). Based on their excellent properties, it is believed that the (Gd_1−xY_x)TaO₄ ceramics will become the next generation of high-temperature thermal protective coatings.