Intrinsic Structural Disorder and Radiation Response of

Jun 17, 2010 - Isometric pyrochlore, A2B2O7, has been extensively studied as a phase for the incorporation of actinides because of its use as an inert...
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Intrinsic Structural Disorder and Radiation Response of Nanocrystalline Gd2(Ti0.65Zr0.35)2O7 Pyrochlore Jiaming Zhang,† Jie Lian,*,‡ Fuxiang Zhang,† Jianwei Wang,† Antonio F. Fuentes,§ and Rodney C. Ewing*,† Departments of Geological Sciences and Materials Science & Engineering, UniVersity of Michigan, Ann Arbor, Michigan 48109-1005, Department of Mechanical, Aerospace & Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180, and CinVestaV Unidad Saltillo, Apartado Postal 663, 25000-Saltillo, Coahuila, Mexico ReceiVed: April 14, 2010; ReVised Manuscript ReceiVed: May 31, 2010

Isometric pyrochlore, A2B2O7, has been extensively studied as a phase for the incorporation of actinides because of its use as an inert matrix nuclear fuel and waste form. A critical issue is the effect of cation and anion disordering on the accumulation of radiation damage due to the alpha decay of actinides. The degree of disordering of A- and B-site cations, as well as oxygen vacancies, in nanocrystalline Gd2(Ti0.65Zr0.35)2O7 pyrochlore was tailored by mechanical milling with subsequent thermal treatments and quantified by X-ray diffraction and Raman spectroscopy. Annealing experiments at 1000 and 1200 °C for different times showed that the nanocrystalline Gd2(Ti0.65Zr0.35)2O7 displayed a gradual decrease in resistance to radiation-induced amorphization that correlates with the increasing degree of ordering of the cations and anion vacancies, highlighting the effect of disordering on the radiation response of nanocrystalline pyrochlore. These results illustrate that the pyrochlore structure type can be designed to function in high-radiation environments by changing the composition, crystal size, and degree of inherent disordering. I. Introduction Advanced nuclear fuel cycles will generate plutonium and minor actinides (Np, Am, Cm) that will have to be incorporated into nuclear fuels for transmutation by fission or immobilized into durable nuclear waste forms for long-term geologic disposal. In either case, it is important to develop materials that can incorporate actinides and are resistant to the radiation-induced amorphization resulting from alpha-decay damage of the incorporated actinides.1-4 Isometric pyrochlore, A2B2O7, has received considerable attention because it is a promising host phase for the incorporation of actinides.5-10 The advantage of using pyrochlore as a potential waste form is that significant amounts of actinides can be incorporated into A and B sites of the pyrochlore structure depending on the oxidation state of the actinide.6 Radiation damage from R-decay events of the incorporated actinides may cause amorphization, defect clustering, vacancy-induced cavities, and concomitant swelling, as well as the formation of new phases.11 A large number of ionirradiation studies have been completed in order to simulate R-decay damage in pyrochlore over a wide range of compositions.9,10,12-14 The results show that the response to ion-beam irradiation varies as a function of composition, particularly in the systems of A2Ti2O7 and A2Zr2O7 (A ) lanthanides and Y). For titanate pyrochlores, with the ideal pyrochlore structure, radiation-induced amorphization occurs concurrently with the formation of a defect fluorite structure. Complete amorphization of titanate pyrochlore occurs at doses less than 0.5 dpa.3,9 However, most zirconate pyrochlores, are highly “resistant” to ion-beam-induced amorphization. For example, Gd2Zr2O7 cannot * To whom correspondence should be addressed. E-mail: [email protected] (J.L.), [email protected] (R.C.E.). † University of Michigan. ‡ Rensselaer Polytechnic Institute. § Cinvestav Unidad Saltillo.

be amorphized with 200 keV Ti+ ions even at doses as high as 1 × 1017 ions/cm2 (∼100 dpa). Instead, Gd2Zr2O7 forms a disordered, defect fluorite structure.6,10 Ideal pyrochlore oxides with a fully ordered structure have the A2B2O6O stoichiometry, such as Gd2Ti2O6O (Fd3m). Pyrochlore is a superstructure of the fluorite structure type, AX2 (Fm3m), with cations ordered on the A site and B site and oneeighth of the anions missing. The eight-coordinated A site (16c) located at the center of a distorted cubic coordination polyhedron, is normally occupied by a larger cation, whereas the sixcoordinated B site (16d), located at the center of a disordered octahedron, is usually occupied by a smaller cation.15-17 The O(1) atoms occupy the 48f site coordinated to two B4+ and two A3+ cations, whereas the O(2) anions occupy the 8a site, being tetrahedrally coordinated to only A3+ cations. Additionally, there is an unoccupied anion tetrahedral site [O(3)], 8b, that is surrounded by four B4+ cations, and it is systematically vacant in ordered pyrochlore. The ordering of the pyrochlore structure depends on the radius ratio of the A and B cations. For example, Gd2Ti2O7 (rA/rB ) 1.74) is more likely to maintain the ordered pyrochlore structure, whereas Gd2Zr2O7 (rA/rB ) 1.46) is easily disordered to the defect fluorite structure. This is generally understood to be the result of the similarity in the ionic radii of the A- and B-site cations, and in this case, the defect fluorite structure is more stable.4 As a result, different degrees of cation disordering can be obtained by using the appropriate substitutions into the A and B sites. The resulting degree of disorder can be used to manipulate the physical and chemical properties of specific pyrochlore compositions. In the binary Gd2(Ti1-yZry)2O7 solid solution, the cation and anion substructures disorder gradually as the Zr content increases, leading to the appearance of oxygen vacancies at 48f sites, which are known to be responsible for oxygen hopping and diffusion.18 Correspondingly, a significant increase in oxygen conductivity is observed in this system when

10.1021/jp103371j  2010 American Chemical Society Published on Web 06/17/2010

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y g 0.3, which has been related with the onset of anion disordering.18 Similarly, an increase in resistance to radiationinduced amorphization by swift heavy ions has also been observed recently in Gd2(Ti1-yZry)2O7 solid solution with increasing Zr content.19-21 When the ternary compounds (including pyrochlore A2B2O7, perovskite ABO3, and spinel AB2O4) are subjected to radiation damage, cation antisite disordering and anion Frenkel disordering will occur depending on their formation and migration energies.22-24 Theoretical studies4,25 have shown that structures exhibit lower energies for disordering when the cation radii are similar. A large number of irradiation experiments have shown a consistent relation between the ratio of the cation radii in the A and B sites and the radiation response for different compositions in pyrochlore systems.3,6,9,10 However, there are very few experimental studies that investigate the influence of intrinsic structural disorder in fluorite-related phases on the energetics of the order-disorder transformation and the related susceptibility to ion-beam-induced amorphization.26 For the pyrochlore structure, the different chemical compositions and degrees of order-disorder affect the energetics (e.g., enthalpy of formation) and behavior under ion-beam irradiation. However, it is not clear how the degree of disorder, an inherent structural parameter, affects the susceptibility to ion-beam-induced amorphization. Here, we report the results of a systematic study of the intrinsic structural disorder in nanocrystalline Gd2(Ti0.65Zr0.35)2O7 powders, tailored by mechanical milling and subsequent thermal treatments. Samples were characterized by X-ray diffraction, Raman spectroscopy, and transmission electron microscopy in order to complete a microstructural characterization of grain size, as well as the degree of cation and anion disordering. Because it has already been demonstrated that crystal size affects the radiation resistance of the pyrochlore structure,27 we have optimized the thermal treatment in order to avoid variation in the grain size and the effect of surface area on the defect annihilation. Thus, these results provide a direct measure of the influence of intrinsic structural disorder on the irradiation response of the pyrochlore. II. Experimental Methods The Gd2(Ti0.65Zr0.35)2O7 pyrochlore was prepared by drymilling stoichiometric mixtures of the constituent oxides (pure monoclinic ZrO2, anatase-TiO2, and cubic Gd2O3) in a planetary ball mill using zirconia vials and balls (a detailed description can be found in ref 28). Post-milling thermal treatments were done at 1000 and 1200 °C for different durations (e.g., 1, 6, and 36 h.). Crystal structures were analyzed with X-ray powder diffraction (XRD) using a Scintag diffractometer with monochromatic Cu KR radiation (λ ) 1.5406 Å). The structural refinements were completed by the Rietveld method using the FullProf program.29 Raman spectra were collected using a HORIBA Jobin Yvon (HR800) micro-Raman spectrometer with a 300 mW HeNe laser as an excitation source (λ ) 632.82 nm). The microstructure and grain size of nanocrystalline Gd2(Ti0.65Zr0.35)2O7 were analyzed by transmission electron microscopy (TEM) using a JEOL 2010F electron microscope. Ion irradiation and in situ TEM observations were completed using the IVEM-Tandem Facility at the Argonne National Laboratory using 1-MeV Kr2+ ions at room temperature. Selected area electron diffraction (SAED) patterns were used to monitor the crystalline-to-amorphous transformation. The critical amorphization fluence, at which complete amorphization occurs, was determined by the disappearance of all of the diffraction maxima in the SAED patterns. The critical

Figure 1. (a) X-ray powder diffraction spectra from Gd2(Ti0.65Zr0.35)2O7 samples after different post-milling thermal treatments. Experimental (circles), calculated (solid lines), and difference (bottom) of the two X-ray diffraction patterns for the Gd2(Ti0.65Zr0.35)2O7 powder sample annealed at 1000 °C for 1 h (b) and 1200 °C for 1 h (c).

amorphization fluence has been converted into a unit of displacements per atom (dpa) using SRIM-2008 simulations30 with displacement threshold energies of 50 eV for Gd, Zr, Ti, and O in Gd2(Ti0.65Zr0.35)2O7. III. Results and Discussion A. Disordering Characterization by XRD and Raman. The X-ray diffraction patterns for the Gd2(Ti0.65Zr0.35)2O7 after annealing at 1000 and 1200 °C are shown in Figure 1a. Because the pyrochlore structure is a superstructure of a fluorite structure, with cation-ordering and anion-deficient atomic arrangements, its diffraction pattern is composed of a set of high-intensity diffraction maxima characteristic of the underlying fluorite-type substructure, plus an additional set of superstructure maxima. The intensities of superstructure diffraction maxima depend on a combination of factors, such as the degree of ordering, difference in the average scattering factors of the elements involved, and the distribution of oxygen vacancies.31 Figure 1a shows that all of the samples have the pyrochlore structure (indexed in the pattern). The result is consistent with the

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TABLE 1: XRD Refinement Results of the Structural Parameters and Grain Size As Measured by TEM for Gd2(Ti0.65Zr0.35)2O7 after Different Post-Milling Thermal Treatments crystal structures treatment T (°C)/ time (h) unit cell, a (nm) Gd/Zr/Ti in 16c (0,0,0), Occ. Gd/Zr/Ti in 16d (0.5,0.5,0.5), Occ. x48f RB Rwp Rexp χ2 grain size calculated from XRD (nm) grain size measured in TEM (nm)

pyrochlore Fd3m (S.P. 227) 1000/1

1000/6

1000/36

1200/1

1.03244(10) 1.03179(7) 1.03147(6) 1.03162(5) 0.56(1)/0.35/0.09(1) 0.59/0.35/0.06 0.63/0.35/0.02 0.74(1)/0.26 (1)/ 0

1200/6 1.03137(4) 0.83(1)/0.17 (1)/ 0

1200/36 1.03131(3) 0.84(1)/0.16 (1)/0

0.44(1)/0/0.56(1)

0.41/0/0.59

0.37/0/0.63

0.26(1)/0.09 (1)/ 0.65 0.17(1)/0.18 (1)/ 0.65 0.16(1)/0.19 (1)/0.65

0.3934(15) 0.092 0.146 0.176 0.683 26.4

0.4048(10) 0.052 0.140 0.166 0.709 28.2

0.4050(11) 0.087 0.153 0.173 0.787 31.7

0.4143(11) 0.12 0.17 0.18 0.832 37.4

0.4196(10) 0.11 0.16 0.12 1.75 44.1

0.4209(11) 0.12 0.18 0.13 2.01 51.5

19 ( 6

21 ( 5

22 ( 6

56 ( 11

71 ( 11

118 ( 22

previous report that the Gd2(Ti0.65Zr0.35)2O7 powders have the pyrochlore structure at temperatures greater than 1000 °C when the mechanically and compositionally induced defects anneal; when annealed at temperatures lower than 800 °C, only the disordered fluorite structure type is retained due to the mixed occupancy of metal cations at the large eight-coordinated 16c site and the smaller six-coordinated 16d site.32 Figure 1b,c shows examples of the refinement fitting using the Rietveld method for the Gd2(Ti0.65Zr0.35)2O7 annealed at 1000 and 1200 °C for 1 h. As a starting model, all the Gd ions are assigned to the 16c sites and, correspondingly, the Ti and Zr ions share the 16d site based on their smaller size. However, the significantly greater scattering from cations in the 16d site, as evidenced in the diffraction pattern, suggests partial occupancy of Gd ions on the 16d sites. Because of the coupled exchange, Zr must also occupy the 16c site. However, refinement of the XRD pattern at 1000 °C based on this assumption results in a negative occupancy of Zr on the 16d sites, which may suggest that only Gd and Ti have exchanged crystallographic sites, and the Zr remained at the 16c sites. The exchange of Gd and Ti cations increased with increasing temperature. At 1200 °C, Zr is also involved in the exchange process from 16c to 16d after all of the Ti cations occupied the 16d sites. The ordering process on the cation sublattice coincides with the redistribution of oxygen vacancies as the bond length and coordination environment change. All of the structural refinement results are summarized in Table 1. The occupancy of Gd in the 16d site and x coordinate of the 48f site are plotted in Figure 2, which shows a gradual

decrease in the disorder of Gd2(Ti0.65Zr0.35)2O7 pyrochlore as a function of annealing time. Raman spectroscopy was used to investigate oxygen disordering in the Gd2(Ti0.65Zr0.35)2O7 pyrochlore after thermal treatments, as the spectral bands are mostly related to the vibrations of the anion substructure.32 Previous studies32-34 have demonstrated the correlation between Raman measurements and the anion disordering in the pyrochlore and fluorite structures. For the pyrochlore structure, vibrations of the oxygen atoms located at the 48f sites, O(1), contribute to five active modes (A1g + Eg + 3T2g), whereas those bonded to the A-site cation, O(2), give a single T2g mode. For the ideal fluorite structure, there is only one vibration mode (T2g). For the sample in this study, the Raman modes are obviously broadened in both the pyrochloreand the fluorite-type structures due to the mixed site occupancy of the cations. Figure 3 shows the comparison of collected spectra from Gd2(Ti0.65Zr0.35)2O7 annealed at 1000 °C (a) and 1200 °C (b). The Raman spectrum of the sample annealed at 1000 °C for 1 h is close to the fluorite spectrum,33 which indicates structural disordering. As the annealing time increases, all the Raman bands near 520 (A1g), 318 (Eg), and 436 cm-1 (T2g) increase in their intensities, and the spectra gradually show the characteristics of the pyrochlore structure. The structural ordering is continuous with the increase in temperature. The samples annealed at 1200 °C have similar spectra, and they are more ordered than the sample annealed at 1000 °C. The overall

Figure 2. Variation of the occupancy of Gd on the 16d site and x coordinate of the 48f oxygen for the thermal treatments at 1000 and 1200 °C.

Figure 3. Raman spectra obtained for the Gd2(Ti0.65Zr0.35)2O7 pyrochlore samples annealed at 1000 °C (a) and 1200 °C (b) for different times.

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Figure 5. Sequence of dark-field TEM images (a-c) and SAED patterns (d-f) taken during in situ 1-MeV Kr2+ ion irradiation of the Gd2(Ti0.65Zr0.35)2O7 after being annealed at 1000 °C for 1 h. The irradiation fluences are 1.5 × 1015 ions/cm2 (∼1.88 dpa), 2.5 × 1015 ions/cm2 (∼3.13 dpa), and 3.8 x1015 ions/cm2 (∼4.7 dpa), respectively. (g) The relationship of the critical amorphization dose depends on the thermal treatment at 1000 and 1200 °C.

Figure 4. (a) A dark-field TEM image of the Gd2(Ti0.65Zr0.35)2O7 pyrochlore annealed at 1000 °C for 1 h (inset is an SAED pattern showing that the nanocrystalline powders have the ordered pyrochlore structure). (b) HRTEM image showing the superlattice in the individual pyrochlore grains (inset shows the associated FFT). Dark-field TEM images show the grain size variation for samples annealed at 1000 °C for 6 (c) and 36 h (d) and 1200 °C for 1, 6, and 36 h (e-g). (h) Plot of grain size as a function of annealing time.

Raman results from the annealed Gd2(Ti0.65Zr0.35)2O7 are consistent with the degree of disordering, as measured by XRD refinements. B. Grain Size Characterization by TEM. The crystal size of the pyrochlore samples after thermal treatment was characterized using transmission electron microscopy. The dark-field TEM images in Figure 4 show the morphology of the Gd2(Ti0.65Zr0.35)2O7 samples after being annealed at 1000 and 1200 °C for different times. The samples annealed at 1000 °C exhibit nanocrystalline particles with a similar grain size, ∼20 nm, assuming a spherical geometry (average particle size is summarized in Figure 4h). The treatment at 1000 °C does not cause any evident change in the grain size over time. An

SAED pattern in the inset (Figure 4a) shows that the nanocrystalline powders have the pyrochlore structure. A HRTEM image (Figure 4b) also confirms the pyrochlore superlattice in the individual grains (evident in the fast Fourier transform in the inset). However, progressive grain growth was observed when the samples were annealed at 1200 °C for increasing periods (as shown in Figure 4e-g), which suggest that the activation energy for grain growth is reached at this temperature. The crystal size was also calculated based on the (133) peak broadening of the XRD profiles using the Scherrer’s formula.35 Both TEM measurements and calculated crystal size by the Scherrer formula show that the grain size changed consistently; however, the calculated value is not accurate once the crystal size is approximately 0.1 µm. C. Irradiation Resistance versus Disordering. The pyrochlore powders were irradiated by 1-MeV Kr+ ions at room temperature. A phase transformation from the pyrochlore structure to an amorphous phase was observed in all of the nanocrystalline Gd2(Ti0.65Zr0.35)2O7 by in situ TEM. An example of the phase transformation represented by the powders after annealing at 1000 °C for 1 h is shown in Figure 5. The darkfield images and SAED patterns were obtained during in situ ion irradiation at a fluence of 1.5 × 1015 ions/cm2 (∼1.88 dpa), 2.5 × 1015 ions/cm2 (∼3.13 dpa), and 3.8 × 1015 ions/cm2 (∼4.7 dpa). The nanocrystalline pyrochlore transformed to an amorphous phase gradually (i.e., decreasing contrast from the nanosize grains in the images, decreasing intensity of the diffraction maxima, and the appearance of the broad, diffuse rings from the amorphous domains). Above the critical amorphization dose, Dc, diffraction maxima from the crystalline

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Figure 6. Critical amorphization dose of Gd2(Ti0.65Zr0.35)2O7 irradiated by 1-MeV Kr2+ as a function of (a) occupancy of Gd on the 16c site and (b) x coordinate of the 48f oxygen.

domains disappear completely, and complete amorphization is achieved. The relation of the critical amorphization dose to the annealing temperature and time is summarized in Figure 5g. The pyrochlore powders are most irradiation resistant after they have been annealed at 1000 °C for 1 h, and their radiation resistance decreased significantly after annealing at 1000 °C for 6 h and further decreased after annealing at 1000 °C for 36 h. The degree of structural ordering/disordering has a significant effect on the response of the pyrochlore structure type to irradiation when the grain size approaches the nanoscale regime, where materials become more radiation resistant due to a greater degree of structural disordering. This enhanced radiation resistance with structural disordering can also be seen for the Gd2(Ti0.65Zr0.35)2O7 pyrochlore annealed at 1200 °C, although it is not as prominent as that observed at 1000 °C, as shown in Figure 5g. When the critical amorphization dose is plotted as a function of the occupancy of Gd in 16c, which represents the level of cation ordering in Gd2(Ti0.65Zr0.35)2O7 pyrochlore, there is an exponential decay with increasing cation order (as shown in Figure 6a). The data can be fit to a function, Dc ) D0 + A exp(-ER/kT), where Dc is the measured critical amorphization dose, R is the occupancy of Gd in 16c (R ) 1 is a perfectly ordered pyrochlore; R ) 0.5 is a completely disordered Gd2(Ti0.65Zr0.35)2O7 with the fluorite structure), k is the Boltzmann constant, T is the absolute temperature (300 K), and D0, A, and E are the constants. The best fit result is Dc ) 0.9 + 1.3e5 exp(-0.5R/kT). A similar fitting process was completed for the critical amorphization dose as a function of the x coordinate of the 48f oxygen (Figure 6b), resulting in the function Dc ) -0.73 + 6.8e8 exp(-1.2x/kT). The formation enthalpy in pyrochlore structures with different compositions is correlated to the tendency of a material to amorphize.36 The present study shows that, for a given structure and chemical composition, a structural parameter, representing the degree of disorder, can be related to the response to high-energy irradiation. The products of E and R (or x) in the exponential decay functions are between 0.25 and 0.5 (0.5 < R < 1) for Dc as a function of the occupancy of Gd on the 16c site and between 0.45 and 0.53 (0.375 < x < 0.4375) for Dc as a function of the x coordinate of the 48f oxygen, both of which may be related to the energy difference between crystalline materials that have different levels of intrinsic disorder as compared with that of the aperiodic state. The radiation resistance of nanocrystalline pyrochlore has been shown to depend on the grain size.26 Consequently, the decreasing tolerance exhibited in the samples annealed at

1200 °C in the present study is a combined effect of structural disordering and change in crystal size, as both of these parameters change during thermal annealing. Recent studies on other materials, for example, spinel, MgGa2O4, and a TiNi alloy,37,38 have demonstrated that the crystal size plays an important role in defect migration and accumulation. Improved radiation performance is achieved in nanocrystalline materials because the grain size is less than the width of the defectdenuded zone; thus, this effect results in minimum defect accumulation within the grain.36 Similarly, enhanced radiation resistance has also been demonstrated in the present study of nanocrystals. Pyrochlore in the Gd2(TixZr1-x)2O7 binary has been shown to systematically change its susceptibility to radiationinduced amorphization with increasing Zr content.3 The critical amorphization dose, ∼0.66 dpa, has been reported for bulk Gd2(Ti0.5Zr0.5)2O7, which is considerably lower than the dose measured in this study (although the Zr composition is lower in the nanocrystalline sample). On the other hand, an optimized annealing temperature has excluded the effects of grain size (nearly constant at 20 nm when annealed at 1000 °C), allowing the study of the effect of intrinsic disorder or radiation resistance. The intrinsic disorder can be manipulated over different compositions in pyrochlore because the cation ionic radius ratio determines the deviation of the ordered structure from the defect fluorite structure. Thus, the irradiation resistance of pyrochlore depends critically on the ability of the structure to sustain cation disorder over the A and B sites, as well as disordering of the oxygen vacancies. Both experimental and theoretical studies show that the completely disordered fluorite structure should accommodate radiation-induced defects far more easily than an ordered fluoritederived structure.4,26 The experimental results of this study have identified an effective means of manipulating the irradiation tolerance to ion-beam-induced amorphization by tailoring the degree of intrinsic structural order/disorder in nanocrystalline pyrochlore. IV. Conclusions We have shown the feasibility of manipulating the level of disorder in the pyrochlore structure type as a means of “tuning” its radiation tolerance. The degree of disorder affects the energy of the crystalline structure as compared with that of the fully damaged aperiodic state. Nanocrystalline Gd2(Ti0.65Zr0.35)2O7 pyrochlore showed a gradual decrease in the degree of disorder for a constant crystal size (∼20 nm) when annealed at 1000 °C for different lengths of time. Correspondingly, a gradual decrease

Nanocrystalline Gd2(Ti0.65Zr0.35)2O7 Pyrochlore in radiation resistance was observed. These results demonstrate the importance of the degree of intrinsic disorder on the radiation response of pyrochlore at the nanoscale. Thus, strategies, including varying compositions,3,19,20 grain size,27 and intrinsic disordering, can be utilized to manipulate the response of pyrochlore to specific radiation environments. Acknowledgment. We thank the staff of the IVEM-tandem Facility at the Argonne National Laboratory for assistance with the irradiation experiments. The work at the University of Michigan was supported as part of the Materials Science of Actinides, an Energy Frontier Research Center, funded by the U.S. Department of Energy, Office of Science, under Award No. DE-SC0001089. J.L. acknowledges support from the National Science Foundation under Grant No. DMR-0906349. References and Notes (1) Taubes, G. Science 1994, 263, 629. (2) Weber, W. J.; Ewing, R. C.; Catlow, C. R. A.; Diaz de la Rubia, T.; Hobbs, L. W.; Kinoshita, C.; Matzke, Hj.; Motta, A. T.; Nastasi, M.; Salje, E. H. K.; Vance, E. R.; Zinkle, S. J. J. Mater. Res. 1998, 13, 1434. (3) Wang, S. X.; Begg, B. D.; Wang, L. M.; Ewing, R. C.; Weber, W. J.; Kutty, K. V. G. J. Mater. Res. 1999, 14, 4470. (4) Sickafus, K. E.; Minervini, L.; Grimes, R. W.; Valdez, J. A.; Ishimaru, M.; Li, F.; McClellan, K. J.; Hartmann, T. Science 2000, 289, 748. (5) Weber, W. J.; Ewing, R. C. Science 2000, 289, 2051. (6) Ewing, R. C.; Weber, W. J.; Lian, J. J. Appl. Phys. 2004, 95, 5949. (7) Lian, J.; Wang, L. M.; Wang, S. X.; Chen, J.; Boatner, L. A.; Ewing, R. C. Phys. ReV. Lett. 2001, 87, 145901. (8) Imaura, A.; Touran, N.; Ewing, R. C. J. Nucl. Mater. 2009, 389, 341. (9) Begg, B. D.; Hess, N. J.; Weber, W. J.; Devanathan, R.; Icenhower, J. P.; Thevuthasan, S.; McGrail, B. P. J. Nucl. Mater. 2001, 288, 208. (10) Lian, J.; Zu, X. T.; Kutty, K. V. G.; Chen, J.; Wang, L. M.; Ewing, R. C. Phys. ReV. B 2002, 66, 054108. (11) Sickafus, K. E.; Grimes, R. W.; Valdez1, J. A.; Cleave, A.; Tang, M.; Ishimaru, M.; Corish, S. M.; Stanek, C. R.; Uberuaga1, B. P. Nat. Mater. 2007, 6, 217–223. (12) Wang, S. X.; Wang, L. M.; Ewing, R. C.; Was, G. S.; Lumpkin, G. R. Nucl. Instrum. Methods Phys. Res., Sect. B 1999, 148, 704. (13) Lian, J.; Wang, L. M.; Ewing, R. C.; Boatner, L. A. Nucl. Instrum. Methods Phys. Res., Sect. B 2005, 241, 365. (14) Lian, J.; Chen, J.; Wang, L. M.; Ewing, R. C.; Farmer, J. M.; Boatner, L. A.; Helean, K. B. Phys. ReV. B 2003, 68, 134107. (15) Subramanian, M. A.; Aravamudan, G.; Rao, G. V. S. Prog. Solid State Chem. 1983, 15, 55.

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