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.

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.

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.

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).

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.

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: (1)Dc=D01−exp⁡[ (Eak)(1Tc−1T) ] 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.

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.