Radiation Damage of LaMgAl11O19 and CeMgAl11O19 Magnetoplumbite

Danju Men,‡,† Maulik K. Patel,§,¶ Igor O. Usov,§ J. C. Pivin,k John R. Porter,††,‡‡ and MarthaL. Mecartney‡

‡Department of Chemical Engineering and Materials Science, University of California, Irvine, 92697-2575, California

§Materials Science & Technology Division, Los Alamos National Laboratory, Mail-Stop G755, Los Alamos, New Mexico 87545

kCentre de Spectrometrie Nucleaire et de Spectrometrie de Masse, CNRS-IN2P3-Universite Paris Sud, UMR 8609, Bat. 108,Orsay 91405, France

††Materials Department, University of California, Santa Barbara 93106-5050, California

Studies of radiation damage in magnetoplumbite-type

LaMgAl11O19 and CeMgAl11O19 are reported. Ion irradiationwas conducted on ceramic composites containing a

LaMgAl11O19 phase at 500°C with 10 MeV Au+ ions and on

ceramic composites containing CeMgAl11O19 phase at 800°Cwith 92 MeV Xe+ ions. The radiation response of these similar

LnMgAl11O19 (Ln = La and Ce) hexaaluminate magnetoplum-

bite phases was evaluated using transmission electron micros-

copy (TEM) and X-ray diffraction (XRD). LaMgAl11O19 wasamorphized by 10 MeV Au ions with swelling of the structure

within an approximate 2 lm radiation depth from the irradiation

surface. CeMgAl11O19 did not amorphize after 92 MeV Xe-ion

irradiation, but ion track damage contrast is seen in approxi-mately 5 lm of the irradiated depth. SRIM Monte-Carlo simu-

lations of nuclear displacements correlate with the experimental

results.

I. Introduction

I NERT matrix nuclear fuels with both fissile and nonfissilephases can be designed to offer (1) increased thermal con-

ductivity, (2) second phases that can accommodate fissionbyproducts while they are being created, and (3) decreasedmicrostructural evolution by providing alternative interfacesthat can be more efficient sinks for defects.1 Recently, four-phase ceramic composites of 3Y-TZP–Al2O3–MgAl2O4–LaPO4 were developed to serve as models for the compositefuel concept.2,3 Under certain process conditions, a fifth phase,LaMgAl11O19, could be formed by solid-state reaction duringsintering. In a related study, CeMgAl11O19 was observed toform using an initial model composite of composition CeO2–Al2O3–MgAl2O4. These two phases can be classified asmagnetoplumbite-type LnMAl11O19 (Ln = lanthanide,M = divalent spinel-forming ion),4 a hexagonal crystal struc-ture with a space group of P63/mmc (Space Group No. 194).5

Magnetoplumbite is a naturally occurring mineral with thegeneral chemical formula PbM12O19, where M has a + 3valence. The crystal lattice of Ln-magnetoplumbite consistsof close-packed oxygen in spinel blocks separated by perov-skite-like layers that contain lanthanide cations (Fig. 1).6,7

(For more details including exact Wyckoff positions see8).

These materials are also known as hexaaluminates, a broaderclass that includes not only the alumina-rich magnetoplum-bites in which large cations are 12-fold coordinated but alsob-alumina (NaAl11O17), a structure with similar spinel blocksbut with ninefold coordination of the large cations.9,10 Inmagnetoplumbite-type structures, the layers containing thelarge cations (such as lanthanides) are mirror planes, so theunit cell in the c-direction is twice that for b-alumina.

The radiation damage of magnetoplumbite is of consider-able interest as it has been proposed as a potential compo-nent in nuclear waste due to its ability to incorporate anextraordinary variety of fission products (including the highyield nuclides of Sr, Ba, Cs, La, and Ce) and actinide ele-ments.11–17 It is also structurally similar to other ionic mate-rials with close-packed oxygen that resist radiation damage.18

Morgan et al.11 demonstrated that Al2O3 added to radioac-tive waste could form a solid solution magnetoplumbitestructure, with multiple large ions in the waste accommo-dated in the magnetoplumbite crystalline lattice sites. ExcessAl2O3 can form insoluble spinel and remain asα-alumina,phases that microstructurally isolate radionuclides in themagnetoplumbite phase to help to prevent leaching andincrease the thermal conductivity. Although these studieshave suggested that Ln-containing magnetoplumbite phasesshould be considered for fission product immobilization innuclear waste forms, to date there are scant data frompublished radiation studies to support such proposals.

Despite the lack of information on swift heavy-ion irradia-tion behavior of Ln-magnetoplumbites, studies conducted insimilar crystal systems may be relevant. Hibonite, a mineralof nominal composition CaAl12O19, has a similar structure tomagnetoplumbite and also has a lathlike grain morphology.Hibonite is found in meteorites formed from presolar stardust19,20 and despite containing fission tracks for Pu244 fromnatural fission, the hibonite grains remain crystalline, indicat-ing that this material is fairly robust against radiation dam-age.21 Vance et al.22 followed up with a study on syntheticCaAl12O19 irradiated with 3 MeV Ar ions at room tempera-ture and found superior damage resistance against amorph-ization when compared with other crystal structuresincluding monazite.23 Planes close to the basal plane orienta-tion, however, showed damage with the (008) X-ray diffrac-tion (XRD) reflections disappearing after 1018 ions/cm2

irradiation whereas (110) reflections remained unchanged anda 1% lattice expansion along the c-axis for 3 9 1017 ions/cm2 irradiation. Hexagonal ferrite BaFe12O19, used for per-manent magnets, also has a magnetoplumbite-related crystalstructure.6 Studies with highly energetic heavy ions (Ar andKr > 1 GeV and fluences of 1013 and 1012) in BaFe12O19

created disordered and amorphized zones in some regionsand defects including stacking faults and a loss of periodicity

T. Mitchell—contributing editor

Manuscript No. 32593. Received January 6, 2013; approved June 12, 2013.¶Present address: Department of Materials Science and Engineering, University of

Tennessee, Knoxville, 37996-2100, Tennessee.‡‡Present address: UES, Dayton, 45432, Ohio.†Author to whom Correspondence should be addressed: e-mail: martham@uci.edu

1

J. Am. Ceram. Soc., 1–8 (2013)

DOI: 10.1111/jace.12503

© 2013 The American Ceramic Society

Journal

along the c-axis for other regions, the latter similar to thebasal plane damage reported for CaAl12O19.

24

To evaluate the potential of lanthanide magnetoplumbitesfor either use in inert matrix fuel or as a waste form for lan-thanide fission product retention, it is important to under-stand their specific irradiation response. Thus in theexperiments detailed here, the microstructural response ofLaMgAl11O19 and CeMgAl11O19 phases at the surface of acomposite under irradiation by energetic heavy ions is evalu-ated. These two magnetoplumbite phases have the same crys-tal structure and similar lattice parameters. La and Ce arenext to each other in the periodic table, and have similar cat-ion outer electron shell configuration and ionic sizes (1.36 �Afor La3+ and 1.34 �A for Ce3+),5,25so it is expected that theradiation damage behavior of one phase should be a fairlygood predictor for the other. Two different types of irradia-tion were deliberately used to show the differences that canresult.

II. Experimental Procedures

(1) Sample FabricationFive-phase and three-phase ceramic composites were fabricatedconsisting of LaMgAl11O19–3Y-TZP–Al2O3–MgAl2O4–LaPO4

(“LaMgAl11O19 composite”) and CeMgAl11O19–MgAl2O4–CeO2 (“CeMgAl11O19 composite”). Ceramic powder mixtureswere sintered at 1773–1873 K (1500°C–1600°C) for 3 h. Theformation of magnetoplumbite can be identified on theXRD patterns by the (114) characteristic peak for the magneto-plumbite structure (Fig. 2). The solid-state reactions were asfollows:

9Al2O3 þ 2MgAl2O4 þ 2LaPO4

! 2LaMgAl11O19 þ P2O5

9Al2O3 þ 2MgAl2O4 þ 2CeO2 ! 2CeMgAl11O19

Samples were cut into disks of 1 mm in thickness, with adiameter of 10 mm. The surfaces of the samples werepolished with 60 nm colloidal silica for later microscopy

observation and ion irradiation. The morphology of LaMg-Al11O19 grains shown in Fig. 3 indicates exaggerated andanisotropic grain growth in the presence of a liquid phase. Itis important to note that the magnetoplumbite grains format the surface so they can be directly exposed to irradiation.

(2) Ion Irradiation and SRIM Monte-Carlo SimulationsTwo sets of ion irradiation experiments were performed onthe samples to evaluate the response of magnetoplumbite todifferent types of radiation damage sources. The LaMg-Al11O19 composite was irradiated with 10 MeV Au ions to afluence of 1 9 1016 ions/cm2 at 773 K (500°C),26 which atthe end of the range simulate the damage caused by primaryknock-on atoms (PKA) created by neutrons. The CeM-gAl11O19 composite was irradiated with 92 MeV Xe ions at1073 K (800°C) with fluences ranging from 1012 ions/cm2 to1013 ions/cm2 to simulate the damage caused by fission frag-ments. Ion beams usually cause accumulated damage at arate faster than that by irradiation in a reactor or in anuclear waste, hence the use of higher temperature allowsone to slowdown the rate of damage and also create near torealistic modifications. Figure 4–show SRIM Monte-Carlosimulated ion irradiation characteristics for the specific ionenergy and material types in this study.27,28 The densities fordifferent phases used for the SRIM calculation are listed inTable I. These experiments do generate more damage(~20 dpa) in LaMgAl11O19 comparing with natural damageaccumulation in waste forms within 1000 yr (~10 dpa). How-ever, as the half-life of some of the minor actinides is1 9 104 yr, hundreds of dpa’s damage is expected toaccumulate in waste forms from radionuclides, so there is

Fig. 2. XRD pattern of pristine LaMgAl11O19 composite, showingthe formation of LaMgAl11O19.

Fig. 1. Unit cell of LnMgAl11O19 (magnetoplumbite), consisting ofspinel blocks separated by mirror planes containing Ln3+ ions.

Fig. 3. SEM image of LaMgAl11O19 large grains in composite priorto irradiation.

2 Journal of the American Ceramic Society—Men et al.

scientific relevance to using ion beams for simulating damageby radionuclides.

The five phases present in the LaMgAl11O19 composite(3Y-TZP, Al2O3, LaPO4, MgAl2O4, and LaMgAl11O19) exhi-bit different ion penetration depth and peak damage dosesdue to the difference in ion stopping power (Fig. 4). For92 MeV Xe irradiation, the displacement damage is greaterat larger depths than near the surface. Figure 5 shows thecomparison of estimated energy loss partitioned betweennuclear (Sn) and electronic stopping (Se) for 10 MeV Auions in the LaMgAl11O19 composite and 92 MeV Xe ions inthe CeMgAl11O19 composite as a function of target depth.Figure 6 shows the electronic-to-nuclear stopping powerratio for 10 MeV Au in LaMgAl11O19 composite and92 MeV Xe ions in CeMgAl11O19 composite (based onFig. 5 results). The dominating role of Se is more importantin 92 MeV Xe ion irradiation comparing with that in10 MeV Au ion irradiation (Fig. 5 and 6). Thus, althoughthe displace damage is small, the damage (microstructuralmodifications) due to ionization of the target atoms can besignificant.

(3) Microstructural CharacterizationCrystalline phases were identified before and after ion irradi-ation by XRD, Siemens/Bruker D5000 XRD (Siemens/Bruker, Karlsruhe, Germany). Plan-view microstructure evo-lution before and after ion irradiation was investigated usingscanning electron microscope (SEM), Zeiss Ultra Plus 55(Zeiss, Jena, Germany). Chemical compositions of grainswere identified by energy-dispersive X-ray spectroscopy(EDS). Accordingly, cross-section transmission electronmicroscopy (TEM) samples of ion-irradiated composites wereprepared using focused ion beam (FEI Quanta 3D FEGDual Beam, FEI, Eindhoven, the Netherlands) to includeboth pristine and radiation-damaged regions for comparison.TEM observations were carried out to study radiation dam-age at different depths of the materials [FEI Tecnai F20(FEI, Eindhoven, the Netherlands) and Philips CM20(Philips, Eindhoven, the Netherlands) operated at 200 keV,JEOL 3010 (JEOL, Tokyo, Japan) operated at 300 keV].

III. Results and Discussion

(1) LaMgAl11O19 Irradiated by 10 MeV AuMicrostructural evolution induced by the ion beam damagewas investigated by plan-view SEM (Fig. 7). Although Al2O3,MgAl2O4, and ZrO2 phases are unchanged, the monaziteappears to have melted. Before irradiation, the LaMgAl11O19

grain surface was flat and the same height as other neighbor-ing phases [Fig. 7(a)]. After 10 MeV Au ion irradiation, thetop surface of the LaMgAl11O19 swelled [Fig. 7(b)], suggesting

Table I. Density for Different Phases Present in the System (unit: g/cm3)

Material 3Y-TZP Al2O3 CeMgAl11O19 CeO2 LaMgAl11O19 LaPO4 MgAl2O4

Density 6.05 3.89 4.24 7.00 4.27 5.08 3.58

(a)

(b)

Fig. 4. SRIM Monte-Carlo simulations of nuclear displacements inunits of displacements per atom (dpa),27 as a function of targetdepth for (a) 10 MeV Au ion irradiations (fluence = 1 9 1016 Au/cm2) of five different phases present in LaMgAl11O19 composite: 3Y-TZP, Al2O3, LaPO4, MgAl2O4, and LaMgAl11O19, and (b) 92 MeVXe ion irradiations (fluence = 1 9 1012 Xe/cm2) of three differentphases present in CeMgAl11O19 composite:CeO2, MgAl2O4, andCeMgAl11O19.

(a)

(b)

Fig. 5. SRIM simulation estimates for Sn and Se as a function ofdepth for (a) 10 MeV Au ions in five phases present inLaMgAl11O19 composite and (b) 92 MeV Xe ions in three phasespresent in CeMgAl11O19 composite. Note that although the damagerange of the incident Au ions is much more shallow compared withXe ion irradiation (1.8–2.0 lm vs 8–9 lm), the Au ions actuallycause more atomic displacements.

Radiation Damage LnMgAl11O19 3

possible irradiation-induced volume expansion due to amor-phization. XRD results on the Au-irradiated LaMgAl11O19

composite could not be obtained due to the small irradiatedarea. Instead, the degree of crystallinity of the LaMgAl11O19

phase after Au irradiation was verified by TEMA cross-section TEM sample of Au-irradiated LaM-

gAl11O19 shows a large LaMgAl11O19 grain, which stretchesfrom the irradiated surface over to the end of the irradiationzone (Fig. 8). High-resolution TEM (HRTEM) imaging andselected-area diffraction (SAD) were performed on three dif-ferent locations within the LaMgAl11O19 grain (a, b, and c)to investigate the microstructure with respect to irradiationdepth. The HRTEM image obtained from position a inFig. 8, 500 nm in the Au irradiation surface, shows an amor-phous structure [Fig. 9(a)], corresponding to diffuse ringsboth in the fast Fourier transform (FFT) pattern of theHRTEM image and in the SAD pattern from this region[Fig. 9(a′) and (a″)]. Further away from the surface, 1.5 lmin depth (position b in Fig. 8), diffraction maxima dimlyappears both in the FFT pattern and SAD, indicating theexistence of short-range order in the structure [Fig. 9(b′) and(b″)]. In position c at 3 lm radiation depth, clear latticefringes are observed in the HRTEM image and sharp diffrac-tion maxima in the SADP, both of which are characteristicof a highly crystalline state [Fig. 9(c–c″)]. Therefore, it canbe concluded that LaMgAl11O19 is amorphized by 10 MeVAu ions in regions within the approximately 2 lm radiationdepth from the irradiated surface, the depth predicted to beaffected by Au ions according to SRIM simulations.

All other phases present in this LaMgAl11O19 composite,namely 3Y-TZP, Al2O3, MgAl2O4, and LaPO4, were verified

to be crystalline after 10 MeV Au irradiation with no amor-phization detected throughout the ion beam damage zone.2

(2) CeMgAl11O19 irradiated by 92 MeV XeFigure 10 compares XRD patterns of the CeMgAl11O19 com-posite before and after 92 MeV Xe irradiation. The only

(a)

(b)

Fig. 6. The ENSP (Electron-to-nuclear Stopping Power) ratio for(a) 10 MeV Au ions in five phases present in LaMgAl11O19

composite and (b) 92 MeV ions in three phases present inCeMgAl11O19 composite as a function of target depth, based on theSRIM simulation results in Fig. 5. This plot indicates that themagnitude of ENSP for 10 MeV Au irradiation is nearly 40 timesthat of 92 MeV Xe irradiation.

(a)

(b)

Fig. 7. Plan-view SEM images of the LaMgAl11O19 composite(a) before irradiation and (b) after 10 MeV Au ion irradiation. Thelath-like gray LaMgAl11O19 appears with a volume expansion afterirradiation. (Bright phase: ZrO2, light gray phase: LaPO4, darkphases: MgAl2O4 or Al2O3).

Fig. 8. Cross-section TEM bright-field images of LaMgAl11O19

composite irradiated by 10 MeV Au ions. HRTEM imaging andselected-area diffraction performed within the large LaMgAl11O19

grain on the left at three different depths from the irradiationsurface, as marked by a, b, c on the image.

4 Journal of the American Ceramic Society—Men et al.

existing XRD pattern of CeMgAl11O19 (JCPDS # 00-026-0872) shows inaccurate relative intensity ratios of the peaks,so the pattern for LaMgAl11O19 (JCPDS 78-1854) was usedinstead for identification. No changes such as peak broaden-ing or peak shifts occur in the XRD patterns after irradia-tion, suggesting that all phases are stable under swiftheavy-ion irradiation without amorphization. Similarly, noobvious changes in the microstructure can be detected inplan-view SEM for the CeMgAl11O19 composite before andafter 92 MeV Xe irradiation (Fig. 11).

Cross-sectional TEM and SAD on the irradiation zone ofCeMgAl11O19 composite after irradiation by 92 MeV Xe ionsreveals that the three phases present, namely CeMgAl11O19,MgAl2O4, and CeO2, are all crystalline and can be indexedto the initial crystalline phases (Fig. 12 and 13). This is truefor all regions from the surface to the interior and shows that

CeMgAl11O19 and the other phases are not amorphized bythe Xe ion beam.

Although there was no amorphization, multiple paralleldark lines of contrast were observed by cross-section TEMimaging in CeMgAl11O19 grains within a 5 lm depth fromthe irradiation surface (Fig. 14). These lines are parallel tothe Xe ion direction, marked by arrows in Fig. 14. This con-trast and the direction of the damage is similar to thatexpected for ion track damage, although high-resolutionnanodiffraction and additional TEM (HRTEM) studies aremost likely required to provide definitive proof. Figure 14(e′)shows a HRTEM image of the CeMgAl11O19 grain e on theirradiation surface in Fig. 12. The morphology of ion trackscannot be clearly delineated in this image from the magneto-plumbite grain at the surface, which might be partly due tothe fact that the high fluence of the Xe ion irradiation

(a) (b) (c)

(a′) (b′) (c′)

(a′′) (b′′) (c′′)

Fig. 9. (a)–(c) HRTEM images obtained from the three locations marked a, b, c in the magnetoplumbite LaMgAl11O19 grain in Fig. 8,exhibiting a transition from an amorphous structure (a) to crystalline lattice fringes (c). (a′)–(c′) The corresponding fast FFT diffraction patterns.(a″)–(c″) SAD patterns, showing amorphous rings (a″) turning into sharp diffraction spots (c″). The zone axis is [1�100]. The dominant latticefringes are imaged using (11�20) planes with a d-spacing of 2.8 �A. (The sample tilt for (a)–(c) and (a′)–(c′) correspond, but is not exactly the sametilt angle for (a″)–(c″) due to using different TEMs for the imaging and SAD).

Radiation Damage LnMgAl11O19 5

(1012 Xe/cm2) could produce numerous, overlapped damagetracks that are difficult to distinguish. Dislocations and straincontrast appear in CeMgAl11O19 grains deeper in the sample,where there is no ion track contrast. The ion track damageappears to end approximately 5 lm into the material.Although the SRIM Monte-Carlo Simulation shows that thedisplacement damage in CeMgAl11O19 is greater at around8 lm depths than near the surface [Fig. 4(b)], grain f (diffrac-tion not shown) is 7.5 lm away from the surface and the cplanes were still strongly diffracting with no indication ofamorphization.

(3) ComparisonsWhen comparing these current results on lanthanide-contain-ing magnetoplumbites with the earlier results onCaAl12O19,

22 several important differences need to be noted.Ca hexaaluminate contains no heavy lanthanide ions.Although the ion fluxes used for the irradiation study onCaAl12O19 are in a similar range to those used in this study,Ar+ ions used in that study are much smaller and lighterthan Xe+ or Au+. The irradiation temperature is also notreported for the CaAl12O19 studies so direct comparisonscannot be made, but as minimal damage was observed bothin that study and in this study for Xe irradiation of CeM-gAl11O19, this indicates the stability of the magnetoplumbitecrystal structure under certain conditions.

Dark line contrast found by TEM in BaFe12O19 irradiatedwith 3 GeV Xe ions has also been attributed to nucleartracks.29 The ion track damage in that magnetic magneto-plumbite BaFe12O19 was postulated to be due to the domi-nance of electronic stopping,30 similar to this study of92 MeV Xe+ irradiation in Ce-magnetoplumbite. Single crys-tal BaFe12O19 irradiated with 1.8 GeV Ar+ ions at 90–160 Kand a fluence of 1015 ions/cm2 is reported to have less dam-age in the high-energy zones near the surface than in regionsdeeper into the sample that experienced lower energy, butbecame amorphous.31 Similarly, a higher degree of radiationdamage is seen in these studies of 10 MeV Au+-irradiatedLa-magnetoplumbite compared with the 92 MeV Xe+-irradi-ated Ce-magnetoplumbite that did not amorphize. Anothermagnetic magnetoplumbite ferrite, SrFe12O19, became par-tially amorphized when irradiated at room temperature with50 MeV Li3+ ions and a fluence of 1014 ion/cm2 (evaluatedby XRD).32 The studies on other types of magnetoplumbiteare difficult to directly correlate, however, as the temperatureof those experiments is usually at room temperature in

contrast to the higher temperatures used in these currentexperiments. Temperature can play a significant role asdescribed in the next paragraph.

As the two magnetoplumbites are expected to be chemi-cally similar, observation that 10 MeV Au irradiation causesamorphization in LaMgAl11O19, whereas 92 MeV Xe resultsin presumed ion track damage in CeMgAl11O19 may beattributed to (1) difference in energy loss of the two irradia-tion damage sources and (2) higher irradiation temperaturesduring the 92 MeV Xe ion irradiation. The irradiation dam-age zone simulated by SRIM is approximately 1.8–2.0 lmdeep for 10 MeV Au irradiation, and 8–9 lm deep for92 MeV Xe irradiation (Fig. 5). Although the damage rangeof the Au ions is much shallower in LaMgAl11O19 as com-pared with Xe ions in CeMgAl11O19 (1.5 lm vs 9 lm), theratio of electronic-to-nuclear stopping is lower in the case of10 MeV Au than 92 MeV Xe. As all energetic ions loseenergy and cause modifications by both Se and Sn, and mostof the researchers usually state one or the other, thus to berealistic, it is important to compare damages to the ratio of

(a)

(b)

(c)

Fig. 11. Plan-view SEM images of CeMgAl11O19 composite(a) before and (b) after Xe ion irradiation to fluence of 1012 ions/cm2, and (c) 1013 ions/cm2. The microstructures look similar.Backscattered contrast due to different densities of the three phasesgives black—MgAl2O4, dark gray—CeMgAl11O19, light gray—CeO2.

(a)

(b)

(c)

Fig. 10. XRD patterns of CeMgAl11O19 composite (a) Before ionirradiation, (b) after 92 MeV Xe ion irradiation to fluence of1012 ions/cm2, and (c) 1013 ions/cm2. No change observed except adecrease in the (111) peak intensity for CeO2.

6 Journal of the American Ceramic Society—Men et al.

Se/Sn. In our case, this is hugely different for Xe ascompared with Au. The Au ions cause more atomic displace-ments compared with 92 MeV Xe ion, which predominantlyloses energy via ionization of the target atoms.

The higher energy ions can create a large transient rise inlattice temperatures, which could lead to rapid annealing ofthe damage. In addition, higher irradiation temperatures

Fig. 12. Cross-section TEM bright-field image of CeMgAl11O19

composite irradiated by 92 MeV Xe ions to a fluence of 1012 Xe/cm2.One grain from each phase is marked for corresponding selected-areadiffraction observations in (Fig. 13). Grain a = MgAl2O4; Grainb = CeO2; Grain c = CeMgAl11O19. Imaging at higher magnificationwas done on three additional CeMgAl11O19 grains (d, e, f) to bettershow ion track morphology (Fig. 14).

(a)

(b)

(c)

Fig. 13. SAD patterns obtained from the three grains representingas marked in Fig. 12, and indexed based on the crystal structures ofeach phase. (a) MgAl2O4 with [2�11] zone axis, (b) CeO2 with [�111]zone axis, (c) CeMgAl11O19 with [�1100 ] zone axis.

(d)

(e)

(f)

(e′)

Fig. 14. (d)–(f) Cross-section TEM bright-field images ofCeMgAl11O19 regions d, e, and f, as marked in Fig. 12. Thedirection of the Xe ions entering the sample is indicated by arrows ineach micrograph. Regions d and e show dark line contrastinterpreted to be Xe ion irradiation ion track damage. Further in theinterior, no such contrast is observed in grain f, just dislocations andstrain contrast. (e′) is a high-resolution TEM (HRTEM) imageobtained from grain e.

Radiation Damage LnMgAl11O19 7

used for Xe irradiations can further facilitate the damagerecovery. If the damage is recovered by high irradiation tem-peratures then the critical temperature for amorphizationcould be between 500°C and 800°C. On the other hand, ifthe damage is self-annealed due to deposition of energy pre-dominantly in the electronic stopping regime by ions similarto fission fragments (in this case 92 MeV Xe), then magneto-plumbites could be used as a potential candidate for inertmatrix fuels where the damage caused by PKA would beannealed by the fission fragments and high reactor operatingtemperatures. To further support these conclusions, studiesof the detailed irradiation response as a function of tempera-tures and the stability of (001) magnetoplumbite planes needto be conducted, not possible in this study using XRDdue to the low intensities relative to other phases in thecomposites.

IV. Conclusions

Radiation damage studies were carried out on two types ofmagnetoplumbites, LaMgAl11O19 and CeMgAl11O19.LaMgAl11O19 became amorphous after 10 MeV Au ion irra-diation at 500°C to depths of approximately 2 lm. Radia-tion-induced dark lines of contrast were observed inCeMgAl11O19 after irradiation by 92 MeV Xe ions at 800°C,interpreted to be due to ion track damage at depths up to5 lm from the surface, but in this case the magnetoplumbiteremained crystalline. It can be concluded that radiation dam-age in magnetoplumbites can cause amorphization, however,irradiations at temperatures near 800°C with swift heavy ionsmay also facilitate the recovery of damage.

Acknowledgment

The material study was partially supported by National Science Foundationresearch, funding under grant NSF DMR 0606063 for the development ofsuperplastic multiphase ceramics. The opinions expressed are the authors’alone and do not reflect any official endorsement by NSF. The authors wouldlike to thank Dr. Kurt E. Sickafus, Los Alamos National Laboratory, for hissupport on the initial studies. The Xe beamtime was obtained at GrandAcc�el�erateur National d’Ions Lourds. Special thanks to Moidi Toiammou formaking the high-temperature stage, and Isabelle Monnet for carrying out theXe ion irradiations. We would like to acknowledge Jesse Angle for the lantha-nide magnetoplumbite structure (Fig. 1). The TEM work on the FEI Tecnai,JEOL 3010, CM200, assisted by Dr. Chengyu Song, was performed at Law-rence Berkeley Laboratory National Center for Electron Microscopy, which issupported by the Office of Science, Office of Basic Energy Sciences of the U.S.Department of Energy under Contract No. DE-AC02-05CH11231.

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8 Journal of the American Ceramic Society—Men et al.