New class of high-entropy rare-earth niobates with high thermal expansion and oxygen insulation

Tailoring the structure and properties of materials using the high-entropy (HE) effect is of significant interest in the fields of environmental and thermal barrier coatings (TBCs). In this work, a new class of dense HE rare-earth niobates was successfully prepared by a solid-phase reaction method, including (Sm_1/5Dy_1/5Ho_1/5Er_1/5Yb_1/5)NbO₄ (5HERN), (Sm_1/6Dy_1/6Ho_1/6Er_1/6Yb_1/6Lu_1/6)NbO₄ (6HERN), (Sm_1/7Dy_1/7Ho_1/7Er_1/7Yb_1/7Lu_1/7Gd_1/7)NbO₄ (7HERN), and (Sm_1/8Dy_1/8Ho_1/8Er_1/8Yb_1/8Lu_1/8Gd_1/8Tm_1/8)NbO₄ (8HERN), along with eight single rare-earth niobates (RENbO₄, RE = Sm, Dy, Ho, Er, Yb, Lu, Gd, and Tm). X-ray diffraction analysis showed that 5–8HERN are single-phase solid solutions with a monoclinic structure (space group C12/c1). The thermal expansion coefficients of 7HERN and 8HERN exceed 11 × 10⁻⁶ K⁻¹ at 1200°C and are much higher than those of the RENbO₄ compositions (10.13–10.74 × 10⁻⁶ K⁻¹) and other some HE rare-earth oxides (10.27–10.87 × 10⁻⁶ K⁻¹). Importantly, 5–8HERN have lower oxygen-ion conductivity and higher activation energy than yttrium-stabilized zirconia (YSZ) and the RENbO₄ compositions. The oxygen-ion conductivity of 5HERN (7.52 × 10⁻⁷ S cm⁻¹, 900°C) was 10⁵ times lower than that of YSZ (0.01 S cm⁻¹, 750°C). The hardness of 5–8HERN is ∼7.81–8.46 GPa and these compositions have low intrinsic lattice thermal conductivity at high temperature (1.28–1.69 W m⁻¹ K⁻¹ at 900°C). The mechanism by which the HE effect improved the material properties was elucidated. Young's modulus, hardness, thermal expansion coefficient, and intrinsic lattice thermal conductivity are linearly related to the mass, size, and distortion degree of samples. In contrast, the oxygen-ion conductivity depends on both the degrees of disorder and distortion and the oxygen-ion vacancy concentration. Based on their overall performance, especially their high thermal expansion coefficients and excellent oxygen-barrier performance, HE rare-earth niobates show potential for further development as TBC materials.     

Mechanical properties,oxygen barrier property,and chemical stability of RE3NbO7 for thermal barrier coating

Rare earth niobate (RE₃NbO₇, RE = Dy, Y, Er, Yb) ceramics have shown extremely low thermal conductivity but remain questionable in high temperature thermal barrier coating (TBC) applications with high thermal, mechanical, and chemical loads. Herein, we comprehensively characterize the properties of rare earth niobates, including mechanical properties, oxygen barrier properties, chemical stability, etc. It is found that the oxygen conductivities of the rare earth niobates are three orders of magnitude lower than 7wt.% yttria-stabilized zirconia (YSZ), indicating a remarkable oxygen barrier property to avoid oxidation of underlying metallic components. The corrosion resistance of rare earth niobate against calcium-magnesium-aluminum silicate (CMAS) is also significantly better than that of YSZ. Together with the extremely low thermal conductivity, the rare earth niobates exhibit a combination of excellent high temperature properties, which may become a promising candidate material of high temperature TBC of next generation gas turbines.     

Features of crystal structures and thermo-mechanical properties of weberites RE3NbO7 (RE=La,Nd, Sm,Eu, Gd) ceramics

The features of crystal structures, thermo-mechanical properties and their dominant mechanisms of weberites RE₃NbO₇ were studied as high-temperature oxides. We concentrated on connections between structures and thermo-mechanical properties, the influences of bond lengths, lattice distortion degrees and microstructures on these properties were estimated. The shortening of bond length and increment of bonding strength would lead to the increase of mechanical properties. The Vickers hardness (4.5-7.8 GPa) and toughness (0.5-1.6 MPa·m^1/2) of weberites RE₃NbO₇ are enhanced by grain refinement and increment of bond strength, while crystal structures, bond lengths, and lattice distortion degrees influenced their Young's modulus (100-170 GPa). Nano-indentation was applied to test the influence of microstructures on modulus and hardness. The dominant mechanisms for mechanical properties and thermal conductivity were proposed, which was conducive to properties tailoring and engineering applications of weberites RE₃NbO₇ oxides.     

Highly porous (La1/5Nd1/5Sm1/5Gd1/5Yb1/5)2Zr2O7 ceramics with ultra-low thermal conductivity

The high-entropy rare earth zirconate (La_1/5Nd_1/5Sm_1/5Gd_1/5Yb_1/5)₂Zr₂O₇ porous ceramics ((5RE_1/5)₂Zr₂O₇ PCs) were prepared using a foam-gel casting-freeze drying method combined with segmented calcination process. The results of SEM, TEM, and XRD analyses of the (5RE_1/5)₂Zr₂O₇ PCs indicated the formation of a defective fluorite crystal structure, with the rare earth elements homogeneously distributed. Meanwhile, the as-prepared (5RE_1/5)₂Zr₂O₇ PCs exhibited high porosity, low bulk density, low thermal conductivity, and relatively high compressive strength. Moreover, the high-temperature thermal conductivity of the samples was evaluated, and the results showed that the (5RE_1/5)₂Zr₂O₇ PCs maintain a thermal conductivity of 0.150 ± 0.002 W m^?1 K^?1 even at 1000 °C. The strategy used in this paper can be extended to the synthesis of other high-entropy porous ceramics with high porosity and low thermal conductivity, which is suitable for applications as thermal insulation materials.     

Novel high entropy (Y0.2Sm0.2Gd0.2Er0.2Ho0.2)3NbO7 nanofibers with ultralow thermal conductivity

Thermal insulation materials can provide thermal protection in extreme environments. Ceramic fibers have played an important role in the thermal protection field at high temperatures due to the advantages of low density, high strength, low thermal conductivity, and excellent thermal stability. In this work, high entropy (Y_0.2Sm_0.2Gd_0.2Er_0.2Ho_0.2)₃NbO₇ (5RE₃NbO₇) nanofibers were fabricated by electrospinning and subsequent calcination. Defective fluorite-structured 5RE₃NbO₇ nanofibers were obtained when heated at 900°C. The research indicates that 5RE₃NbO₇ nanofiber based porous ceramics present an ultralow thermal conductivity (0.0992 W/m·K, porosity of 78.18%), good thermal stability, and high spectral reflectance, which establish the foundation for applications in thermal insulation.     

Multiphonon scattering mechanisms to limit thermal conductivity in weberite RE3NbO7: A case study of (La1-xGdx)3NbO7 ceramics

The management of thermal conductivity is of significant scientific interest, particularly for thermal barrier coatings (TBCs). Multifarious strategies have been used to regulate heat transportation, but it is hard to achieve limit thermal conductivity at elevated temperatures. A systematical investigation of weberite (La_1-xGd_x)₃NbO₇ was thus performed, and multiphonon scattering mechanisms were introduced to achieve limit thermal conductivity (0.92 W m^?1 K^?1). Phonon point defect scattering process accounted for thermal conductivity reduction at low temperatures. Additionally, lattice softening strongly contributed to the reduction of high-temperature thermal conductivity, and solid and stiff chemical bonds were beneficial for inhibiting thermal radiative conductivity. A novel strategy was presented to modify thermal transportation property of weberite RE₃NbO₇ ceramics. Also, the hardness, toughness, and modulus were improved to promote engineering applications of weberite RE₃NbO₇. This study also illuminates novel paths for thermal management and mechanical properties manipulation of TBCs, thermoelectric materials, and microelectronics.     

High‐Temperature Properties and Ferroelastic Phase Transitions in Rare‐Earth Niobates (LnNbO4)

Phase transition and high‐temperature properties of rare‐earth niobates (LnNbO₄, where Ln = La, Dy and Y) were studied in situ at high temperatures using powder X‐ray diffraction and thermal analysis methods. These materials undergo a reversible, pure ferroelastic phase transition from a monoclinic (S.G. I2/a) phase at low temperatures to a tetragonal (S.G. I4₁/a) phase at high temperatures. While the size of the rare‐earth cation is identified as the key parameter, which determines the transition temperature in these materials, it is the niobium cation which defines the mechanism. Based on detailed crystallographic analysis, it was concluded that only distortion of the NbO₄ tetrahedra is associated with the ferroelastic transition in the rare‐earth niobates, and no change in coordination of Nb⁵⁺ cation. The distorted NbO₄ tetrahedron, it is proposed, is energetically more stable than a regular tetrahedron (in tetragonal symmetry) due to decrease in the average Nb–O bond distance. The distortion is affected by the movement of Nb⁵⁺ cation along the monoclinic b‐axis (tetragonal c‐axis before transition), and is in opposite directions in alternate layers parallel to the (010). The net effect on transition is a shear parallel to the monoclinic [100] and a contraction along the monoclinic b‐axis. In addition, anisotropic thermal expansion properties and specific heat capacity changes accompanying the transition in the studied rare‐earth niobate systems are also discussed.     

Thermal properties of (Zr0.2Ce0.2Hf0.2Y0.2RE0.2)O1.8 (RE= La,Nd and Sm) high entropy ceramics for thermal barrier materials

The high-entropy ceramic materials (Zr_0.25Ce_0.25Hf_0.25Y_0.25)O_1.875 (H-0) and (Zr_0.2Ce_0.2Hf_0.2Y_0.2RE_0.2)O_1.8 (H-RE) (RE = La, Nd and Sm) with fluorite structure and homogeneous element distribution were prepared. With fluorite structure, fine grain size and high density, the H-0 and H-RE ceramics displayed low thermal conductivity, suitable thermal expansion coefficient, high hardness and fracture toughness. The effect of La, Nd and Sm on the mechanical, heat conductivity and heat expansion properties of high entropy ceramics were discussed. The single-phase high-entropy ceramic materials in this work are very suitable for application as thermal barrier materials.     

Spark plasma sintering of (5RE.2)Ta3O9 high-entropy ceramics and their thermal properties

Six rare-earth tantalate high-entropy ceramics of (5RE_.2)Ta₃O₉ (RE represents any five elements selected from La, Ce, Nd, Sm, Eu, Gd) were designed and prepared by spark plasma sintering process at 1400°C in this study. The (5RE_.2)Ta₃O₉ ceramics only consist of a single-phase solid solution with perovskite structure. Their relative densities are all above 90%, and the average grain size is in the range of 1.47–2.92 μm. The thermal conductivity of (5RE_.2)Ta₃O₉ ceramics is in 2.24–1.90 W m⁻¹ K⁻¹ (25°C–500°C), which is much lower than that of yttria-stabilized zirconia. In six samples, (La_.2Nd_.2Sm_.2Gd_.2Eu_.2)Ta₃O₉ possesses a thermal conductivity of 1.90 W m⁻¹ K⁻¹, a thermal expansion coefficient of 3.47 × 10⁻⁶ K⁻¹ (500°C), a Vickers hardness of about 7.33 GPa, and a fracture toughness of about 5.20 MPa m^1/2, which are suitable for its application as thermal barrier coatings.     

The thermo-mechanical properties and ferroelastic phase transition of RENbO4 (RE = Y,La, Nd,Sm, Gd,Dy, Yb) ceramics

In this work, RENbO₄ (RE = Y, La, Nd, Sm, Gd, Dy, Yb) ceramics with low density, low Young's modulus, low thermal conductivity, and high thermal expansion have been systematically investigated, the excellent thermo-mechanical properties indicate that RENbO₄ ceramics possess the potential as the new generation of thermal barrier coatings (TBCs) materials. X-ray diffraction and Raman spectroscopy phase structure identification reveal that all dense bulk specimens obtained by high-temperature solid-state reaction belonged to the monoclinic (m) phase with C12/c1 space group. The ferroelastic domains are detected in the specimens, revealing the ferroelastic transformation between tetragonal (t) and monoclinic (m) phases of RENbO₄ ceramics. The Young's modulus and hardness of the RENbO₄ ceramics measured by the NanoBlitz 3D nanoindentation method are discussed in details, and the lower Young's modulus (60-170 GPa) and higher hardness (the maximum value reaches 11.48 GPa) indicating that higher resistance of RENbO₄ ceramics to failure and damage. Lower thermal conductivity (1.42-2.21 W [m k]⁻¹ at 500°C-900°C) and lower density (5.330-7.400 g/cm³) than other typical TBCs materials give RENbO₄ ceramics the unique advantage of being new TBCs materials. Meanwhile, the thermal expansion coefficients of RENbO₄ ceramics reach 9.8-11.6 × 10⁻⁶ k⁻¹ and are comparable or higher than other typical TBCs materials. According to the first-order derivative of the thermal expansion rate, the temperature of the ferroelastic transformation of RENbO₄ ceramics can be observed.     

Potential thermal barrier coating materials: RE3NbO7 (RE=La, Nd,Sm, Eu,Gd, Dy) ceramics

In this work, RE₃NbO₇ ceramics are synthesized via solid‐state reaction and the phase structure is characterized by X‐ray diffraction and Raman spectroscopy. The relationship between crystal structure and thermophysical properties is determined. Except Sm₃NbO₇, each RE₃NbO₇ exhibits excellent high‐temperature phase stability. The thermal expansion coefficients increase with the decreasing RE³⁺ ionic radius, which depends on the decreasing crystal lattice energy and the maximum value reaches 11.0 × 10^?6 K^?1 at 1200°C. The minimum thermal conductivity of RE₃NbO₇ reaches 1.0 W m^?1 K^?1 and the glass‐like thermal conductivity of Dy₃NbO₇ is dominant by the high concentration of oxygen vacancy and the local structural order. The outstanding thermophysical properties pronounce that RE₃NbO₇ ceramics are potential thermal barrier coating materials.     

High-entropy rare earth titanates with low thermal conductivity designed by lattice distortion

High-entropy single-phase rare earth titanates (RE_0.2Gd_0.2Ho_0.2Er_0.2Yb_0.2)₂Ti₂O₇ (RE = Sm, Y, Lu) were designed and synthesized successfully, in which their lattice distortion was quantitatively described by mass disorder and size disorder. It is worth mentioning that (Y_0.2Gd_0.2Ho_0.2Er_0.2Yb_0.2)₂Ti₂O₇ could obtain the low thermal conductivity (1.51 W·m⁻¹·K⁻¹, 1500°C), high thermal expansion coefficient (average, 11.69×10⁻⁶ K⁻¹, RT ∼1500°C) and excellent high-temperature stability. In addition, the relationship between the microstructure and thermal transport behaviors has been studied at the atomic scale. Due to the disorder of A-site ions, severe lattice distortion occurred in specific crystal planes, and the large mass difference between Y³⁺ and other RE³⁺ further causes mass fluctuation and results in lower thermal conductivity. Compared with YSZ, the high-entropy rare earth titanate (Y_0.2Gd_0.2Ho_0.2Er_0.2Yb_0.2)₂Ti₂O₇ has lower thermal conductivity, higher thermal expansion coefficient, and excellent high-temperature stability, which has great potential for application in the thermal protection field.     

Rare-earth-tantalate high-entropy ceramics with sluggish grain growth and low thermal conductivity

A series of rare-earth-tantalate high-entropy ceramics ((5RE_0.2)Ta₃O₉, where RE = five elements chosen from La, Ce, Nd, Sm, Eu and Gd) were prepared by conventional sintering in air at 1500 ^°C for 10 h. The (5RE_0.2)Ta₃O₉ high-entropy ceramics exhibit an orthogonal structure and sluggish grain growth. No phase transition occurs in the test temperature of 25–1200 ^°C. The thermal conductivities of all (5RE_0.2)Ta₃O₉ ceramics are in the range of 1.14–1.98 W m^?1 K^?1 at a test temperature of 25–500 ^°C, approximately half of that of YSZ. The sample of (Gd_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)Ta₃O₉ exhibits a low glass-like thermal conductivity with a value of 1.14 W m^?1 K^?1 at 25 ^°C. The thermal expansion coefficient of (5RE_0.2)Ta₃O₉ ceramics ranges from 5.6 × 10^?6 to 7.8 × 10^?6 K^?1 at 25–800 ^°C, and their fracture toughness is high (3.09–6.78 MPa·m^1/2). The results above show that (5RE_0.2)Ta₃O₉ ceramics could be a promising candidate for thermal barrier coatings.     

Excellent CMAS resistance of a newly developed equiatomic high entropy (Dy1/4Ho1/4Tm1/4Yb1/4)2Si2O7 ceramic pyrosilicate

In this work, novel equiatomic high entropy (Dy_1/4Ho_1/4Tm_1/4Yb_1/4)₂Si₂O₇ or (4RE_1/4)₂Si₂O₇ ceramic pyrosilicate was fabricated through a single solid solution method to use as environmental barrier coating. The SEM analysis of high entropy powders shows the homogenous mixing and XRD proves the formation of single β-phase after milling and sintering. The coefficient of thermal expansion was reported as (2.3–4.8 × 10⁻⁶ K⁻¹) from 400 K⁻¹ to 1723 K⁻¹. The ultra-low thermal diffusivity (0.4 mm² s⁻¹) and thermal conductivity (0.8 W/m°C) were reported at 1500 °C for this novel HE ceramic disilicate. The as fabricated (4RE_1/4)₂Si₂O₇ pyrosilicate shows an excellent CMAS resistant for even up to 48 h and negligible amount of Ca is able to penetrate in the substrate. Rare earth disilicate species with intermediate radii such as Tm³⁺ helps in maintaining phase stability along with passive element Yb³⁺ of smaller radii which also protect the interface from severe CMAS attack. However, the rare earth species with larger radii such as Dy³⁺ and Ho³⁺ actively take part in apatite formation leading to reduced corrosion activity of CMAS melt by changing its composition. This result confirms the application of (4RE_1/4)₂Si₂O₇ as a potential candidate to be used as protecting coating material in harsh combustion environments.     

Rare-earth high-entropy aluminate-toughened-zirconate dual-phase composite ceramics for advanced thermal barrier coatings

Superb toughening is achieved by incorporating a secondary ferroelastic phase in high-entropy rare-earth zirconate 5RE₂Zr₂O₇ (HZ). Here, we report an enhancement of 64% in fracture toughness through the addition of 30mol% high-entropy rare-earth aluminate 5REAlO₃ (HA) to the HZ matrix (30HA). The aforementioned rare-earth elements RE are La, Sm, Eu, Gd, and Yb. The present dual-phase composite ceramic 30HA has a large fracture toughness of 2.77 ± 0.14 MPa m^1/2, along with excellent high-temperature phase stability, resulting in good usage for potential thermal barrier coating applications. Particularly, the fracture toughness of the dual-phase composite ceramics at first increases to a maximum and then drops suddenly, as the mole fraction of HA increases from 0 to 50%. A clear definition of fitting parameters and their physical significance is provided for a better interpretation of the experimental data. The present toughening mechanism sheds light on microstructure engineering in high-entropy ceramics for excellent mechanical properties.     

High-entropy rare-earth disilicate (Lu0.2Yb0.2Er0.2Tm0.2Sc0.2)2Si2O7: A potential environmental barrier coating material

A new high-entropy ceramic (Lu_0.2Yb_0.2Er_0.2Tm_0.2Sc_0.2)₂Si₂O₇ ((5RE_0.2)₂Si₂O₇) was proposed as a potential environmental barrier coating (EBC) material for ceramics matrix composites in this work. Experimental results showed that the (5RE_0.2)₂Si₂O₇ synthesized by solid-phase sintering was a monoclinic solid solution and had good phase stability proved by no obvious absorption/exothermic peak in the DSC curve from room temperature to 1400 °C. It performed a lower coefficient of thermal expansion (2.08 ×10^?6-4.03 ×10^?6 °C^?1) and thermal conductivity (1.76–2.99 W?m^?1?°C^?1) compared with the five single principal RE₂Si₂O₇. In water vapor corrosion tests, (5RE_0.2)₂Si₂O₇ also exhibited better water vapor corrosion resistance attributed to the multiple doping effects. The weight loss was only 3.1831 × 10^?5 g?cm^?2 after 200 h corrosion at 1500 °C, which was lower than that of each single principal RE₂Si₂O₇. Therefore, (5RE_0.2)₂Si₂O₇ could be regarded as a remarkable candidate for EBCs.     

High-entropy (Ho0.2Y0.2Dy0.2Gd0.2Eu0.2)2Ti2O7/TiO2 composites with excellent mechanical and thermal properties

High-performance ceramics with low thermal conductivity, high mechanical properties, and idea thermal expansion coefficients have important applications in fields such as turbine blades and automotive engines. Currently, the thermal conductivity of ceramics has been significantly reduced by local doping/substitution or further high-entropy reconfiguration of the composition, but the mechanical properties, especially the fracture toughness, are insufficient and still need to be improved. In this work, based on the high-entropy titanate pyrochlore, TiO₂ was introduced for composite toughening and the high-entropy (Ho_0.2Y_0.2Dy_0.2Gd_0.2Eu_0.2)₂Ti₂O₇-xTiO₂ (x = 0, 0.2, 0.4, 1.0 and 2.0) composites with high hardness (16.17 GPa), Young's modulus (289.3 GPa) and fracture toughness (3.612 MPa·m^0.5), low thermal conductivity (1.22 W·m⁻¹·K⁻¹), and thermal expansion coefficients close to the substrate material (9.5 ×10⁻⁶/K) were successfully prepared by the solidification method. The fracture toughness of the composite toughened sample is 2.25 times higher than that before toughening, which exceeds most of the current low-thermal conductivity ceramics.     

Novel (Sm0.2Lu0.2Yb0.2Y0.2Dy0.2)3TaO7 high-entropy ceramic for thermal barrier coatings

A novel (Sm_0.2Lu_0.2Dy_0.2Yb_0.2Y_0.2)₃TaO₇ (SLT-5RE_0.2) oxide with a single-fluorite structure was synthesized via an optimized sol-gel and sintering method, and its crystal structure, mechanical and thermophysical properties were investigated. The results indicate that the calcined nanoscale powder is of high crystallinity, and bulk sample is of a uniform elemental distribution. Compared to YSZ (6–8 wt.% Y₂O₃ partially stabilized by ZrO₂), SLT-5RE_0.2 exhibits lower Young's modulus, less mean acoustic velocity, and higher Vickers microhardness. Owing to the strengthened anharmonic vibration and phonon scattering, SLT-5RE_0.2 exhibits low thermal conductivity (1.107 W K^?1·m^?1, 900 °C). The high thermal expansion coefficient (11.3 × 10^?6 K^?1, 1200 °C) of SLT-5RE_0.2 ceramic can be ascribed to the reduced lattice energy and ionic spacing as well as the cocktail effect of high-entropy ceramics. The excellent mechanical and thermophysical properties, and excellent phase steadiness during the whole testing temperature cope, indicate that SLT-5RE_0.2 high-entropy ceramic can be a candidate material for thermal barrier coatings.     

Dendrite microstructure formation and enhanced toughness in high-entropy REAlO3–RE2Zr2O7 eutectic oxide

In this study, complex GdAlO₃–Gd₂Zr₂O₇ and high-entropy REAlO₃–RE₂Zr₂O₇ (RE = Nd, Sm, Gd, Eu, and Dy) composites with an equiaxed dendrite structure at eutectic composition are successfully fabricated using a gas levitation containerless solidification method. The unique microstructure of the composites is characterized, and the evolution process of the dendritic structure is explained. The formation of dendrites at eutectic composition is attributed to the rapid cooling induced by the shutoff of lasers and the homogeneous temperature field and nucleation achieved through gas levitation. The GdAlO₃–Gd₂Zr₂O₇ and high-entropy oxide composites exhibit enhanced fracture toughness compared to the bulk samples fabricated by solid-state sintering methods. The fracture toughness increases by 44% for GdAlO₃–Gd₂Zr₂O₇ bulk sample and 34% for high-entropy REAlO₃–RE₂Zr₂O₇ bulk sample, which can be attributed to the complicated interfaces introduced by the equiaxed dendritic microstructure and the high thermal mismatch stress between two phases. Additionally, the high-entropy REAlO₃–RE₂Zr₂O₇ oxides exhibit excellent high-temperature stability, with no significant change in dendritic microstructure or fracture toughness even after holding at 1573 K for 100 h. These findings suggest the potential of high-entropy eutectic oxide ceramics with dendrite microstructure for advanced engineering applications.     

High-entropy ferroelastic rare-earth tantalite ceramic: (Y0.2Ce0.2Sm0.2Gd0.2Dy0.2)TaO4

In this study, high-entropy rare-earth tantalate ceramics (Y_0.2Ce_0.2Sm_0.2Gd_0.2Dy_0.2)TaO₄ ((5RE_0.2)TaO₄) have been successfully fabricated. The possibility of formation of (5RE_0.2)TaO₄ was verified via first-principles calculations. In addition, the phase structure, ferroelastic toughening mechanism, thermophysical, and mechanical properties were systematically investigated. The (5RE_0.2)TaO₄ ceramics have lower phonon thermal conductivity (1.2–2.6 W·m^–1·K^–1) in the entire temperature range than that of RETaO₄ and YSZ. (5RE_0.2)TaO₄ has a higher fracture toughness and lower brittleness index than YSZ. The thermal expansion coefficients of (5RE_0.2)TaO₄ are as high as 10.3 × 10^-6 K^–1 at 1200°C and Young's modulus is 66–189 GPa, and thus, (5RE_0.2)TaO₄ possesses great potential for application in thermal barrier coatings (TBCs).