Microstructure and Cs Behavior of Ba-Doped Aluminosilicate Pollucite

Article pubs.acs.org/JPCC

Microstructure and Cs Behavior of Ba-Doped Aluminosilicate Pollucite Irradiated with F+ Ions Weilin Jiang,* Libor Kovarik, Zihua Zhu, Tamas Varga, Mark H. Engelhard, and Mark E. Bowden Pacific Northwest National Laboratory, Richland, Washington 99352, United States

Tina M. Nenoff and Terry J. Garino Sandia National Laboratories, Albuquerque, New Mexico 87185, United States S Supporting Information *

ABSTRACT: Radionuclide 137Cs is one of the major fission products that dominate heat generation in spent fuels over the first 300 years. A durable waste form for 137Cs that decays to 137Ba is needed to minimize its environmental impact. Aluminosilicate pollucite CsAlSi2O6 is selected as a model waste form to study the decay-induced structural effects. Whereas Ba-containing precipitates are not present in charge-balanced Cs0.9Ba0.05AlSi2O6, they are found in Cs0.9Ba0.1AlSi2O6 and identified as monoclinic Ba2Si3O8. Pollucite is susceptible to electron-irradiation-induced amorphization. The threshold density of electronic energy deposition for amorphization was determined to be ∼235 keV/nm3. Pollucite can be readily amorphized under F+ ion irradiation at 673 K. A significant amount of Cs diffusion and release from the amorphized pollucite occurs during the irradiation. However, cesium is immobile in the crystalline structure under He+ ion irradiation at room temperature. The critical temperature for amorphization is not higher than 873 K under F+ ion irradiation. If kept at or above 873 K all the time, the pollucite structure is unlikely to be amorphized; Cs diffusion and release are improbable. A general discussion regarding pollucite as a potential waste form is provided in this report. transforms to the ground state of 137Ba (T1/2 = 2.6 min) by giving off a 0.662 MeV γ-photon. It can also decay directly to 137 Ba with a decay energy of 1.176 MeV, but the probability is much smaller (5.4%). In addition to generating heat, the decay changes the cation valence and ionic radius in the host structure, potentially leading to instability of its microstructure and an increase in the mobility of the radionuclide. Pollucite compositions, such as CsAlSi2O6, have a zeolite-like aluminosilicate framework with small pore openings and have been considered as a potential ceramic waste form for immobilization of 137Cs because of their low leach rates and good thermal stability.10,11 Furthermore, their natural analogues suggest durability on a geologic time scale. However, radiation resistance and decay effects in pollucite must be investigated before an informed management decision can be made. A previous TEM study12,13 showed that a diluted, aged sample of 137CsAlSi2O6·0.5H2O is susceptible to electronirradiation-induced amorphization at 200 keV; volatilization of Cs in the sample under electron irradiation was also observed. A later study3 found that an aged 137Cs-containing pollucite, after decay of nearly 16% of the total 137Cs over the elapsed 20 years, exhibited a homogeneous, crystalline matrix with no

1. INTRODUCTION Fission products of uranium in spent fuels from commercial nuclear power plants contain large yields of 137Cs and 90Sr, which are highly radioactive, with half-lives of 30.2 and 29.1 years, respectively.1 As a result of β− decay and self-electron irradiation, these two radionuclides are primarily responsible for the heat generation in spent fuel during the first 300 years of storage. One of the options for dealing with this problem is to separate the radioisotopes from the complex composition for confinement, long-term storage, or permanent disposal. Removal of 137Cs and 90Sr from the spent fuel could significantly reduce the repository size for nuclear wastes.2 Spent fuels are chemically reprocessed by ion exchange for 137 Cs and by solvent extraction for 90Sr.3 Prior to disposal, the radionuclides must be immobilized in a solid form to prevent their release to the environment. Ideal waste forms are mechanically strong, chemically stable, resistant to selfirradiation-induced damage, and highly thermally conductive. Since the Blue Ribbon Commission Report,4 the United States has launched a new direction to manage nuclear wastes.5 Over the past several years, we have focused on SrTiO3 as a model waste form for 90Sr, and the results have been published.6−9 This article reports on the results of aluminosilicate pollucite CsAlSi2O6 as a potential 137Cs waste form. Isotope 137Cs decays to an excited state of 137mBa by emission of a β− particle with a probability of 94.6% and quickly © 2014 American Chemical Society

Received: May 7, 2014 Revised: June 11, 2014 Published: June 24, 2014 18160

dx.doi.org/10.1021/jp5045223 | J. Phys. Chem. C 2014, 118, 18160−18169

The Journal of Physical Chemistry C

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× 14 mm with an average ion flux on the order of 5 × 1012 F+ cm−2 s−1. Sample temperature was measured at the front surface through the use of a customized chromel−alumel thermocouple system. The ion implantation conditions are given in Table 1. The implanted samples were cut into smaller

evidence of distinct Ba phases. It should be noted that the initial pollucite contained only a small mass fraction (0.0375) of 137Cs substitution for stable 133Cs. Thus, the total Ba concentration in the aged sample was small. Synthesis of high-quality pure and doped pollucite compositions has been an active subject of research,14−17 in addition to studies of natural pollucite minerals.18,19 In this study, we simulated decay-induced chemical effects by doping Ba into a nonradioactive 133CsAlSi2O6 pollucite structure and study irradiation effects on structure. Without any specific concerns of radiological contamination, the samples were characterized using various methods. Herein, we report on the results of Ba-doped pollucite20 before and after ion irradiation and post-thermal annealing.

Table 1. Ion Implantation and Thermal Annealing Conditions for Ba-Doped Pollucites Cs0.9Ba0.05AlSi2O6 and Cs0.9Ba0.1AlSi2O6a

a

2. EXPERIMENTAL PROCEDURES 2.1. Pollucite Synthesis. A hydrothermal technique21 was used to synthesize the base pollucite, CsAlSi2O6, as well as the Ba-doped pollucites,20 Cs0.9Ba0.05AlSi2O6 and Cs0.9Ba0.1AlSi2O6. In the basic procedure to synthesize CsAlSi2O6, distilled H2O (2.0 mL) was added to 3.00 g of CsOH solution (50 wt % aqueous solution; Alfa Aesar, Ward Hill, MA), after which fine aluminum powder (0.27 g, Aluminum Metal Finest Powder; Fisher, Pittsburgh, PA) was added carefully, as the aluminum dissolution is very exothermic. The silicon source, tetraethylorthosilicate (4.17 g, 99.9%; Alfa Aesar) was subsequently added to the solution, along with several milliliters of ethanol to facilitate dissolution of the tetraethylorthosilicate in the solution. At this point, the solution, which was in a polytetrafluoroethylene (PTFE) acid digestion liner (23 mL, Parr Instrument Company, Moline, IL), was gelled while being magnetically stirred in an ultrasound bath. It was then ready to undergo the hydrothermal treatment after the liner had been capped with a PTFE lid and placed in the acid digestion vessel (Parr 4749). The sealed vessel was heated at 10 K/min to 493 K and kept at this temperature for 2 h. During this time, the vessel was held horizontally and rotated at 16 rpm. Once the vessel had cooled to room temperature, the product was vacuum-filtered, rinsed with acetone, and then dried in air at 363 K. To remove residual organic species and to crystallize the powder, it was heated in air at 1373 K for 2 h. Barium hydroxide [Ba(OH)2·8H2O; Fisher] was used as the barium source to synthesize the Ba-doped pollucites. For the Cs0.9Ba0.05AlSi2O6 composition, 0.158 g of barium hydroxide was added to 2.70 g of the CsOH solution, and for the Cs0.9Ba0.1AlSi2O6 composition, 0.316 g of barium hydroxide was added to 2.70 g of the CsOH solution. The rest of the synthesis process was identical to that described for the base CsAlSi2O6 synthesis. To fabricate dense pellets of the materials, uniaxial hot-pressing was used. The powder was loaded into a graphite die and placed in the hot press (Centorr, Nashua, NH). After evacuation, the hot press was backfilled with nitrogen and heated to 1773 K for 1 h. Once the maximum temperature was attained, a pressure of 41 MPa was applied. 2.2. Ion Implantation and Sample Characterizations. Ion implantation was conducted with a NEC 9SDH-2 pelletron 3.0 MV electrostatic tandem accelerator.22 Ba-doped pollucites Cs0.9Ba0.05AlSi2O6 and Cs0.9Ba0.1AlSi2O6 were implanted with 1.2 MeV F+ ions up to a fluence of 1.36 × 1017 F+/cm2, corresponding to an atomic concentration of 5 at. % at the maximum of the profile peak. The implantation was performed at temperatures of 673 and 873 K. A beam rastering system was utilized to achieve uniform implantation over an area of 14 mm

fluence (1016 cm−2)

F ratio at peak (at. %)

Timp (K)

Tann (K)

tann (h)

8.2 13.6 2.7

3 5 1

673 673 873

1423 873, 1073 1073

10 4 2

In all cases, 1.2 MeV 19F+ ions were used.

pieces for thermal annealing at 873, 1073, and 1423 K. The temperature ramp time was 1 h, and the annealing time was 2, 4, or 10 h. Sample cooling to ∼343 K took over 10 h in the furnace. All furnace annealing was conducted in a flowing Ar gas environment. The thermal annealing conditions are also given in Table 1. Sample characterizations were performed before, during, and after implantation and annealing using various spectroscopic and microscopic methods. The as-synthesized samples were analyzed for crystalline phases using a symmetric scan of X-ray diffraction (XRD). Grazing-angle-incidence XRD (GIXRD) was conducted to probe the implanted surface layer. The measurements were conducted using a Philips X’Pert multipurpose diffractometer (MPD) with a fixed Cu anode operating at 45 kV and 40 mA. The depth profile of the implanted F was measured by use of time-of-flight secondary-ion mass spectrometry (TOF-SIMS), where 2 keV O2+ and 25 keV Bi+ ion beams were used for sputtering and analysis, respectively. The depth scale was calibrated by measuring the crater depth with a Veeco Dektak 150 stylus profilometer. Elemental mapping and composition analysis for the sintered samples were accomplished based on wavelength-dispersive X-ray spectroscopy (WDS) and energy-dispersive X-ray spectroscopy (EDS), respectively, using a JEOL JXA-8530F field-emission electron probe microanalysis (EPMA) instrument operating at 20 kV with a probe current of 20 nA. Spot mode was used for elemental mapping with a size of 60 nm × 60 nm and a dwell time of 200 ms per spot. The elemental ratio was also measured using inductively coupled plasma atomic emission spectroscopy (ICP-AES). The microstructures are examined using helium ion microscopy (HIM). Compared to conventional scanning electron microscopy (SEM), HIM has demonstrated a greatly enhanced imaging capability with a better surface sensitivity, higher spatial resolution, larger depth of field, and sharper Z contrast. An accelerating voltage of 25 or 30 keV was used for imaging, with a probe current ranging from 5 to 15 pA. No conductive coatings were applied for the insulating pollucite materials, and charge compensation was achieved using an electron gun integral to the HIM instrument. Cross-sectional thin specimens were prepared by use of a focused ion beam (FIB, FEI Helios NanoLab 600), where 30 and 5 keV Ga+ ions were used for cutting and polishing the sample, respectively, and 1 keV Ar+ ions were for nanomilling. The specimens were examined with an FEI aberration-corrected Titan 80−300 scanning transmission electron microscope (spatial resolution = 0.1 nm) at 300 keV. X-ray photoelectron spectroscopy (XPS; 18161



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cubic pollucite close to the (111) projection is illustrated in Figure 2a, with the Cs lattice on the (111) projection plane shown in Figure 2b. Undoped pollucite was examined using high-angle annular dark-field (HAADF) atomic-level-resolution scanning transmission electron microscopy (STEM). The Cs atoms in the crystal structure are clearly exhibited as bright spots, as shown in Figure 2c. Because of a significant contrast, the Si, Al, and O lattices are not visible. The observed Cs lattice arrangement exactly matches the simulated one. The Cs atoms located in wider channels are brighter, corresponding to a higher electron scattering yield received by the high-angle detector. This is mainly because the atomic column has a larger Cs occupancy, as illustrated by the three-dimensional view in Figure 2a. According to Figure 2c, the lattice constant is determined by the distance of two nearest Cs atoms (∼1.03 nm) in the wide channel divided by a factor of (2/3)−1/2, namely, 1.26 nm, which is slightly smaller than the theoretical value of 1.367 nm. Sample drift could contribute to the error, but slightly compressed local structure also cannot be ruled out. 3.1.2. Composition and Impurities. The estimated primary composition of the sintered pollucite samples was analyzed using EDS integral to an EPMA instrument with the analytical results given in Table 2. The results are not expected to be very

PHI Quantera SXM) was also used to study elemental depth profiles and impurities. The XPS experiments were performed based on Al Kα irradiation (hν = 1486.7 eV) in a clean base vacuum at 2 × 10−7 Pa using oil-free turbo and ion pump systems. A wide scan in the binding energy range from 0 to 1350 eV for a survey was performed at an angle of 45° relative to the normal X-ray incidence without ion sputtering to avoid any changes in surface chemistry. For elemental depth profiles of Cs and F, 4 kV Ar+ ions at 3.7 uA were used with 22 sputter cycles.

3. RESULTS AND DISCUSSION 3.1. As-Synthesized Pollucites CsAlSi2O6, Cs0.9Ba0.05AlSi2O6, and Cs0.9Ba0.1AlSi2O6. 3.1.1. Crystalline Phase. The undoped and Ba-doped samples were structurally analyzed using XRD based on symmetric scans. A representative XRD pattern for the as-sintered CsAlSi2O6 pollucite is shown in Figure 1. The major crystalline phase matches the

Table 2. Stoichiometries of Pollucites Determined by EDS and ICP-AES composition

Cs

Ba

Al

Si

O

note

CsAlSi2O6 Cs0.9Ba0.05AlSi2O6 Cs0.9Ba0.1AlSi2O6 CsAlSi2O6 Cs0.9Ba0.05AlSi2O6

1.0 0.9 0.9 1.0 0.9

0.0 0.06 0.11 0.0 0.06

0.94 0.80 0.96 1.0 0.99

2.1 2.2 2.1 2.4 2.5

7.0 7.2 7.0 − −

EDS EDS EDS ICP-AES ICP-AES

accurate because of some peak overlapping and the general inability to exactly correct ZAF (atomic number, X-ray absorption, and fluorescence) effects. Furthermore, the large excitation volume (involving grain boundaries) near the surface induced by the 20 keV electrons used in the measurement could also contribute to the uncertainty. The data in Table 2 suggest that the compositions in the three samples are nearly stoichiometric with a slightly higher concentration of oxygen. More accurate chemical analysis based on ICP-AES was also performed for all of the cations in CsAlSi 2 O 6 and Cs0.9Ba0.05AlSi2O6. The results are included in Table 2. Based

Figure 1. Typical XRD pattern from a symmetric scan for an assintered CsAlSi2O6 pollucite.

diffraction pattern of cubic pollucite structure without water. The dry phase is expected because the samples were sintered at high temperature (1773 K). The 5 and 10 at. % Ba-doped pollucite samples exhibit similar patterns (Figure S1, Supporting Information). The atomic arrangements in the pollucite from simulation and experiment are shown in Figure 2. A threedimensional view of the simulated atomic arrangement in the

Figure 2. Atomic arrangement of (a) cubic CsAlSi2O6 pollucite in three-dimensional view and (b,c) Cs lattice on the (111) projection plane from (b) simulation and (c) experiment. 18162



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from 0% in black and 100% in white. The backscattering electron (BSE) image obtained prior to scans for mapping is also included in the figure, which shows Z contrast. It was observed that the contrast in the BSE image becomes darker after repeated scans, probably because of amorphization of the material followed by Cs loss (see below). With increasing dwell time, a darker BSE image in the scanned area was observed. For an increased Ba doping concentration of 10 at. %, the brightest areas in the BSE image correspond to Ba-aggregated regions where the Cs and Al have a lower concentration, as shown in Figure 4. The data also suggest that, where there is an enrichment of Cs, there is a deficiency of Ba. It should be noted that, because 20 keV electrons were used for the elemental mapping, the excitation volume was relatively large and the elemental concentrations on the map were measured from the total volume near the surface. 3.1.4. Morphology of the Cleaved Surface. The undoped and Ba-doped pollucite samples were examined by HIM. Figure 5 shows the freshly cleaved cross sections of the three

on the average weight ratio of the original data, elemental ratios, referenced to Cs, were estimated. The compositions of the two samples were confirmed to be nearly stoichiometric, but with a slight enrichment of Si. In addition, surface analysis was performed using XPS without prior sputtering of the sample surface, and the data for Cs0.9Ba0.1AlSi2O6 are shown in Figure 3. A quantitative analysis

Figure 3. Survey XPS spectrum of as-sintered Cs0.9Ba0.1AlSi2O6.

of elemental concentrations in the sintered samples is not highly reproducible. This is expected because of possible surface contamination and the high surface sensitivity of XPS analysis. However, it is possible to use XPS to identify impurities of heavy elements that are not likely to be from surface contamination. In these samples, it was determined that very small amounts of Na and Ca impurities were present, as shown in Figure 3. 3.1.3. Elemental Mapping. Mapping of elements in undoped and Ba-doped pollucite was performed using EPMA. Beam scans were conducted over 6 × 6 μm2 with a spot size of 60 × 60 nm2 and a dwell time of 200 ms at each spot. Both the EDS and WDS methods for elemental mapping were applied simultaneously for a direct comparison. In general, EDS has a higher yield and better statistics, whereas WDS has a better elemental resolution. In this study, Cs and Ba peaks could be well-resolved by WDS but not by EDS (Figure S2, Supporting Information). Thus, the WDS method was employed to map major elements, including Ba, in doped pollucites. As an illustration, Figure 4 shows maps of Cs, Ba, Al, Si, and O in Cs0.9Ba0.1AlSi2O6 with the concentrations ranging

Figure 5. Cross-sectional HIM micrographs of as-sintered (a) CsAlSi2O6, (b) Cs0.9Ba0.05AlSi2O6, and (c) Cs0.9Ba0.1AlSi2O6 (field of view = 10 μm).

representative samples with a field of view of 10 μm. Interconnected crystallites of CsAlSi2O6 with sizes ranging from 100 nm to 1 μm can be seen (Figure 5a). Distinct facets of the crystallites appear. For Cs0.9Ba0.05AlSi2O6, shown in Figure 5b, the average crystallite size appears to be slightly larger than for the undoped pollucite. For an increased Ba doping concentration of 10 at. %, the crystallites become even larger, as shown in Figure 5c. Some small particles with brighter contrast located mostly at the grain boundaries are observed,

Figure 4. Backscattering electron image and WDS elemental maps of C-coated Cs0.9Ba0.1AlSi2O6 (field of view = 6 μm). 18163



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imental electron diffraction patterns along the three zone axes were made. The best match of the diffraction patterns between experiment and simulation suggests that the precipitate is a monoclinic Ba2Si3O8 crystal with space group P21/c; lattice parameters a = 1.2477 nm, b = 0.4685 nm, and c = 1.3944 nm; and interplanar angles α = 90°, β = 93.54°, and γ = 90°. The zone axes were identified to be [2̅01], [4̅81], and [1̅20], respectively, as indicated in Figure 7. Note that some of the small, weak diffraction spots originated from pollucite diffraction, and others might be associated with dynamic diffraction effects and a possible superlattice structure of Al substitution for Si (Figure 6b). Atomic-level-resolution HAADF STEM images were also recorded for lattice patterns and are shown in Figure 7. Results from lattice simulations using the software “CrystalMaker”26 are also shown in the figure. A direct comparison confirms that the precipitate is the monoclinic Ba2Si3O8. 3.2. Ion and Electron Irradiation in Cs0.9Ba0.1AlSi2O6, and Cs0.9Ba0.1AlSi2O6. 3.2.1. SRIM Predictions and F+-IonBeam-Induced Amorphization. Computer simulation of F+ ion implantation in CsAlSi2O6 was performed using the Stopping and Range of Ions in Matter (SRIM) code27 for the depth profiles of atomic displacements and implanted species. The results are shown in Figure 8a. In the simulation, the

which are Ba-containing aggregates as also observed by TEM (see below). 3.1.5. Ba-Containing Precipitates in Cs0.9Ba0.1AlSi2O6. No Ba-containing precipitates were observed in 5 at. % Ba doped pollucite Cs0.9Ba0.05AlSi2O6, which is consistent with previous reports.23,24 However, precipitates were found to be present in the Cs0.9Ba0.1AlSi2O6 phase. HAADF STEM and EDS on the pollucite (area 1) and precipitate (area 2) were performed, and the data are shown in Figure 6. During alignment of the

Figure 6. (a) Low-resolution HAADF STEM micrograph of Cs0.9Ba0.1AlSi2O6 pollucite. (b) EDS spectra in the areas of (1) pollucite and (2) precipitate.

pollucite crystal to the electron beam, electron-beam-induced amorphization in pollucite is evident, as shown in Figure 6a, which will be further discussed below. The EDS data in Figure 5b clearly indicate that the precipitate contains mainly Ba, Si, and O, but not Cs and Al, consistent with the data shown in Figure 4. A detailed STEM study of the Ba-containing precipitate was performed to identify the composition and crystal phase of the precipitate. For a chosen precipitate, three zone axes were aligned with the electron beam, as shown in Figure 7. The corresponding electron diffraction patterns are also shown in the figure. Based on the EDS data (Figure 6b), which suggest that the precipitate consists of mainly Ba, Si, and O, an extensive search of the current database for all barium silicates and simulations of the diffraction patterns were conducted using the software “jems”.25 Comparisons with the exper-

Figure 8. (a) Depth profiles of the atomic displacement rate and implanted F in CsAlSi2O6 from SRIM simulation. (b) Low-resolution HAADF STEM micrographs of Cs0.9Ba0.1AlSi2O6 implanted with F+ ions to 1.36 × 1017 F+/cm2 at 673 K and annealed at 1073 K.

specific gravity of the material was taken as 2.9 g/cm3, a measured value of the reference pollucite (PDF 00-029-0407) in Figure 1. This value is about 10% smaller than the calculated one (3.25 g/cm3) because of the porosity of the sintered samples. The threshold displacement energies for all of the sublattices were assumed to be 50 eV. This assumption might not be accurate, which affects the absolute value of dose in displacements per atom (dpa) but not the data interpretation in this study. Figure 8b shows a low-resolution HAADF STEM micrograph of Cs0.9Ba0.1AlSi2O6 pollucite implanted with 1.2 MeV F+ ions to 1.36 × 1017 F+/cm2 at 673 K and annealed at 1073 K for 4 h. Selected-area electron diffraction inside the surface implantation layer indicates that the layer of ∼1.7 μm in thickness beneath the protective/conductive coating layer is completely amorphized. The crystalline−amorphized interface is shown in the inset of Figure 8b. Beyond the amorphization layer is the unirradiated area, showing crystalline grains with precipitates (brighter areas) mostly at the grain boundaries. Clearly, the amorphization at the damage peak is produced by damage accumulation and structural collapse at a critical dose

Figure 7. Electron diffraction patterns and lattice sites for three different crystallographic axes of a precipitate in Cs0.9Ba0.1AlSi2O6 pollucite. Simulations and comparison with database indicate that the precipitate is monoclinic Ba2Si3O8, shown along the (a) [2̅01], (b) [4̅81], and (c) [1̅20] axes. 18164



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largely immobile in the sintered pollucite Cs0.9Ba0.1AlSi2O6 during ion implantation at 673 K. After annealing at 873 K, the shape becomes skewed toward the surface, indicating that there is some out-diffusion of the implanted F. However, F loss is negligible at 873 K within 4 h, as its concentration at the surface is still near zero. A more significant F out-diffusion occurs at 1073 K as the F profile becomes flatter with a relatively high concentration at the surface. As a result, a considerable amount of F escapes from the sample. A complete loss of F was observed in Cs0.9Ba0.1AlSi2O6 annealed at 1423 K (Figure S3, Supporting Information). It should be mentioned that the F peak is located at a depth of ∼1.5 μm and ends at ∼1.9 μm (Figure 10a, asimplanted). This depth range is comparable to, but slightly shallower than, the prediction (1.725 and 2.1 μm, respectively) by SRIM simulation (Figure 8a), probably because of underestimation of the stopping powers in the SRIM database or of the assumed specific gravity (2.9 g/cm3). Further measurements of F using XPS in combination with Ar+ ion sputtering were also conducted, and the data are shown in Figure 10b. The results confirm the F depth profiles and also indicate a significant diffusion and release of Cs during ion implantation and post-thermal annealing in the amorphized layer (see below). The microstructures of the F+-ion-implanted and thermally annealed samples were examined using HIM. Figure 11 shows

for amorphization through elastic-collision-induced damage cascades. In contrast, the precipitates within the amorphization band are crystalline, exhibiting more resistance to ion-beamirradiation-induced amorphization at the elevated temperature. 3.2.2. Electron-Beam-Induced Amorphization. It is shown in Figure 6a that the pollucite was amorphized by 300 keV electron irradiation during STEM for alignment of the crystal zone axis. The electron-irradiation-induced amorphization is also demonstrated in a magnified view in Figure 9, which shows

Figure 9. HAADF STEM micrograph showing 300 keV electronbeam-induced amorphization in CsAlSi2O6 pollucite.

two amorphization spots induced by the electron irradiation. Similar behavior was also reported for 137Cs containing pollucite under TEM examination using 200 keV electrons.12 Because the maximum energy imparted to the lattice atoms (up to a few electronvolts) by 300 keV electrons upon elastic collision is well below the threshold displacement energy (on the order of 10−100 eV) for the sublattices in CsAlSi2O6, accumulation of atomic displacements is not responsible for the amorphization. Instead, the amorphization is attributed to ionization-induced radiolysis (material decomposition) due to electronic energy deposition from the intense electron beam. 3.2.3. F Diffusion, Gas Formation, and Release. Implantation of as-sintered samples with 1.2 MeV F+ ions was conducted at 673 K to an ion fluence of 1.36 × 1017 F+/cm2, corresponding to 5 at. % F at the profile peak. Post-thermal annealing was carried out at 873 and 1073 K for 4 h each in flowing Ar environment. SIMS analysis was subsequently performed, and the data are shown in Figure 10a. As SIMS detection efficiency for F is extremely high, there is a saturation of the signals around the F peak (∼5 at. %). Dashed lines are drawn to extrapolate the F profiles. The presence of the welldefined, somewhat symmetric F peak is an indication that F is

Figure 11. Cross-sectional HIM micrographs of (a) Cs0.9Ba0.05AlSi2O6 pollucite implanted with F+ ions to 1.36 × 1017 F+/cm2 at 673 K and (b,c) the same postannealed at (b) 873 and (c) 1073 K (field of view = 20 μm).

cross-sectional views of Cs0.9Ba0.05AlSi2O6 at different stages of the implantation and annealing. An amorphized layer at the surface is clearly visible in each case. Inside the layer, there are holes of various sizes up to 500 nm. These holes are attributed to F2-gas-filled bubbles formed during ion implantation at an elevated temperature (673 K). Upon annealing at 873 K for 4 h, the bubbles grow to larger sizes and become internally

Figure 10. (a) SIMS spectra and (b) XPS depth profiles of F and Cs in Cs0.9Ba0.1AlSi2O6 after F+ ion implantation to 1.36 × 1017 F+/cm2 at 673 K and thermal annealing at 873 and 1073 K. The dashed lines in panel a are the data extrapolation (see text). 18165



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implantation, the Cs concentrations at the sample surfaces of Cs0.9Ba0.05AlSi2O6 and Cs0.9Ba0.1AlSi2O6 are 1.2 and 4.2 at. %, respectively. Compared to the initial values (∼9%) in both unimplanted samples, Cs loss at the surface corresponds to 87% and 53%, respectively. Part of the reason for the greater Cs loss from Cs0.9Ba0.05AlSi2O6 might be bubble rupture, leading to more surface area that can release Cs to vacuum more efficiently. In addition to the different Cs retentions in the two samples, the thermal annealing behaviors are also different. The 5% Ba doped pollucite shows no significant changes during annealing at 873 and 1073 K. The loss of Cs is saturated, and the Cs depth distribution remains largely the same except for some possible accumulation at the surface after annealing at 1073 K. In contrast, Cs in the 10% Ba doped sample exhibits a concentration gradient along the depth in the amorphized layer of the as-implanted and 873 K annealed samples. Accumulation of Cs at the surface is evident with a flatter depth profile after annealing at 1073 K, which is consistent with the XPS data (Figure 10b). These results suggest that Cs loss occurs mainly during ion irradiation at 673 K. 3.2.5. Critical Temperature and Critical Density for Amorphization in Pollucite. To avoid full amorphization in pollucite due to damage accumulation, the sample temperature must be raised to increase the simultaneous recovery rate. When a total balance between the rate of defect production and the rate of defect recovery is reached, amorphization through damage accumulation will not occur. Figure 14a shows the near-surface region of Cs0.9Ba0.1AlSi2O6 implanted with 1.2 MeV F+ ions to 2.73 × 1016 F+/cm2 at 873 K. The corresponding dose (∼5 dpa) at the damage peak (∼1.55 μm deep) is relatively high, and the material is still not amorphized, suggesting that the irradiation temperature (873 K) is not below the critical temperature for amorphization in pollucite. However, a continuous amorphization band from the surface to a depth of 460 nm is observed in the material, as shown in Figure 14a. The electron diffraction patterns in the crystalline and amorphized regions are shown in the Supporting Information (Figure S4). The results suggest that the amorphization in pollucite is a direct result of radiolysis through ionization in the near-surface region, where the electronic energy deposition by the incident F+ ions exceeds the critical density for amorphization. This result is consistent with electron irradiation of pollucite (Figures 6a and 9). It should be noted that, similarly to what has been observed in the nuclear collision processes, the precipitate within the amorphized pollucite band is not amorphized, showing a greater resistance to radiolysis. In addition, there is a depletion of Cs in the amorphized layer (Figure S5, Supporting

interconnected; some bubbles rupture at the surface, leading to release of the pressurized F2 gas, as can be seen from Figure 11b. With increasing temperature to 1073 K, the average size of the bubble becomes larger in the amorphized layer (Figure 11c). The layer thickness remains unchanged, and recrystallization within the layer does not occur. In sharp contrast to Cs 0 . 9 Ba 0 . 0 5 AlSi 2 O 6 , sample Cs0.9Ba0.1AlSi2O6 implanted and annealed under the same conditions exhibited different behavior. No bubbles were observed in the amorphized layer even after annealing at 1073 K, as shown in Figure 12. Although Cs0.9Ba0.05AlSi2O6 is

Figure 12. Cross-sectional HIM micrographs of (a) Cs0.9Ba0.1AlSi2O6 pollucite implanted with F+ ions to 1.36 × 1017 F+/cm2 at 673 K and (b,c) the same postannealed at (b) 873 and (c) 1073 K (field of view = 20 μm).

charge balanced in the structure due to vacancies,23,24 it is yet unknown why F2 bubbles are not formed in Cs0.9Ba0.1AlSi2O6. The results might suggest that the implanted F is trapped in the structure by forming chemical bonds with host atoms, such as Ba. Further studies are needed to understand the behavior and possible chemical impact, as BaF+ might be able to replace Cs+ and stabilize the pollucite structure. 3.2.4. Cs Diffusion and Release in Amorphized Pollucite. The depth profiles of Cs in Cs 0.9 Ba 0.05 AlSi 2 O 6 and Cs0.9Ba0.1AlSi2O6 pollucite samples before and after F+ ion implantation and thermal annealing at 873 and 1073 K were determined by 2.0 MeV He+ ion Rutherford backscattering spectrometry (RBS), and the data were analyzed using the code SIMNRA.28,29 Cs and Ba, as well as Al and Si, are not resolvable in the RBS spectra. For this reason, we do not distinguish Cs and Ba. Because Cs has a significantly higher concentration than the doped Ba, we use Cs to represent both Cs and Ba. The depth scale is estimated from the backscattering ion energy using surface energy approximation.30 The depth profiles of Cs in the pollucite samples are shown in Figure 13. In general, there is significant Cs diffusion to the surface and release to the vacuum during F+ ion implantation at 673 K. At the end of the

Figure 13. Depth profiles of Cs and Ba determined by RBS for (a) Cs0.9Ba0.05AlSi2O6 and (b) Cs0.9Ba0.1AlSi2O6 pollucite samples before and after F+ ion implantation to 1.36 × 1017 F+/cm2 at 673 K and thermal annealing at 873 and 1073 K. 18166



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sequences in the same sample area were carried out, each of which had an accumulated charge of 10 μC over the beam spot area of ∼0.8 × 0.8 mm2, corresponding to an ion fluence of ∼1016 He+/cm2. Figure 15 shows the RBS spectra for selected

Figure 15. In situ 2.0 MeV He+ ion RBS spectra taken repeatedly in the same area on a Cs0.9Ba0.1AlSi2O6 sample at room temperature, showing no loss of Cs during He+ ion irradiation. Figure 14. Surface amorphization of (a) Cs0.9Ba0.1AlSi2O6 and (b) Cs0.9Ba0.05AlSi2O6 at 873 K, induced by electronic energy deposition of 1.2 MeV F+ ions to 2.73 × 1016 ions/cm2 at 873 K. (c) SRIM simulation of ionization rate as a function of depth.

ion fluences of 1016, 4 × 1016, and 7 × 1016 He+/cm2. According to the RBS data, no changes occur in the spectra, suggesting that Cs loss is insignificant under the irradiation conditions up to the highest fluence applied (7 × 1016 He+/cm2). Compared to megaelectronvolt F+ ions, megaelectronvolt He+ ions deposit much less electronic (and nuclear) energy in the near-surface region because of a lower Z value and a higher velocity. The material is not fully amorphized at the end of the irradiation. The results in Figure 15 suggest that, prior to amorphization of pollucite, Cs diffusion and release are negligible under He+ ion irradiation at room temperature. 3.3. General Discussion. Pollucite has been widely investigated as a potential waste form to host radionuclide 137 Cs. The results from this study indicate that the pollucite structure can be readily amorphized under ion or electron irradiation through damage accumulation or radiolysis. Once amorphized, the material cannot be recrystallized at temperatures up to 1073 K and Cs is readily released from the amorphized material through out-diffusion. The self-irradiation effects of electrons and γ-rays from the β− decays in real pollucite waste form 137CsAlSi2O6 are expected to be extremely small in terms of generation of displaced atoms. The primary decay effects on pollucite structural stability should be the change in chemistry, including charge imbalance and change in ionic radii. However, caution must be exercised if the waste form contains impurities of actinide elements that undergo α decays. Unlike β− decay, both the energetic α particles and the daughter recoils can produce atomic displacements. Accumulation of simple and extended defects can lead to amorphization of the material below 673 K. Consequently, Cs diffusion to the surface and release to the environment become likely. If the waste form is kept at or above 873 K all the time, the pollucite structure cannot be amorphized through the damage accumulation process; Cs diffusion and release are not probable even at elevated temperature. After the pollucite waste form is aged for hundreds of years, barium-containing precipitates could be formed, possibly including monoclinic Ba2Si3O8 that is more resistant to irradiation-induced amorphization.

Information), which agrees with the results in Figures 10b and 13. Similar behavior of ionization-induced material decomposition and amorphization in as-implanted Cs0.9Ba0.05AlSi2O6 also occurs, as shown in Figure 14b. The electron diffraction pattern in the amorphized region is shown in the Supporting Information (Figure S6). The thickness (460 nm) of the amorphization band in the Ba-doped pollucite under identical irradiation conditions is the same as for the as-implanted Cs0.9Ba0.1AlSi2O6 (Figure 14a). In addition, Cs depletion in the amorphized layer and Cs enrichment at the sample surface are observed by EDS (Figure S7, Supporting Information) for the pollucite irradiated at 873 K to 1.36 × 1017 F+/cm2, as shown in the inset of Figure 14b. This result is consistent with the fact that Cs is mobile in amorphized pollucite and tends to diffuse to the surface (Figures 10b and 13). The holes observed in the pollucite grains were formed during the sintering process and might be associated with local Cs aggregation and evaporation. According to SRIM simulation with the results shown in Figure 14c, the observed thickness (460 nm) of the amorphized surface layer at an ion fluence of 2.7 × 1016 F+/cm2 corresponds to an electronic energy deposition rate of 860 eV nm−1 ion−1, or a threshold density of electronic energy deposition for amorphization of ∼235 keV/nm3 for CsAlSi2O6. Because this value is much larger than that from self-electron irradiation in 137 CsAlSi2O6 over its lifetime, pollucite crystal is not expected to be fully amorphized because of ionization by the self-electron irradiation from the β− decay process. After hundreds of years, a significant fraction of 137Cs (half-life = 30.2 years) decays to 137 Ba, resulting in a dramatic change in the composition. New crystalline phases can form in the structure, possibly including the observed precipitate phase Ba2Si3O8 (Figure 7) that is more resistant to radiolysis. 3.2.6. He+ Ion Irradiation in Cs0.9Ba0.1AlSi2O6. To evaluate the loss rate of Cs in Cs0.9Ba0.1AlSi2O6 as a function of ion fluence, an in situ RBS experiment was performed. The sample was irradiated with 2.0 MeV He+ ions at room temperature, and at the same time, RBS data were recorded. A total of seven

4. CONCLUSIONS The safe capture and storage of fission products is of vital interest in terms of both legacy nuclear waste and the future evolution of nuclear power. In particular, there is a great need 18167



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to discover and develop stable waste forms for 137Cs, which decays to 137Ba by emission of β− particles and γ-rays. To that end, we have undertaken a multidisciplinary experimental study to address the technical issues associated with the decayinduced structural effects and radiation resistance of aluminosilicate pollucite as a model waste form for 137Cs. Major results from this study are summarized as follows: (1) T h e s y n t h e s i z e d s a m p l e s o f C s A l S i 2 O 6 , Cs0.9Ba0.05AlSi2O6, and Cs0.9Ba0.1AlSi2O6 have a cubic pollucite structure without water. The overall composition of each sample is nearly stoichiometric. Impurities of Na and Ca are present at very low levels. The pollucite grain size ranges from 100 nm to 1 μm. Microscopically, there are regions of elemental enrichment and deficiency in Cs0.9Ba0.1AlSi2O6. Monoclinic Ba2Si3O8 precipitates, located mostly at the grain boundaries, were identified in the 10 at. % Ba-doped pollucite and were found to be more irradiation-resistant to amorphization. (2) High-dose F+ ion implantation leads to amorphization of pollucite crystal even at temperatures as high as 673 K. Apparent recrystallization does not occur during postannealing up to 1073 K; diffusion of the implanted F toward the surface occurs at 873 K and becomes more significant at 1073 K with F loss. The implanted F in pollucite is completely released at 1423 K. (3) Bubbles of F2 gas are formed in F+-ion-implanted Cs0.9Ba0.05AlSi2O6. In contrast, no gas bubbles are found in Cs0.9Ba0.1AlSi2O6 under identical implantation conditions. Even after thermal annealing at 1073 K, bubble formation does not occur. Although further studies are still needed to fully understand the behavior, the results could suggest that the implanted F in Cs0.9Ba0.1AlSi2O6 is trapped in the structure by forming chemical bonds with host atoms, such as Ba. (4) The pollucite is also susceptible to electron-irradiationinduced amorphization due to ionization-induced material decomposition (radiolysis). Complete amorphization through radiolysis can be avoided below the threshold density of electronic energy deposition, which was determined to be ∼235 keV/nm3 in this study. (5) Cesium is mobile and can diffuse to the surface in amorphized pollucite under ionizing irradiation at elevated temperatures. It can be readily released from the sample. However, prior to amorphization, there is no evidence from this study that shows Cs loss during He+ ion irradiation at room temperature. (6) The critical temperature for amorphization in pollucite is higher than 673 K but lower than or equal to 873 K under F+ ion irradiation. The results from this study indicate that the stability of the pollucite with respect to fission-product decay depends on environmental temperature and inclusion of radioactive impurities, such as actinides. If kept at or above 873 K all the time, the pollucite structure is unlikely to be amorphized; Cs diffusion and release to the environment are improbable even at elevated temperature. After hundreds of years of storage, barium-containing precipitates could be formed in pollucite, possibly including monoclinic Ba2Si3O8 that is more resistant to irradiation-induced amorphization.

Article

ASSOCIATED CONTENT

S Supporting Information *

XRD patterns of Cs0.9Ba0.05AlSi2O6 and Cs0.9Ba0.1AlSi2O6 (Figure S1). Comparison of EDS and WDS maps for Cs and Ba (Figure S2). Depth profiles of F in Cs0.9Ba0.1AlSi2O6 implanted at 673 K and annealed at 1423 K (Figure S3). Electron diffraction patterns inside and outside amorphized Cs0.9Ba0.1AlSi2O6 (Figure S4). EDS spectra showing depletion of Cs in amorphized Cs0.9Ba0.1AlSi2O6 (Figure S5). Electron diffraction pattern inside amorphized Cs0.9Ba0.05AlSi2O6 (Figure S6). EDS spectra showing Cs enrichment at the surface of amorphized Cs0.9Ba0.05AlSi2O6 (Figure S7). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: (509) 371-6491. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Nuclear Energy Research & Development, U.S. Department of Energy. The research was performed using EMSL, a national scientific user facility sponsored by the U.S. DOE’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory, a multidisciplinary national laboratory operated by Battelle for the U.S. DOE under Contract DEAC05-76RL01830. W.J. is grateful to John Vienna and Joseph Ryan at PNNL for stimulating discussions. T.M.N. and T.J.G. acknowledge support from the U.S. DOE/NE/FCRD-SWG and thank David Rademacher and Clay Newton at SNL for their assistance with sample preparation. Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia Corp., a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. DOE’s NNSA under Contract DE-AC04-94AL85000.

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REFERENCES

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Microstructure and Cs Behavior of Ba-Doped Aluminosilicate Pollucite

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