Structures, and Thermophysical Properties Characterizations of (La1-xHox)3NbO7 Solid Solutions as Thermal Barrier Coatings

A sequence of (La_1-xHo_x)₃NbO₇ solid solutions were fabricated in this work, which were studied as candidate for thermal insulation materials. The lattices were identified via XRD, when SEM and EDS were used to characterize the microstructures and element distributions. The results showed that the highest modulus, hardness, and toughness of (La_1-xHo_x)₃NbO₇ were 196 GPa, 9.2 GPa, and 1.6 MPa m^1/2, respectively, and they accorded with the mechanical property requirements. Also, a low thermal conductivity (1.06 W m⁻¹ K⁻¹) and high thermal expansion coefficients (TECs: 11.3 × 10⁻⁶ K⁻¹) were simultaneously realized in (La_3/6Ho_3/6)₃NbO₇, at high temperatures. No phase transition was detected up to 1,200°C, which proved their good high-temperature lattice stability. The intense anharmonic lattice vibrations might contribute to the outstanding thermal properties of (La_1-xHo_x)₃NbO₇ ceramics. The suitable modulus, high hardness, low thermal conductivity, and high TECs of (La_1-xHo_x)₃NbO₇ solid solutions proclaimed that they were exceptional thermal insulation ceramics.

Introduction

Diverse oxide ceramics are studied as candidate for thermal barrier coatings (TBCs), which are used to reduce the temperatures of superalloy parts (Padture et al., 2002; Clarke, 2003; Chen et al., 2011; Zhang et al., 2016; Zhang et al., 2017; Chen et al., 2018a; Liu et al., 2018; Chen et al., 2019a; Chen and Feng, 2019; Liu et al., 2019; Zhang et al., 2019; Zhou et al., 2019; Zhang et al., 2020; Zhou et al., 2020; Zhou et al., 2021). Good thermal insulations and a long service life are researched in TBCs, which require that oxides should possess low thermal conductivity, high thermal expansion coefficients (TECs), high hardness, and so on (Padture et al., 2002; Clarke, 2003; Yang et al., 2016; Zhang et al., 2016; Chen et al., 2018a; Zhou et al., 2019). Yttria-stabilized zirconia (YSZ) ceramics are widely researched and applied as TBCs, and they have good comprehensive thermophysical properties, including high hardness and fracture toughness, comparatively low thermal conductivity, and low density (Padture et al., 2002; Clarke, 2003; Chen et al., 2011). Since the phase transition of YSZ reduces its application temperature below 1,200°C, new materials are researched to replace it. La₂Ce₂O₇, LnMgAl₁₁O₁₉, La₂Zr₂O₇, and other oxides are extensively researched as TBCs to replace YSZ, and low thermal conductivity, high hardness, and good lattice stability are found in some oxides (Wang et al., 2012; Yang et al., 2016; Zhang et al., 2016; Zhang et al., 2017; Chen et al., 2019a; Liu et al., 2019). Among various ceramics, the ones with pyrochlore-/fluorite-type structures have lower thermal conductivity and better phase stability than YSZ. Hence, oxides having pyrochlore-/fluorite-type structures are broadly studied, and thermophysical property improvements are essential for various materials (Ma et al., 2008; Wang et al., 2012; Wright et al., 2020; Adjaoud and Albe, 2021; Irum et al., 2021; Wang et al., 2021; Xiang et al., 2021; Zhang et al., 2021; Zhu et al., 2021).

The excellent thermophysical properties of pyrochlore-/fluorite-type RE₂Zr₂O₇ ceramics include low thermal conductivity (1–2 W · m⁻¹ · K⁻¹, 25–1,000°C), high hardness (9–12 GPa), and good high-temperature phase stability (Wang et al., 2012; Yang et al., 2016; Liu et al., 2019; Wright et al., 2020; Xiang et al., 2021). One shortage of RE₂Zr₂O₇ is the inadequate fracture toughness (∼1 MPa m^1/2), which stops it from being used as TBCs. Therefore, other oxides possessing pyrochlore-/fluorite-type structures are studied as TBCs. A₃BO₇-type rare earth niobates (RE₃NbO₇) have been studied in the past several years, and they have orthorhombic weberite and cubic fluorite lattices, which rely on RE³⁺ ionic size (Ma et al., 2008; Zhang et al., 2017; Yang et al., 2019; Chen et al., 2021a; Chen et al., 2021b; Xiang et al., 2021). The crystal structures of RE₃NbO₇ derive from fluorite A₄O₈, when the four A⁴⁺ ions are substituted by three RE³⁺ and one Nb⁵⁺ cation, and one oxygen vacancy is produced in the lattice to maintain charge balance (Wang et al., 2012; Chen et al., 2018a; Chen et al., 2021a; Chen et al., 2021b). RE₃NbO₇ is a weberite lattice when RE³⁺ ionic size is longer than 1.03 Å, and others are cubic fluorite with RE³⁺ size less than 1.03 Å. There are many similarities between crystal structures of RE₃NbO₇ and RE₂Zr₂O₇ ceramics. The lattices of RE₃NbO₇ and RE₂Zr₂O₇ originate from prototype fluorite A₄O₈; high concentration of oxygen vacancy (12.5%), [REO] polyhedrons, and excellent lattice stability are found in these two-type ceramics. Therefore, RE₃NbO₇ and RE₂Zr₂O₇ may have similar thermophysical properties.

Different structural and thermophysical property features are documented in weberite and fluorite RE₃NbO₇ oxides, and they display good thermal insulation performance (Chen et al., 2018b; Yang et al., 2019; Chen et al., 2021a; Chen et al., 2021b). The advantages of RE₃NbO₇ as TBCs include ultralow thermal conductivity, high TECs, and competitive modulus. The features of weberite and fluorite RE₃NbO₇ have been reported, and the differences of structures and properties between these two types of niobates are distinct. To study the variations of properties of RE₃NbO₇ along with the changes of lattice structures and to provide good understanding on these series niobates, the relevant study should be performed. The lowest thermal conductivity of weberite niobates is detected in La₃NbO₇, and the minimum thermal conductivity of fluorite niobates is found in Ho₃NbO₇ oxides (Chen et al., 2018b; Chen et al., 2019b; Yang et al., 2019; Chen et al., 2021a; Chen et al., 2021b). Dense (La_1-xHo_x)₃NbO₇ solid solutions are fabricated in this work, and their lattices, thermal conductivity, hardness, and other properties are characterized. The variation trends of structures and thermophysical properties of (La_1-xHo_x)₃NbO₇ are documented, which enables scholars to study their inner dominated mechanism from a good aspect.

Experiments and Methods

The bulk samples of (La_1-xHo_x)₃NbO₇ (x = 0/6, 1/6, 2/6, 3/6, 4/6, 5/6, and 6/6) solid solutions were fabricated via a high-temperature sintering process using La₂O₃, Ho₂O₃, and Nb₂O₅ powders (purity beyond 99.9% and a particle size smaller than 20 µm). The RE₂O₃ and Nb₂O₅ powders were weighted and then mixed in alcohol via a ball mill at a speed of 300r/min for 10 h, and then they were heated at 90°C for 6 h to remove alcohol. Approximately 1–2 g dry mixtures were taken and compressed via a tablet press under a condition of 300 MPa for 2 min. The green bodies of (La_1-xHo_x)₃NbO₇ were finally sintered at 1,600–1700°C for 5–10 h to obtain their dense bulk samples. The lattice structures were identified via XRD (X-ray diffraction), and their surface microstructures were observed via a SEM (scanning electron microscope) and an EDS (energy dispersive spectrometer). The microhardness (H) and toughness (K_IC) were tested by an indentation test (DHV-1000Z-CCD, China), when Young’s modulus (E) and mean acoustic velocity (V_a) were obtained by using an ultrasonic pulser/receiver equipment (UMS-100, France). The hardness and toughness of oxides were affected by bonding strengths, which were reflected via Young’s modulus and Debye temperature (Ɵ_D) (Anderson, 1963; Schlichting et al., 2001):

${\theta }_D=\frac{h}{k_B}{\left(\frac{3m}{4\pi V}\right)}^{1/3}{V}_a,(1)$

where h, k_B, and m represented Plank’s constant, the Boltzmann constant, and the lattice weight, respectively. As for thermal properties, TECs were measured via a TMA (thermal mechanical analysis), and thermal diffusivity (λ) was tested via a LFA to calculate thermal conductivity (k) (Schlichting et al., 2001):

$k=\frac{\lambda ·\rho ·C_P}{1-4\varphi /3},(2)$

where ρ, C_P, and ϕ represented density, heat capacity, and porosity, respectively. Porosity was tested via an Archimedes balance, the weight (w₁) of sample was measured first, and then the sample was put in water to obtain its weight (w₂) under the action of water buoyancy; finally, the sample was taken out from water and wiped up to measure its weight (w₃). The porosity was calculated via a formula: ϕ = 1−w₁/(w₃−w₂). Thermal conductivity was calculated via phonons, and the corresponding phonon mean free path (l) was obtained via thermal diffusivity and average acoustic velocity (Kingery, 1955; Anderson, 1963; Kittle, 1996; Schlichting et al., 2001; Qu et al., 2012; Zhao et al., 2016; Wang et al., 2018; Ye et al., 2019):

$l=3\lambda /V_a,(3)$

where V_a was a constant, and it indicated that the temperature dependence of l relied on λ (Bruls et al., 2005). More details about sample preparations, structures identifications, and properties measurements could be found in our previous articles (Chen et al., 2018b; Chen et al., 2019b; Chen et al., 2021a).

Results and Discussions

Structures Analysis

In the current work, the variations of properties along with the changes of lattices in (La_1-xHo_x)₃NbO₇ oxides are studied. The normalized XRD peaks of prepared specimens are depicted in Figure 1, and it is found that three types of lattices are formed. When r_RE is 1.060–1.160 Å, the prepared samples (0/6 ≤ x ≤ 4/6) are in an orthorhombic phase with the space group (SG) of Cmcm; when r_RE is 1.039 Å, the prepared sample (x = 5/6) is in an orthorhombic phase with the SG of C222₁; and Ho₃NbO₇ is a cubic fluorite phase with $\text{Fm}\overline{3}\text{m}$SG. The main XRD peaks shift to the right along with the decline of La content displayed in Figure 1B, which indicates the shrinkage of the lattice. Table 1 exhibits the exact information of lattices of (La_1-xHo_x)₃NbO₇ solid solutions. Some certain relationships can be constructed between cubic and orthorhombic (La_1-xHo_x)₃NbO₇ lattice parameters. In cubic fluorite Ho₃NbO₇, it is found that a_c = b_c = c_c, when the lattice constants of orthorhombic (La_1-xHo_x)₃NbO₇ are derived from the relationships of a_o ≈ 2a_c, b_o ≈ 2^0.5b_c, and c_o ≈ 2^0.5c_c. The shrinkages of lattices and increments of unit cell mass lead to their theoretical density increases with the increasing Ho content. Also, Table 1 exhibits that the porosity of each sample is 1–4%, and dense bulk samples are made in this work. The impacts of porosity on thermophysical properties are not taken into account.

FIGURE 1

FIGURE 1. The normalized XRD peaks of (La_1-xHo_x)₃NbO₇ (x = 0/6, 1/6, 2/6, 3/6, 4/6, 5/6, and 6/6) ceramics: (A) 20˚≤2-Theta≤60˚ and (B) 27˚≤2-Theta≤30˚.

TABLE 1

TABLE 1. The effective RE³⁺ size (rRE/Å), space group (SG), lattice constants (a, b, c/Å), unit cell volume (V/ų), theoretical density (ρ₀/g · cm⁻³), and porosity (ϕ/%) of (La1-xHox)₃NbO₇ (x = 0/6, 1/6, 2/6, 3/6, 4/6, 5/6, and 6/6) ceramics.

The grain sizes of (La_1-xHo_x)₃NbO₇ are a micron scale, and most grains have sizes of 1–20 µm shown in Figure 2. The good combinations among neighboring grains and a small grain size may result in good mechanical properties. The sintering temperatures of (La_1-xHo_x)₃NbO₇ increase from 1,600 to 1700°C in conjunction with the increasing Ho content. Navrotsky’s research showed that the formation enthalpy of RE₃NbO₇ became more exothermic with the increases of RE³⁺ ionic radius, which indicated that the sintering temperature decreased with the increasing RE³⁺ ionic size (Mielewczyk and Navrotsky, 2015; Chen et al., 2018b). Furthermore, the crystal structure is affected by RE³⁺ ionic radius, and fluorite RE₃NbO₇ has a smaller grain size than weberite RE₃NbO₇. Therefore, the grain size of (La_1-xHo_x)₃NbO₇ is affected by the sintering temperatures and crystal structures, and they are dominated by RE³⁺ ionic radius. The increments of sintering temperature and order–disorder (weberite–fluorite) transition of crystal structure lead to a decrease of the grain size. Figure 3 exhibits the backscattered electron (BSE) photo and the corresponding element mappings of (La_3/6Ho_3/6)₃NbO₇. No precipitated phase is found, and each element is evenly distributed in this sample. The XRD, SEM, and EDS results prove that dense and high-purity (La_1-xHo_x)₃NbO₇ samples are made via current methods.

FIGURE 2

FIGURE 2. Surface microstructure of (La_1-xHo_x)₃NbO₇ (x = 0/6, 1/6, 2/6, 3/6, 4/6, 5/6, and 6/6) ceramics.

FIGURE 3

FIGURE 3. SEM and EDS mapping photos of (La_3/6Ho_3/6)₃NbO₇ ceramics: (A) BSE photo and (B–F) element mappings.

Mechanical Properties

Equation 1 expresses that Debye temperature is proportional to the average acoustic velocity. Figures 4A,B exhibit that (La_1-xHo_x)₃NbO₇ (0/6 ≤ x ≤ 2/6) oxides have similar Young’s modulus (100–110 GPa) and Debye temperatures (310–340 K), implying that their bonding strengths are approximate. With the further increments of Ho content, (La_1-xHo_x)₃NbO₇ (3/6 ≤ x ≤ 6/6) oxides have far higher Young’s modulus (180–200 GPa) and Debye temperatures (390–420 K) than those of the aforementioned samples. Similar variation trends are found in hardness and toughness of the prepared samples, as shown in Figures 4C,D. (La_1-xHo_x)₃NbO₇ (3/6 ≤ x ≤ 6/6) oxides have higher hardness and toughness than the remaining samples and their maximums are 9.2 ± 0.35 GPa and 1.6 ± 0.28 MPa m^1/2, respectively. The detail values of various mechanical properties are listed in Table 2. One can see that (La_1-xHo_x)₃NbO₇ ceramics have higher hardness than LaNbO₄ (∼3 GPa), while they have a lower value than YSZ (12–14 GPa), RE₂Zr₂O₇ (9–12 GPa), and LaMgAl₁₁O₁₉ (∼14 GPa) (Wang et al., 2012; Chen et al., 2019a; Liu et al., 2019). Furthermore, (La_1-xHo_x)₃NbO₇ ceramics have lower Young’s modulus than YSZ (∼250 GPa), RE₂Zr₂O₇ (240 GPa), and LnMgAl₁₁O₁₉ (300 GPa), while they have higher values than LaNbO₄ (60 GPa), Ho₂SiO₅ (150 GPa), and REPO₄ (140 GPa) (Vassen et al., 2000; Wang et al., 2012; Feng et al., 2013; Li et al., 2018; Chen et al., 2019a; Liu et al., 2019; Wu et al., 2020). The high modulus produces poor thermal stress fracture resistance, which is indicated by a so-called TSR parameter (Kingery, 1955):

$TSR=\frac{{\sigma }_f\left(1-\upsilon \right)}{E·TECs},(4)$

where σ_f is the flexural strength and υ is Poisson’s ratio. Evidently, comparatively low modulus is necessary for maintaining high hardness and good thermal stress fracture resistance, which is essential for the lifetime of TBCs. Besides, the highest fracture toughness of (La_1-xHo_x)₃NbO₇ is lower than that of YSZ (3.5 MPa · m^1/2), but it is better than that of La₂Zr₂O₇ (1.0 MPa · m^1/2) (Schlichting et al., 2001; Yang et al., 2016; Liu et al., 2019).

FIGURE 4

FIGURE 4. Mechanical properties of (La_1-xHo_x)₃NbO₇ (x = 0/6, 1/6, 2/6, 3/6, 4/6, 5/6, and 6/6) ceramics: (A) Young’s modulus, (B) Debye temperature, (C) Vickers hardness, and (D) fracture toughness.

TABLE 2

TABLE 2. The average acoustic velocity (V_a/m · s⁻¹), Young’s modulus (E/GPa), hardness (H/GPa), Debye temperature (Ɵ_D/K), Poisson’s ratio (υ), and fracture toughness (KIC/MPa · m^1/2) of (La1-xHox)₃NbO₇ (x = 0/6, 1/6, 2/6, 3/6, 4/6, 5/6, and 6/6) ceramics.

The variations of aforementioned properties are affected by bonding strengths, which are dominated by the following factors. First, lattice shrinkages caused by the decrements of r_RE result in the enhancements of bonding strength as that bonding strengths increase with the decrease of bonding lengths (Zhao et al., 2016; Wang et al., 2018). Second, the Ho–O bonds have higher strengths than La–O bonds, which is reflected by their modulus and Debye temperatures. Third, Figure 3 shows that the grain size of (La_1-xHo_x)₃NbO₇ is decreased by the increased Ho content. The decrements of grain size will lead to increases of grain boundary density, fracture toughness, and hardness.

Thermal Properties

Figures 5A,B,D exhibit that thermal diffusivity and conductivity, and phonon mean free path of (La_1-xHo_x)₃NbO₇ solid solutions have the similar temperature dependences because they are dominated by phonons. At room temperature, Ho₃NbO₇ has the lowest thermal diffusivity (0.34 mm²/s), and conductivity (0.96 W · m⁻¹ · K⁻¹) contributed to its disorder cubic fluorite lattice (Chen et al., 2021a; Yang et al., 2019; Chen et al., 2018b). Amorphous thermal conductivity is found in Ho₃NbO₇, when the remaining prepared samples exhibit different temperature dependences of thermal conductivity. The low thermal conductivity originates from the short phonon mean free path shown in Figure 5D. The lowest value of k (1.06 W · m⁻¹ · K⁻¹) at 900°C is found in (La_3/6Ho_3/6)₃NbO₇, and it has the shortest l (0.29 nm). It is evident that (La_1-xHo_x)₃NbO₇ has much lower thermal conductivity (0.96–1.42 W · m⁻¹ · K⁻¹) than YSZ (2.50–3.45 W · m⁻¹ · K⁻¹), La₂Zr₂O₇ (1.45–2.58 W · m⁻¹ · K⁻¹), LaNbO₄ (1.43–3.18 W · m⁻¹ · K⁻¹), and other candidate oxides (Vassen et al., 2000; Feng et al., 2013; Zhao et al., 2016; Chen et al., 2018b; Li et al., 2018; Liu et al., 2019; Ren et al., 2019; Liu et al., 2020a; Liu et al., 2020b; Chen et al., 2020; Ren et al., 2020; Wu et al., 2020; Chen et al., 2021a; Chen et al., 2021b).

FIGURE 5

FIGURE 5. Thermal properties of (La_1-xHo_x)₃NbO₇ (x = 0/6, 1/6, 2/6, 3/6, 4/6, 5/6, and 6/6) ceramics: (A) thermal diffusivity, (B) temperature dependence of thermal conductivity, (C) composition dependence of thermal conductivity, and (D) phonon mean free path.

During the substitution process of (La_1-xHox)₃NbO₇, misfits of atomic mass and ionic size are introduced to the lattice to enhance phonon scattering strength, and thermal conductivity is reduced (Wang et al., 2012; Feng et al., 2013; Zhao et al., 2016; Liu et al., 2019; Liu et al., 2020b). Normally, thermal conductivity decreases with the increasing phonon scattering strength caused by point defects, which can be reflected by the phonon scattering coefficient. It is believed that the phonon scattering coefficient of (La_3/6Ho_3/6)₃NbO₇ reaches the maximum based on phonon point defect scattering theory. However, besides phonon point defect scattering, influence of phonon propagation speed on thermal conductivity should be considered according to the evident variations of sound speed. The average acoustic velocities of (La_1-xHo_x)₃NbO₇ (x = 3/6, 4/6, 5/6, 6/6) (3,100–3,400 ms⁻¹) are far higher than those of rest samples (2,500–2,700 ms⁻¹), indicating that (La_1-xHo_x)₃NbO₇ (x = 3/6, 4/6, 5/6, 6/6) oxides have far higher phonon propagation speed than the others. Therefore, the thermal conductivity of (La_1-xHo_x)₃NbO₇ (x = 0/6, 1/6, 2/6) decreases with the increasing Ho content, and it increases with the further increments of Ho content in (La_1-xHo_x)₃NbO₇ (x = 3/6, 4/6, 5/6) (Figure 5C). Both point defect phonon scattering strength and changes in phonon propagation speed play essential roles on regulations of thermal conductivity of (La_1-xHo_x)₃NbO₇ ceramics.

Besides thermal conductivity, high TECs are essential for the service of TBCs. Figure 6A displays that the deformation of each sample increases with the increasing temperature, and their slopes are almost constant, which are further proved via the first-order differential curves depicted in Figure 6B. The phase transition and corresponding temperature can be detected by TMA, which have been proven in our previous research studies on RE₃NbO₇ and RENbO₄ ceramics (Chen et al., 2018b; Wu et al., 2020). It is believed that (La_1-xHo_x)₃NbO₇ ceramics have a stable lattice up to 1,200°C. The TECs of (La_1-xHo_x)₃NbO₇ increase with the increments of temperature, as shown in Figure 6C, and the maximal value (11.3 × 10⁻⁶K⁻¹) at 1,200°C is found in (La_3/6Ho_3/6)₃NbO₇. One drawback of YSZ (TECs: 10 × 10⁻⁶ K⁻¹) and RE₂Zr₂O₇ (TECs: 9 × 10⁻⁶ K⁻¹) is the low TECs, which produce high thermal stress during high-temperature applications (Yang et al., 2016; Schlichting et al., 2001; Kingery, 1955; Vassen et al., 2000). The lowest high-temperature thermal conductivity and highest TECs are simultaneously achieved in (La_3/6Ho_3/6)₃NbO₇ because of the strongest anharmonic lattice vibrations. Normally, the anharmonic lattice vibrations increase with the increasing temperature, which leads to decrease of thermal conductivity and increase of TECs. The TECs and thermal conductivity of (La_1-xHo_x)₃NbO₇ are shown in Figure 6D to compare with other TBC oxides. The prepared (La_1-xHo_x)₃NbO₇ samples have lower thermal conductivity and higher TECs than current YSZ, RE₂Zr₂O₇, RETaO₄, and distinct high-entropy ceramics (HECs) (Vassen et al., 2000; Schlichting et al., 2001; Zhao et al., 2016; Chen et al., 2018b; Ren et al., 2019; Liu et al., 2020a; Liu et al., 2020b; Chen et al., 2020; Ren et al., 2020; Chen et al., 2021a; Chen et al., 2021b). It is believed that good thermal insulation performances and low thermal stress can be provided, when (La_1-xHo_x)₃NbO₇ ceramics are applied as TBCs.

FIGURE 6

FIGURE 6. Thermal properties of (La_1-xHo_x)₃NbO₇ (x = 0/6, 1/6, 2/6, 3/6, 4/6, 5/6, and 6/6) ceramics: (A) thermal expansion rate, (B) first-order differential curve, (C) thermal expansion coefficients; and (D) comparison of thermal conductivity and TECs among various oxide TBCs (Vassen et al., 2000; Schlichting et al., 2001; Zhao et al., 2016; Chen et al., 2018; Ren et al., 2019; Chen et al., 2020; Liu et al., 2020; Liu et al., 2020; Ren et al., 2020; Chen et al., 2021; Chen et al., 2021).

Conclusion

Dense (La_1-xHo_x)₃NbO₇ oxide bulks have been fabricated in this work, and their structures and thermophysical properties are characterized. The excellent thermophysical properties indicate that the prepared samples are candidate TBCs, and following conclusions are obtained:

1) Dense and high-purity (La_1-xHo_x)₃NbO₇ ceramics are obtained by a solid-state sintering process, and phase transitions of Cmcm→C222₁→$\text{Fm}\overline{3}\text{m}$SG are detected along with the increments of Ho content. The grain sizes are a micron scale, and each element is evenly distributed in these samples.

2) The mechanical properties are affected by their bonding strengths and grain sizes, and the changes of modulus and Debye temperatures prove that their bonding strengths enhance with the increments of Ho content, which is caused by the shortening of bonding length and addition of stiff Ho–O bonds. The highest modulus, hardness, and toughness are 196 GPa, 9.2 GPa, and 1.6 MPa m^1/2, respectively.

3) Good thermal insulations (k: 1.06 W m⁻¹ K⁻¹ at 900°C), high TECs (11.3 × 10⁻⁶ K⁻¹ at 1,200°C), and excellent high-temperature phase stability are simultaneously realized in (La_3/6Ho_3/6)₃NbO₇ ceramics, which are far better than those of YSZ, RE₂Zr₂O₇, RETaO₄, and other TBCs. The intense anharmonic lattice vibrations lead to obvious thermal property improvements.

Data Availability Statement

The original contributions presented in the study are included in the article/Supplementary Material; further inquiries can be directed to the corresponding author.

Author Contributions

LC conducted the experiments and wrote this paper, YW and QZ conducted partial experiments on thermal conductivity and thermal expansions, and JF designed the detailed experiments, and discussed and improved this article.

Funding

The current work was supported by the National Natural Science Foundation of China (No. 91960103 and 51762028) and Yunnan Province Materials Genome Engineering (No. 2018ZE019).

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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