Ion beam irradiation-induced amorphization of nano-sized KxLnyTa2O7-v tantalate pyrochlore

Nano-sized (~10–15 nm) tantalate pyrochlores K_xLn_yTa₂O_7-v (Ln = Gd, Y, and Lu) were irradiated with 1 MeV Kr²⁺ beams at different temperatures and their radiation response behaviors were studied by in situ transmission electron microscopy observations. All of these nano-sized K_xLn_yTa₂O_7-v pyrochlores are sensitive to radiation-induced amorphization with low-critical doses (~0.12 dpa) at room temperature and high-critical amorphization temperatures above 1160 K. The K⁺ plays a key role in determining the radiation response of tantalate pyrochlores, in which the K⁺-rich KLuTa₂O₇ displays greater amorphization susceptibility than K_0.8GdTa₂O_6.9 and K_0.8YTa₂O_6.9 with lower K⁺ occupancy at the A-site. The reduced amorphization tolerance of the composition with a greater K⁺ content is consistent with the prominently larger K⁺/Ta⁵⁺ cationic radius ratio, which may result in more structural deviation from the parent fluorite structure and less capability to accommodate radiation-induced defects. An empirical correlation between critical amorphization temperature and ionic size was derived, generally describing the dominant effect of the cation ionic size in controlling radiation response of a wide range of pyrochlore compounds as potential nuclear waste forms. The results of the tantalate pyrochlore in this work highlight that nanostructured pyrochlores are not intrinsically radiation tolerant and their responses are highly compositional dependent.

Introduction

The A₂B₂O₇ pyrochlore with a cubic structure of Fd-3m space group is a derivative of the fluorite structure but with two cation sites and one eighth of the anions absent. The A and B cation sites of A₂B₂O₇ pyrochlores can accommodate a wide variety of metal elements, resulting in more than 500 compositions. With further loss of anions, the structure can become a defect pyrochlore structure of A₂B₂O₆. The A₂B₂O₇ pyrochlores exhibit high-chemical durability, excellent compatibility with actinides, and tunable radiation tolerance by controlling their cationic compositions and electronic bonding, and therefore, are considered as important host materials for nuclear waste forms and fuel matrix (Raison et al., 1999; Ewing et al., 2004). The radiation stability of pyrochlores can be greatly affected by their cationic compositions. One classical example is that the pyrochlore radiation resistance can be improved dramatically by substituting the Ti⁴⁺ cations in the Gd₂Ti₂O₇ pyrochlore with Zr⁴⁺ cations, which results in a smaller A site to B site cation ionic radius ratio (r_A/r_B), and amorphization cannot be reached in Gd₂Zr₂O₇ even at an extremely high-damage level up to 100 dpa at room temperature (Wang et al., 1999). The discovery of the radiation-resistant materials based on pyrochlore structure-type has significant environmental impacts on effective nuclear waste management, critical for developing advanced nuclear energy systems. The fundamental physics governing the phase and structural stability under intensive radiation conditions is critical for designing radiation tolerant materials. Extensive research activities on pyrochlore materials are ongoing from both experimental and atomistic modeling perspectives, revealing the importance of cation ionic size, structural disorder, and electronic structure in controlling the radiation performance of pyrochlore materials (Lian et al., 2002, 2003; Patel et al., 2008; Zhang et al., 2009a,b, 2010). Specifically, the smaller ionic radius ratio of A-site to B-site will lead to less structural deviation from the parent fluorite structure and thus a greater tendency toward an order-disorder structural transition from pyrochlore to fluorite upon intensive radiations due to the lower formation energies of cation anti-site defects. As a result, materials become more radiation tolerant (Sickafus et al., 2000).

Recently, for the purpose of mitigating the radiation damage, nano-scale materials design strategy has been applied in order to develop advanced nuclear materials with enhanced radiation performance. The rationale is that nanostructured materials are more radiation resistant as the high densities of surfaces, interfaces, and grain boundaries may behave as sinks for defect recovery. The enhanced radiation performance was demonstrated by previous studies on spinel, TiN, and ZrO₂ (Rose et al., 1995, 1997; Shen et al., 2007; Wang et al., 2007). It was also reported that the radiation performance of nano-sized pyrochlores with a composition of Gd₂(Ti_0.65Zr_0.35)₂O₇ increases drastically as the grain size decreases below 20 nm (Zhang et al., 2009a,b). As compared with zirconate (Wang et al., 1999; Lian et al., 2002; Ewing et al., 2004; Patel et al., 2008), titanate (Lian et al., 2003; Patel et al., 2008), and stannate (Lian et al., 2006) pyrochlores, nano-sized tantalate pyrochlores have not been studied previously, and no experimental data are available for understanding the impact of their cationic compositions on their radiation stability. In this study, we investigate the response of nano-sized K_xLn_yTa₂O_7-v pyrochlores (Ln = Y, Gd, Lu) under intensive radiation environments. Our results indicate that tantalate pyrochlores with average grain sizes of ~10–15 nm are radiation sensitive with high-critical amorphization temperatures and the potassium occupancy at the pyrochlore A-site significantly affects materials resistance against radiation-induced amorphization. These results also highlight that nanostructured pyrochlores are not intrinsically radiation tolerant and their radiation performance is highly compositional dependent.

Experimental

The rare earth-doped tantalate pyrochlores of K_xLn_yTa₂O_7-v used in this study were synthesized in Sandia National laboratory using hydrothermal methods upon aqueous reaction between K⁺-salts of the [Ta₆O₁₉]⁸⁻ polyoxometalate ion and lanthanide citrate complexes at 220 °C (Nyman et al., 2009). A PANalytical X’pert Pro X-ray diffractometer was used for X-ray diffraction (XRD) characterization, and the Rietveld refinement was performed in TOPAS program (Version 3.0 Bruker AXS, Karlsruhe, Germany) to determine the structures and phase compositions of these pyrochlores. Quantitative energy-dispersive X-ray spectra (EDS) were collected at random locations of each sample and analyzed statistically in order to provide complementary information on the chemical compositions of the tantalate pyrochlores. Transmission electron microscopy (TEM) characterization was conducted using a Hitachi H-9000NAR TEM with 300 KeV electron beam.

The ion irradiation experiments were conducted using the IVEM-tandem facility at Argonne National Laboratory, which combines a Hitachi H-9000NAR TEM with an NEC Tandem ion accelerator, enabling in situ TEM observation of material behaviors under intensive ion bombardment. A heating stage equipped with the TEM allows irradiation experiments in a wide temperature range from 300 to 1023 K. The K_xLn_yTa₂O_7-v nanoparticles were irradiated with 1 MeV Kr²⁺ ions at an ion flux of 6.25 × 10¹¹ ions/cm² and their microstructural evolution was monitored in situ by selected area electron diffraction (SAED) and dark field TEM imaging. The dark field imaging was obtained by inserting an objective aperture to cover the low-index diffraction rings of the SAED. The ion species and energy chosen for the irradiation were aimed at simulating the radiation effects of alpha decay on K_xLn_yTa₂O_7-v. The critical amorphization fluences were obtained when diffraction rings completely faded and amorphous halos appeared in the SAED patterns. The full cascade calculations using SRIM 2008 program were performed in order to convert the critical fluence to a universal damage unit of dpa.

Results and Discussion

Materials Characterization

The crystal structures of the three different samples are characterized by XRD and the patterns are compared side by side in Figure 1. The XRD pattern of the Gd³⁺-doped tantalate pyrochlore fits a single-phase pyrochlore structure, whereas the Y³⁺ and Lu³⁺-doped tantalate pyrochlores exhibit peak-doublings at 2-Theta positions of around 29°, 34°, 49°, 58°, suggesting that the existence of dual phases. Quantitative EDS analysis also reveals that the chemical composition of the Gd³⁺-doped tantalate pyrochlore sample is uniform throughout the sample; whereas the Y³⁺ and Lu³⁺-doped samples each shows two distinct chemical compositions with quite different K⁺ contents, consistent with the XRD results. Accordingly, the EDS data are grouped into five phases, and their average chemical compositions in atom percentage are listed in Table 1.

FIGURE 1

Figure 1. (Color online) XRD patterns of nano-sized tantalate pyrochlore samples and the corresponding Rietveld refinement fittings. The x-axes are 2-Theta position and the y-axes are intensity in counts. The dual phases in the Lu³⁺- and Y³⁺-doped samples result in the doubling of peaks. The chemical compositions are determined by Rietveld refinement results.

TABLE 1

Table 1. EDS analysis of the tantalate pyrochlores give five different groups of chemical compositions, each with a distinct atomic percentage of cation composition.

Rietveld refinements of the XRD patterns were performed based on the nominal chemical compositions by EDS analysis. A 2-Theta range of 25°–70° was used due to the high background before 25°. The refinement results, as shown in Table 2, reveal that the Gd³⁺-doped pyrochlore powders consist of a single phase as face-centered cubic (fcc) K_0.8GdTa₂O_6.9 with Fd-3m space group. In contrast, both Y³⁺- and Lu³⁺-doped samples consist of two different phases. Specifically, the Y³⁺-doped samples consist of K_0.8YTa₂O_6.9 (27.2 wt%) and K_0.4Y_0.8Ta₂O_6.4 (72.8 wt%), and the Lu³⁺-doped pyrochlore samples consist of KLuTa₂O₇ (12.8 wt%) and K_0.4Lu_0.8Ta₂O_6.4 (87.2 wt%), all being fcc structure with Fd-3m group space. As for the dual-phase Y³⁺ and Lu³⁺-doped compounds, the x parameter of 48f oxygen is 0.332 for K_0.8YTa₂O_6.9 and 0.316 for KLuTa₂O₇, indicating that the structures are close to A₂B₂O₇ pyrochlore; in contrast, the x parameter is 0.305 for K_0.4Y_0.8Ta₂O_6.4 and 0.306 for K_0.4Lu_0.8Ta₂O_6.4, smaller than that of perfectly ordered pyrochlores (x = 0.3125) (Subramanian et al., 1983) indicating that these two phases are closer to the A₂B₂O₆ defect pyrochlore structure. The slight differences in lattice parameters between the two phases of Y³⁺ and Lu³⁺-doped samples result in the doubling of peaks in XRD patterns as shown in Figure 1. However, the difference in the lattice parameter is too small (≤0.05 Å) to be differentiated by electron diffractions in TEM. The morphology, crystal size, and microstructure of the K_xLn_yTa₂O_7-v samples were further characterized by TEM. The TEM images in Figure 2 show that these tantalate pyrochlores are in the form of nanoparticles with average particle sizes of ~10–15 nm, consistent with the XRD refinement results.

TABLE 2

Table 2. Rietveld refinement parameters of the nano-sized tantalate pyrochlore: nominal compositions, weight percentage of each phase, x parameter of oxygen 48f, related structure, lattice constant a (Å), R factors (R_exp, R_wp, R_p), and goodness of fit (χ²).

FIGURE 2

Figure 2. Transmission electron microscopy images of (A) K_0.8GdTa₂O_6.9; (B) K_0.8YTa₂O_6.9/K_0.4Y_0.8Ta₂O_6.4; (C) KLuTa₂O₇/K_0.4Lu_0.8Ta₂O_6.4.

Ion Irradiation-Induced Amorphization

The radiation damage in the K_xLn_yTa₂O_7-v induced by 1 MeV Kr²⁺ bombardment was simulated in SRIM 2008 program using a chemical formula of K_0.8GdTa₂O_6.9 for the Gd-doped sample and the average chemical formulae of K_0.50Y_0.85Ta₂O_6.53 and K_0.47Lu_0.82Ta₂O_6.47 for the Y and Lu-doped dual-phase samples, respectively, and a displacement energy of 50 eV was applied to all the tantalate pyrochlores. The damage levels were given in dpa by converting the corresponding ion fluences based on the SRIM 2008 calculations, and the calculated nuclear and electron stopping powers are listed in Table 3. The nuclear stopping powers of each sample are higher than the corresponding electron stopping powers, which suggests that the elastic collision may dominate the ion irradiation and create high-displacements defects in the materials. These displacement defects will contribute to the microstructural changes of the samples as the key factor.

TABLE 3

Table 3. The nuclear stopping power (S_n) and electron stopping power (S_e) of the incident 1 MeV Kr²⁺ in different K_xLn_yTa₂O_7-v samples; x parameters of oxygen 48f; critical amorphization doses at 300 and 873 K and the critical amorphization temperatures of the tantalate pyrochlores.

As observed with in situ TEM, upon 1 MeV Kr²⁺ ion irradiations at room temperature, K_0.8GdTa₂O_6.9, K_0.8YTa₂O_6.9/K_0.4Y_0.8 Ta₂O_6.4, and KLuTa₂O₇/K_0.4Lu_0.8Ta₂O_6.4 nanoparticles all undergo a crystalline-to-amorphous phase transformation. The in situ TEM dark field images and SAED patterns in Figure 3 illustrate a similar room temperature ion beam-induced amorphization process of the three samples. The crystallinity of the original nanoparticles is indicated by the bright contrast in the dark field image and the diffraction ring patterns, characteristic for nanostructured tantalate pyrochlores (inset in Figures 3A,D,G). Upon 1 MeV Kr²⁺ irradiation with a dpa of 0.05, the contrast of the dark field images of all three samples was reduced gradually and the diffraction rings became vague as shown in Figures 3B,E,H. The nanoparticles were completely amorphized by 1 MeV Kr²⁺ at a critical dose of 0.12 dpa, as evidenced in Figures 3C,F,I. These results suggest that nano-sized tantalate pyrochlores are highly radiation sensitive and can be amorphized at a relatively low-damage level (~0.12 dpa).

FIGURE 3

Figure 3. Transmission electron microscopy dark field images and SAED patterns show the 1 MeV Kr²⁺ irradiation-induced amorphization process of the nano-sized tantalate pyrochlores at room temperature. Irradiation process of K_0.8GdTa₂O_6.9: (A) unirradiated, (B) 0.05 dpa (1 × 10¹⁴ ions/cm²), (C) amorphized at 0.12 dpa (2.5 × 10¹⁴ ions/cm²); irradiation process of K_0.8YTa₂O_6.9/K_0.4Y_0.8Ta₂O_6.4: (D) unirradiated, (E) 0.05 dpa (1 × 10¹⁴ ions/cm²), (F) amorphized at 0.12 dpa (2.5 × 10¹⁴ ions/cm²); irradiation process of KLuTa₂O₇/K_0.4Lu_0.8Ta₂O_6.4: (G) unirradiated, (H) 0.05 dpa (1 × 10¹⁴ ions/cm²), (I) amorphized at 0.12 dpa (2.5 × 10¹⁴ ions/cm²).

A decrease in potassium content in the three pyrochlore samples was observed by post-irradiation quantitative EDS analysis. The average atomic percentage of potassium decreased from the original 7.2 to 3.5 at % in the irradiated K_0.8GdTa₂O_6.9, from 6.2 to 2.6 at % in K_0.8YTa₂O_6.9/K_0.4Y_0.8Ta₂O_6.4, and from 3.9 to 1.7 at % in KLuTa₂O₇/K_0.4Lu_0.8Ta₂O_6.4. The loss of potassium is likely due to increased release rate of the highly volatile potassium in the radiation-induced amorphous matrix as a result of volume changes, increased surface area, reduced atomic bonding constraints, changes in local coordination, and the pathways for ion exchange.

The critical amorphization doses of the three K_xLn_yTa₂O_7-v samples all increase at elevated temperature. Figure 4 shows their amorphization processes under 1 MeV Kr²⁺ at 873 K. As the ion irradiation-induced amorphization process is mainly a competition between the accumulation of defects upon radiation and the recovery process of these defects, elevated temperature will lead to the increase of defect mobility in the materials and consequently the enhanced dynamics for defect annealing. As a result, the critical amorphization dose increases with temperature. It is also noted that the critical doses of K_0.8GdTa₂O_6.9, K_0.8YTa₂O_6.9/K_0.4Y_0.8Ta₂O_6.4, and KLuTa₂O₇/K_0.4Lu_0.8Ta₂O_6.4 are similar at room temperature, but differentiate at elevated temperature. At 873 K, the critical doses of both K_0.8GdTa₂O_6.9 and K_0.8YTa₂O_6.9/K_0.4Y_0.8Ta₂O_6.4 increase to ~0.30 dpa; in contrast, the critical dose of KLuTa₂O₇/K_0.4Lu_0.8Ta₂O_6.4 only slightly increases to 0.14 dpa. The difference becomes even larger at higher temperatures, indicating different dynamics of defect annealing.

FIGURE 4

Figure 4. Transmission electron microscopy dark field images and SAED patterns show the 1 MeV Kr²⁺ irradiation-induced amorphization process of the nano-sized tantalate pyrochlores at 873 K. Irradiation process of K_0.8GdTa₂O_6.9: (A) unirradiated, (B) 0.10 dpa (2 × 10¹⁴ ions/cm²), (C) amorphized at 0.30 dpa (6.25 × 10¹⁴ ions/cm²); irradiation process of K_0.8YTa₂O_6.9/K_0.4Y_0.8Ta₂O_6.4: (D) unirradiated, (E) 0.10 dpa (2 × 10¹⁴ ions/cm²), (F) amorphized at 0.30 dpa (6.25 × 10¹⁴ ions/cm²); irradiation process of KLuTa₂O₇/K_0.4Lu_0.8Ta₂O_6.4: (G) unirradiated, (H) 0.07 dpa (1.5 × 10¹⁴ ions/cm²), (I) amorphized at 0.14 dpa (3 × 10¹⁴ ions/cm²).

The temperature dependence of radiation damage-induced amorphization of the nano-sized tantalate pyrochlores K_xLn_yTa₂ O_7-v upon 1 MeV Kr²⁺ irradiation is fitted with the following equation derived based on the direct-impact model:

$D_c=\frac{D_0}{1-\exp \left[\left(\raisebox{1ex}{$E_a$}\!\left/ \!\raisebox{-1ex}{$k$}\right.\right)\left(\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$T_c$}\right.-\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$T$}\right.\right)\right]}$ (1)

where D₀ is the critical amorphization dose obtained by extrapolating the amorphization dose at T = 0 K; E_a is the dynamic annealing activation energy; T_c is the critical amorphization temperature above which the radiation-induced amorphization cannot occur (Weber et al., 1994; Wang et al., 1998, 2001). The fitted curve is shown in Figure 5. Based on the temperature dependence fitting, the critical amorphization temperatures, above which complete amorphization cannot be achieved, are 1167 ± 41 K for K_0.8GdTa₂O_6.9, 1165 ± 34 K for K_0.8YTa₂O_6.9/K_0.4Y_0.8Ta₂O_6.4, and ~1291 K for KLuTa₂O₇/K_0.4Lu_0.8Ta₂O_6.4.

FIGURE 5

Figure 5. Temperature dependence of the critical amorphization dose of the K_xLn_yTa₂O_7-v samples.

Composition Dependence of Radiation Tolerance

Table 3 summarizes the nuclear and electron stopping power (S_n and S_e) of 1 MeV Kr²⁺ in the different tantalate pyrochlore samples, and their 48f oxygen x parameters (Wyckoff position), critical amorphization doses, and critical temperatures (T_c), which clearly show the higher radiation tolerance of K_0.8GdTa₂O_6.9 and K_0.8YTa₂O_6.9/K_0.4Y_0.8Ta₂O_6.4 than KLuTa₂O₇/K_0.4Lu_0.8Ta₂O_6.4. The x parameters of 48f oxygen Wyckoff positions indicate that the special atom positions in a crystal. Since K_0.4Y_0.8Ta₂O_6.4 and K_0.4Lu_0.8Ta₂O_6.4 have small x parameters of 48f oxygen (0.305 and 0.306, respectively) and thus are closer to the A₂B₂O₆ defect pyrochlore structure, they have larger deviation from the parent fluorite structure-type as compared with the A₂B₂O₇-based K_0.8YTa₂O_6.9 (x = 0.332) and KLuTa₂O₇ (x = 0.316) phases. Previous results on a wide range of pyrochlore compositions indicated that materials become more sensitive to radiation damage with increasing structural deviations from the parent fluorite structure, as evidenced by the lower 48f oxygen positional parameter (Lian et al., 2002). Therefore, K_0.8YTa₂O_6.9 and KLuTa₂O₇ with compositions close to A₂B₂O₇ with greater 48f oxygen positional parameter may be more radiation tolerant, which may determine the overall critical amorphization doses of the dual-phase K_0.8YTa₂O_6.9/K_0.4Y_0.8Ta₂O_6.4 and KLuTa₂O₇/K_0.4Lu_0.8Ta₂O_6.4, respectively. Therefore, we only consider the phases of K_0.8GdTa₂O_6.9, K_0.8YTa₂O_6.9, and KLuTa₂O₇ in the following discussion of composition dependence of radiation tolerance.

The greater radiation susceptibility to radiation-induced amorphization for KLuTa₂O₇ as compared with K_0.8GdTa₂O_6.9 and K_0.8YTa₂O_6.9 may be attributed to the variation in K⁺ content, resulting in the change of cation ionic radius ratio and structural deviation from the parent fluorite structure, and thus, affecting their radiation performance. Particularly, the A and B site ionic radius ratio is critical in determining the radiation tolerance of A₂B₂O₇ pyrochlores, as a larger r_A/r_B ratio can lead to an increase in the A–B anti-site defect formation energy (Sickafus et al., 2000). Consequently, the structure is less capable of accommodating the high-displacement defects created by the incident ions, leading to lower amorphization resistance. In these K_xLn_yTa₂O_7-v samples, K⁺/Ta⁵⁺ has the largest r_A/r_B ratio of 2.359, significantly larger than those of Gd³⁺/Ta⁵⁺ (1.645), Y³⁺/Ta⁵⁺ (1.592), and Lu³⁺/Ta⁵⁺ (1.526). Due to the prominently large ionic radius ratio of K⁺/Ta⁵⁺, the K⁺ cation in the tantalate pyrochlores may dominate the structural deviation of pyrochlores, which controls their radiation tolerance. Our ion irradiation experiments show that K_0.8GdTa₂O_6.9 and K_0.8YTa₂O_6.9, both having lower K⁺ occupancy at A-site, are less sensitive to the radiation-induced amorphization at elevated temperature with relatively lower critical amorphization temperature; in contrast, the K⁺-rich KLuTa₂O₇ sample is more sensitive to the radiation-induced amorphization, and thus, has much higher critical temperature.

The lower amorphization doses and higher critical amorphization temperatures of these K⁺-doped tantalate pyrochlore samples as compared with the non-K⁺-doped titanate and zirconate pyrochlores indicate that the tantalate pyrochlores used in our study may be more susceptible to radiation-induced amorphization. The doping of K⁺ cations with large ionic radius may be responsible, at least partially, for the more radiation-sensitive nature of these tantalate pyrochlores as compared to non-K⁺-doped rare earth titanate (Lian et al., 2003), zirconate (Lian et al., 2002), and stannate pyrochlores (Lian et al., 2006) with smaller r_A/r_B. The correlation of the critical amorphization temperature T_c with the A–B site ionic ratio r_A/r_B of pyrochlores is discussed based on our radiation data of nano-sized tantalate pyrochlore and the previous radiation data of various bulk titanate and zirconate pyrochlores (Lian et al., 2002, 2003), as shown in Figure 6. Since we have demonstrated above that the chemical composition effect may overwhelm the size effect and significantly reduce the radiation tolerance of pyrochlore, bulk tantalate pyrochlores with the same chemical compositions may possess similar or slightly lower T_c, which will only have a minor impact on the correlation curve in Figure 6. For the tantalate pyrochlore phases of K_0.8GdTa₂O_6.9, K_0.8YTa₂O_6.9, and KLuTa₂O₇, whose A-sites are occupied by both K and Ln (Ln = Gd, Y, and Lu), the r_A/r_B values are determined by using the average r_A/r_B values weighted by the A-site occupancy of K and Ln. The resulting r_A/r_B versus T_c curve exhibits a near power-law relationship, which can be well fitted based on an empirical power-law function in the form of T_c = T₀ (R_r–R_rc)ⁿ, where R_r is defined as the r_A/r_B radius ratio, R_rc is defined as the threshold of r_A/r_B value at which T_c approaches absolute 0, and T₀ is a temperature constant in Kelvin and n is the exponent constant. The best fitting of the experimental data is T_c = (1545 ± 122 K)[R_r–(1.605 ± 0.007)]^(0.22±0.05) with R² = 0.91550 (coefficient of determination), indicating that the threshold of r_A/r_B value locates at 1.605 ± 0.007 (R_rc) and the exponent constant n = 0.22 ± 0.05. This result is consistent with previous radiation experiments on zirconate pyrochlores, in which La₂Zr₂O₇ (r_A/r_B = 1.611), whose r_A/r_B value is above the 1.605 ± 0.007 threshold, can be amorphized by 1.5 MeV Kr²⁺; whereas Nd₂Zr₂O₇ (r_A/r_B = 1.540), Sm₂Zr₂O₇ (r_A/r_B = 1.497), and Gd₂Zr₂O₇ (r_A/r_B = 1.462) with r_A/r_B values below 1.605 ± 0.007 cannot be amorphized even at low temperatures. The prediction is also consistent with a previous model based on the electronic bonding calculation developed by Lumpkin et al. to predict the radiation tolerance of synthetic pyrochlores (Lumpkin et al., 2007). In general, K⁺ content at the A sites of tantalate pyrochlores will result in an increase of the r_A/r_B value, which in turn can significantly reduce their radiation stability, highlighting important implication of chemical composition control for designing radiation resistant materials for nuclear waste forms and fuel matrix applications.

FIGURE 6

Figure 6. Correlation of A–B site ionic radius ratio (r_A/r_B) with critical amorphization temperature (T_c) for titanate and zirconate A₂B₂O₇ pyrochlores and tantalate K_xLn_yTa₂O_7-v pyrochlores. The different cation compositions of these pyrochlores are indicated in the upper x-axis, where the titanate and zirconate pyrochlores are indicated by the A and B site cations in the form of “A–B,” and the tantalate pyrochlores are indicated by “KGdTa” (representing K_0.8GdTa₂O_6.9), “KYTa” (representing K_0.8YTa₂O_6.9), and “KLuTa” (representing KLuTa₂O₇). The empirical power-law fitting equation of T_c = (1545 ± 122 K) [R_r–(1.605 ± 0.007)]^(0.22±0.05) and the R² of fitting (coefficient of determination) is given at the lower right of the figure.

Conclusion

The radiation response of nano-sized (~10–15 nm) tantalate pyrochlores K_xLn_yTa₂O_7-v (Ln = Gd, Y, and Lu) was investigated using 1 MeV Kr²⁺ ion beam irradiations at different temperatures, and the cation composition dependence of the ion beam-induced amorphization was studied. EDS measurements and XRD refinement reveal that the Y³⁺ and Lu³⁺-doped pyrochlore samples are dual-phase as K_0.8YTa₂O_6.9/K_0.4Y_0.8Ta₂O_6.4 and KLuTa₂O₇/K_0.4Lu_0.8Ta₂O_6.4, respectively; whereas a single phase of K_0.8GdTa₂O₇ exists in the Gd³⁺-doped tantalate pyrochlore. Ion beam-induced amorphization processes were observed in these nano-sized K_xLn_yTa₂O_7-v by in situ TEM. At elevated temperatures, both K_0.8GdTa₂O₇ and K_0.8YTa₂O_6.9/K_0.4Y_0.8Ta₂O_6.4 exhibit higher radiation tolerance than KLuTa₂O₇/K_0.4Lu_0.8Ta₂O_6.4, and the critical temperatures of K_0.8GdTa₂O₇ and K_0.8YTa₂O_6.9/K_0.4Y_0.8Ta₂O_6.4 are estimated to be 1167 ± 41 and 1165 ± 34 K, respectively, lower than that of KLuTa₂O₇/K_0.4Lu_0.8Ta₂O_6.4 (~1291 K). Compared with K_0.4Y_0.8Ta₂O_6.4 and K_0.4Lu_0.8Ta₂O_6.4 who are close to the A₂B₂O₆ defect pyrochlore structure, the K_0.8GdTa₂O₇, K_0.8YTa₂O_6.9, and KLuTa₂O₇ phases, having less structural deviation from the parent fluorite structure, may be responsible for the overall radiation tolerance. The less radiation tolerance of KLuTa₂O₇ as compared to K_0.8GdTa₂O₇ and K_0.8YTa₂O_6.9 is attributed to the high K⁺ occupancy at pyrochlore A sites in K_xLn_yTa₂O_7-v, resulted from the prominently large ionic radius ratio of K⁺/Ta⁵⁺ and consequently the large anti-site defect formation energy. An empirical threshold ionic radius ratio (r_A/r_B) of 1.605 ± 0.007 is determined by analyzing the radiation data of these tantalate pyrochlores and the previous data of titanate and zirconate pyrochlores. Pyrochlores with r_A/r_B smaller than 1.605 ± 0.007 are extremely resistant to displacive radiation-induced amorphization. The radiation-sensitive nature of the nano-sized K_xLn_yTa₂O_7-v tantalate pyrochlores may also be due to the rich K⁺ content, as it significantly increases the average r_A/r_B value. These results highlight that the radiation tolerance of nanostructured materials is highly compositional dependent, and nano-sized tantalate pyrochlores are sensitive to radiation damage.

Conflict of Interest Statement

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.

Acknowledgments

This work was supported by a NSF career award DMR 1151028. We thank the staff of the IVEM-tandem Facility at the Argonne National Laboratory for assistance with the irradiation experiments. We also acknowledge Dr. May Nyman of Sandia National Laboratory in providing tantalate pyrochlores samples for radiation study.

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