Cation Sublattice Disorder Induced by Swift Heavy Ions in MgAl2O4

Oct 13, 2007 - F-13108 Saint Paul Lez Durance, France. ReceiVed: April 3, 2007; In Final Form: August 14, 2007. MgAl2O4 and ZnAl2O4 spinels have been ...
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J. Phys. Chem. B 2007, 111, 12707-12714

12707

Cation Sublattice Disorder Induced by Swift Heavy Ions in MgAl2O4 and ZnAl2O4 Spinels: 27Al Solid-State NMR Study Nadia Pellerin,*,† Catherine Dodane-Thiriet,‡ Vale´ rie Montouillout,† Michel Beauvy,‡ and Dominique Massiot† CNRS-CRMHT, 1D aVenue de la recherche scientifique, 45071 Orle´ ans cedex 2, and Laboratoire des Lois du Comportement du Combustible, DRN/DEC/SESC, CEA, CEA Cadarache, F-13108 Saint Paul Lez Durance, France ReceiVed: April 3, 2007; In Final Form: August 14, 2007

MgAl2O4 and ZnAl2O4 spinels have been irradiated by swift heavy ions (86Kr and 36S) simulating the irradiation by fission products for applications in the transmutation targets. The structures of unirradiated and irradiated spinel samples have been studied by NMR spectroscopy, with 27Al magic angle spinning and multiple-quantum magic angle spinning experiments. The parameters of fluence and electronic stopping power have been compared. For 86Kr ions, the obtained spectra are modified by irradiation: we observe a rise of the amount of aluminum in tetrahedral sites and a widening of the lines associated with the different aluminum environments compared with those of the pristine samples. Site exchange in the cationic sublattice is then observed and can be quantified from NMR spectra, determining the inversion parameter. An inversion parameter of 0.77 is estimated for the MgAl2O4 spinel irradiated with 1013 Kr ions/cm2, for a value of 0.275 in the pristine samples. Moreover, a line attributed to aluminum in 5-fold coordination with oxygen is observed in irradiated spinel samples at the maximum fluence for krypton. These new aluminum environments can characterize a transition layer which could change toward an amorphous layer, increasing the electronic stopping power and/or the fluence.

1. Introduction A proposed solution to reduce the harmfulness of radioactive wastes is transmutation. It consists in breaking down heavy nuclei formed in nuclear reactors, such as actinides (neptunium, americium, curium, ...), under neutron bombardment. To be transmuted, these nuclei should be inserted in a matrix that remains stable under high fast neutron irradiation and under internal irradiation due to fission products. Numerous neutron,1-3 electron,4,5 and ion4,6,7 irradiation studies have concluded that spinel structure materials are highly resistant to radiation damage. This spinel radiation resistance is explained by the annihilation of the vast majority of point defects generated during irradiation by interstitial-vacancy recombination, rather than condensing into defect aggregates such as dislocation loops and voids.2 Here temperature plays a determining part. Spinel compounds then form one of the most interesting families for potential transmutation targets. The spinel ceramics are also attractive electrical isolators for use in a radiation environment such as fusion reactors. Among this family, MgAl2O4 has been extensively studied. The spinel AB2O4 structure consists of a cubic cell which belongs to the space group Fd3hm. It is characterized by two types of cation sites: octahedral for the trivalent cation and tetrahedral for the divalent cation. However, this structure is well-known for exhibiting cation exchange between tetrahedral and octahedral interstices in the fcc anion sublattice. This * To whom correspondence should be addressed. E-mail: [email protected]. Phone: (33) 2 38 25 55 25. Fax: (33) 2 38 63 81 03. † CNRS-CRMHT. ‡ CEA Cadarache.

inversion is a function of temperature, impurity content, and thermal history.8-10 The spinel general chemical formula can be written as (A2+1-xB3+x)[A2+xB3+2-x]O4, where the quantity in parentheses represents the average occupancy of tetrahedral sites, while the quantity in brackets represents the average occupancy of octahedral sites. The variable x is called the inversion parameter. This parameter quantifies the cation disorder, specifying the fraction of trivalent ions that occupy tetrahedral sites and the fraction of divalent ions that occupy octahedral sites. When cations are exchanged, the inversion parameter x alters from 0 in a normal spinel to 0.67 in a random spinel to 1 in an inverse spinel. This parameter is usually estimated by various experimental techniques such as X-ray and neutron diffractometry,2,11,12 calorimetry measurements,9,13 ESR (electron spin resonance),8 or solid-state NMR (nuclear magnetic resonance) of 27Al or 17O.9,14-17 An energetic ion moving through a solid encounters a collection of ions and electrons. This moving ion loses energy in two primary ways: the main part of the energy is given up in exciting and accelerating electrons by inelastic processes, a lesser amount being lost in elastic interactions with ions involving displacement cascades and characterized by the dpa parameter (displacements per atom). The inelastic energy losses are characterized by the (dE/dx)e parameter (energy losses per unit length) and specified for the threshold electronic stopping power, noted Se. High-energy particles introduce a number of interstitials and vacancies in crystalline materials via the displacement processes. Swift ions damage a more important sample depth than low ions. The mechanisms of defect production during irradiation by swift heavy ions in the electronic stopping power dominant regime are still not understood. Moreover, data on the accurate behavior of irradiated

10.1021/jp072620t CCC: $37.00 © 2007 American Chemical Society Published on Web 10/13/2007

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Figure 1. Profiles of inelastic energy losses (dotted lines) and dpa (full lines) as a function of the penetration depth of Kr ions in MgAl2O4 (a) and ZnAl2O4 (b) samples described in the text.

spinels are insufficient to explain or predict a material evolution. However, several models are proposed by simulating the damage by radiations of isolating materials.18-20 Independently of the origin of the phenomena, electronic or thermal, the track formation induces the presence of local constraints and distortions in the target material. When the irradiation fluence is sufficiently high, i.e., superior at ∼1013 ions/cm2, the individual tracks overlap and all the irradiated bulk is perturbed. Structural and microstructural modifications of an irradiated MgAl2O4 spinel have been studied by numerous methods, especially by TEM, electron diffraction,1,6,7,21,22 X-ray diffraction,12 optical absorption,5,21 Rutherford backscattering spectrometry,23 or neutron diffraction.2 In the regions of intense damage, there is continuous track formation by any particle whose (dE/dx)e exceeds a critical value characteristic of the irradiated material (∼1-20 keV/nm for insulators).6 The damage region could offer a swelling, amorphous layer,7,21,22 interstitial dislocation loops, or only a crystalline transition layer or disordered crystalline state,6,22 depending on the irradiation conditions such as the temperature, the threshold level for the electronic stopping power, and the fluence. According to calculated displacement damage, the observed phase transformations in swift heavy ion irradiated MgAl2O4 appear to be due to high electronic stopping power effects rather than elastic collision processes.22

Amorphization could be associated with a chemical disordering process (antisite defects).6,7 One example of such chemical disorder is cation mixing. This process was proposed first by Clinard et al.1 and confirmed by Sickafus et al. from neutron diffraction measurements of the inversion parameter in irradiated samples.2 Nevertheless, on the basis of an analysis of intensity spots of TEM diffraction patterns, some authors claim that a new metastable state is reached by MgAl2O4 under ion irradiation, before amorphization of this spinel.24,25 Large efforts are made to determine the structure of these radiation-damaged crystalline phases reached before amorphization of the spinel.2,7,12,24,25 In the present study, MgAl2O4 and ZnAl2O4 spinel ceramics have been irradiated with swift heavy ions (86Kr32+ of 721.5 MeV and 36Sn+ of 338.7 MeV) to simulate the fission product irradiation and recoil atoms from R fission.12 In these conditions, the damage is mostly caused by energy electronic losses. The purpose of the present study is to analyze the structural modifications of the spinel materials induced by irradiation, thanks to NMR spectroscopy. In this way, irradiated and unirradiated spinel ceramics have been investigated by 27Al NMR spectroscopy. This tool allows the local structural and chemical environment of the nuclei to be probed. NMR spectroscopy can be very efficient to characterize and quantify the different aluminum coordination states in crystalline and amorphous materials.26 We propose a method to evaluate the inversion parameter of these ceramics. The radiation resistance of MgAl2O4 and ZnAl2O4 spinels has been compared, and the influence of the two irradiation parameters fluence and electronic stopping power on the local order has been studied. 2. Experimental Methods 2.1. Sample Preparation.12 MgAl2O4 powder is synthesized by the Baı¨kowsky Co. It contains 4 ppm (a.w.) Na, 77 ppm K (a.w.), 11 ppm (a.w.) Fe, 10 ppm (a.w.) Si, and 2 ppm (a.w.) Ca impurities. Moreover, 1.2% (a.w.) of MgO and 0.3% (a.w.) of Al2O3 remain in this powder. ZnAl2O4 powder is prepared from ZnO (99.99% purity) and Al2O3 (99% purity) powders blended and calcinated for 1 h at 1473 K. 2.2. Sample Irradiation.12 Irradiations are performed at room temperature on MgAl2O4 and ZnAl2O4 packed powders at the GANIL facility. The density of these powders is estimated at 50% of the theoretical density. Spinels are irradiated by 86Kr32+ ions with a kinetic energy of 721.5 MeV. Two fluences are systematically tested: a minimum fluence of 5 × 1011 or 1012 ions/cm2 and a maximum fluence of 1013 ions/cm2. According to Zinkle et al., these fluence values allow the spinel ceramics to be damaged without amorphization.22 MgAl2O4 spinels are also irradiated by 36Sn+ ions with a kinetic energy of 338.7 MeV using two fluences of 1012 and 1013 ions/cm2. The program TRIM-98 is used to calculate the inelastic energy losses and the dpa profiles in the two spinels MgAl2O4 and ZnAl2O4 as a function of the penetration depth of the swift heavy ion (Figure 1). The accuracy of the values of the

TABLE 1: Irradiation Conditions of MgAl2O4 and ZnAl2O4 Spinel Samples at the GANIL Facility MgAl2O4 (50% dth)

ZnAl2O4 (50% dth)

ion

energy (MeV)

fluence (ions/cm2)

Se (keV/nm)

depth (µm)

fluence (ions/cm2)

Se (keV/nm)

depth (µm)

86Kr32+

721.5

6.0-8.2

107.3

5 × 1011 1013

7.0-9.0

96.4

36Sn+

338.7

1012 1013 1012 1013

1.5-3.5

156.4

Cation Sublattice Disorder Induced by Heavy Ions

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TABLE 2: 27Al MAS NMR Parameters Concerning the MgAl2O4 Spinel Sample: Pristine and Irradiated Powder Mixturea irradiation (ions/cm2) no

wIP/wTb (%) 0

species [6]

Al(1) Al(2) [4]Al [6] Al(1) [6]Al(2) [4] Al [6]Al(1) [6] Al(2) [4]Al [5] Al [6]Al(1) [6] Al(2) [5]Al [6] Al(1) [6]Al(2) [4] Al [6]

Kr: 1012

45

Kr: 1013

42

S: 1012

78

S: 1013

63

δisoc (ppm)

δCSAd (ppm)

νQe (kHz)

intensity (%)

14.0 12.0 71.5 14.0 11.9 71.4 14.3 12.0 71.5 36.8 14.1 12.0 71.4 14.0 12.0 71.4

4 1 2 4 1 2 4 1 1

473 91755 2 484 908 566 55399 6 736 / 486 910 550 492 937 561

62 24 14 61 25 14 59 17 22 2 63 23 14 64 22 14

4 1 2 4 1 2

a A Gaussian model has been used for the [5]Al line. b wIP/wT ) weight proportion of the irradiated powder in the analyzed powder mixture. c δiso ) isotropic chemical shift. d δCSA ) width of the Gaussian distribution of δiso. e νQ ) quadrupolar coupling frequency.

penetration depth is (0.8 µm. The irradiation characteristic parameters for studied samples are presented in Table 1. 2.3. NMR Experiments. For NMR experiments, all the samples (typically 300 mg of powder) are ground into a fine powder. Considering the irradiation conditions, samples analyzed after irradiation are then mixtures of irradiated and nonirradiated powders. The weight proportion of irradiated powder, calculated from the irradiation parameters (Table 1 and Figure 1), is indicated in Table 2. The high-resolution 27Al NMR experiments are carried out at room temperature, using a Bruker spectrometer operating at 17.6 T (750 MHz), with a 4 mm ZrO2 rotor spinning at typically 14 kHz. Frequencies are referenced to aluminum resonance in a molar solution of Al(NO3)3 at 0 ppm. The conventional singlepulse detection method is used. A small pulse angle (typically 1 µs at π/24) is applied to ensure a quantitative excitation of the central transition.27,28 The spectra correspond to the summing of 4096 transients with a recycle delay of 1 s. The multiple-quantum magic angle spinning (MQMAS) experiments are acquired using the z-filtered pulse sequence29 and a synchronized acquisition of the indirect dimension.30 Triple-quantum excitation and conversion are achieved under high-power irradiation (rf field of 110 kHz), and z-filtered generation is achieved with a low-power 90° selective pulse (rf field of 30 kHz). 3. Results 3.1. Pristine Spinels. The 27Al MAS NMR spectra at 750 MHz for pristine spinels MgAl2O4 and ZnAl2O4 are presented in Figure 2. Compared to low-field spectra, high-field spectra exhibit high resolution with reduced second-order quadrupolar shifts and broadenings and enhanced chemical shift distribution effects. These spectra are consistent with those of previous workers for MgAl2O4 and ZnAl2O4.9,10,16,31 They show the main peaks of the central transition (CT) 〈-1/2, +1/2〉 and their sets of spinning sidebands relevant, respectively, to the satellite transitions (STs) 〈(3/2, (1/2〉 (the satellite transitions 〈(5/2, (3/2〉 remain unobservable). In both compounds, the isotropic chemical shift of the dominant peak, on the left part of the resonance due to the second quadrupolar order, is in the range

Figure 2. 27Al MAS NMR spectra at 17.6 T of pristine MgAl2O4 and ZnAl2O4 spinels.

Figure 3. 27Al MQMAS NMR spectrum at 17.6 T for the pristine MgAl2O4 spinel.

of 10-20 ppm attributed to a 6-fold-oxygen-coordinated aluminum species (noted [6]Al). For MgAl2O4, a second peak is clearly present with an isotropic chemical shift in the range of 70-80 ppm, characteristic of aluminum in 4-fold coordination with oxygen (noted [4]Al). This peak indicates clearly a Mg2+/Al3+ site exchange within the crystal lattice. Moreover, the MQMAS experiment (Figure 3),32,30 where the 27Al resonances are spread in two dimensions, according to their isotropic chemical shifts and quadrupolar couplings, shows clearly the presence of two different [6]Al components: a sharp line corresponding to [6]Al(1) (at higher chemical shift) and a broad line assigned to [6]Al(2) (at lower chemical shift). These [6]Al components are characterized by a different isotropic chemical shift, and the mean quadrupolar coupling constant of [6]Al(2) seems much higher. This indicates unambiguously the presence of a second, distorted, [6]Al line. This component has previously been proposed by other authors.10,17 Indeed, the presence of Mg2+ in octahedral sites locally modifies the environment of a part

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TABLE 3: Inversion Parameter for the Irradiated Powder Part, xIP, Deduced from the Inversion Parameter for the Total Powder Mixture, xexp MgAl2O4 irradiation (ions/cm2) xexpa xIPb xIP - xNIPc c

no 0.275

ZnAl2O4 Kr: 1012 0.29 0.30 0.02

Kr: 1013 0.48 0.77 0.49

S: 1012 0.27 0.27 0

S: 1013 0.28 0.29 0.01

no 0.035

Kr: 5 × 1011 0.05 0.09 0.05

Kr: 1013 0.23 0.54 0.50

a Inversion parameter for the total irradiated powder mixture: direct experimental value. b Inversion parameter for the irradiated powder part. Increment of the inversion parameter according to the nonirradiated powder.

transitions of the spinning sidebands with the CT and adding the intensity of the CT spinning sidebands ([6]Al species). In the case of pristine ZnAl2O4 spinels, the complex signal obtained at low field, due to the presence of numerous overlapping resonances, is largely reduced at high field. We observe then at 750 MHz a sharp resonance in the range of hexacoordinated aluminum ([6]Al) (15-17 ppm). The presence of a very small amount of tetracoordinated aluminum (less than 2%) can be noticed. The spectra are simulated, as previously described, using the Dmfit program. To quantify the cation exchange in the spinels, the inversion parameter x is calculated. This parameter (Table 3) is deduced from eq 1, where I[6] and I[4] are the amounts of aluminum in, respectively, 6-fold and 4-fold coordination.

x ) 2/(1 + I[6]/I[4])

Figure 4. 27Al MAS NMR spectra at 17.6 T for different fluences of krypton ions: comparison of irradiated and nonirradiated samples of (a) MgAl2O4 and (b) ZnAl2O4. Intensities are normalized on the main peak.

of the [6]Al. The spectra obtained for pristine MgAl2O4 spinels are simulated using the Dmfit program.33 According to the specific asymmetric line shape of the observed peaks (a steep low-field edge and a long tail on the right side), spectra are simulated taking into account a Gaussian distribution of isotropic chemical shifts and a statistic distribution of charges (Gaussian isotropic model (GIM)), leading to a distribution of the electric field gradient.34,35 The full modeling of the 27Al spectrum of the unirradiated MgAl2O4 sample consists in one [4]Al line relative to the tetrahedral units and two [6]Al lines relative to the octahedral sites. The isotropic chemical shift mean value and distribution, and the mean quadrupolar parameters are calculated for each line (Table 2). The proportion of the different species is calculated by integrating the intensities of the central transition (Table 2), taking into account the overlapping outer

(1)

The inversion parameters, calculated with a very good precision, are 0.035 ( 0.002 for pristine ZnAl2O4 and 0.275 ( 0.005 for MgAl2O4. The error is estimated from the reproducibility of the simulation process. These results are in good agreement with values determined by other authors from neutron diffraction, X-ray diffraction, and NMR spectroscopy.15,36 3.2. MgAl2O4 and ZnAl2O4 Spinel Samples Irradiated with Krypton Ions. Figure 4 compares the NMR 27Al spectra of spinels MgAl2O4 and ZnAl2O4 irradiated with krypton at two different fluences to those of the respective nonirradiated spinel reference spectrum. Nearly no difference is observed with the spectrum of the MgAl2O4 compound irradiated with the lowest fluence, but the spectrum of the sample irradiated with a fluence of 1013 Kr ions/cm2 shows clearly a broadening of the [6]Al line and especially of the [4]Al line, as well as the presence of a new peak with a maximum close to 35 ppm. The broadening of the [4]Al and [6]Al lines is due to an increase of the quadrupolar coupling constant, especially in the case of a tetrahedral environment, consistent with an increased disorder. The 35 ppm line unambiguously indicates the presence of a new Al environment which can be assigned to aluminum in 5-fold ([5]Al) coordination with oxygen, according to its chemical shift.27 This [5]Al site has previously been observed in poorly crystalline or amorphous precursors of the MgAl2O4 spinel31 and is a proof of disorder creation as a partial destruction of the spinel structure. Recently, an increase of the AlO5 unit part has also been observed in oxide and oxynitride glasses under Sn irradiation.37 Due to the low intensity of this additional resonance, its signal cannot be observed in the MQMAS experiment (not shown). All the spectra were simulated, as previously described, and the obtained NMR parameters are presented in Table 2. Similarly, the ZnAl2O4 spectrum shows a significant difference after an irradiation of 1013 Kr ions/cm2. We observe a broadening of the [6]Al signal and a new signal at around 65 ppm, obviously assigned to the [4]Al environment. We can also distinguish a weak signal with an isotropic chemical shift in

Cation Sublattice Disorder Induced by Heavy Ions

Figure 5. samples.

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27

Al MAS NMR spectra at 17.6 T for different fluences of sulfur ions: comparison of irradiated and unirradiated MgAl2O4 spinel

the range of 30-40 ppm, probably due to a small amount of pentacoordinated aluminum. Other authors have observed this coordination in an amorphous gel network.38 As in the case of MgAl2O4, these new contributions indicate an increase of chemical disorder compatible with cation mixing and amorphization. For the irradiated samples, x parameter calculation cannot be directly deduced from eq 1. We have to take into account both the presence of [5]Al species and the proportion of powder effectively irradiated. [5]Al species being associated with amorphous structures, they can be considered as structural defects, as well as [4]Al. The contributions of the [4]Al and [5]Al signals are thus added in eq 1. As already mentioned in the Experimental Methods, the powders studied by NMR are mixtures of nonirradiated powder, NIP (inversion parameter equal to that of the reference spinel), and irradiated powder, IP (inversion parameter unknown). Moreover, the weight of irradiated powder, wIP , is deduced from the penetration depth of the ions and the pellet density (estimated at 50% of the theoretical density). Considering that the observed spectrum for the mixture results in the weighted sum of the respective spectrum for the pure irradiated powder part and pure pristine powder, the experimental inversion parameter verifies that wTxexp ) wIPxIP + wNIPxNIP , where xexp is the inversion parameter calculated from the experimental spectrum of the mixture. We thus can estimate the average inversion parameter associated with the irradiated powder part from the inversion parameters calculated for the mixture, xexp, and the pristine sample, xNIP , with eq 2 (Table 3). It should be noticed that the main error in the xIP parameter is due to the determination of the weights wIP and wNIP . We estimate that, for ∆wIP/wIP ) ∆wNIP/wNIP ) 10% and ∆xexp/xexp ) 1%, the relative error ∆xIP/ xIP approaches 20%.

[ (

xIP ) xexp -

)]

xNIPwNIP wNIP + wIP wNIP + wIP wIP

(2)

We observe for both spinel samples that the calculated inversion parameter of the irradiated part increases with the fluence, up to the rather high values of 0.77 for the MgAl2O4 spinel and 0.54 for the ZnAl2O4 spinel. At 1013 Kr ions/cm2, the fluence parameter appears very efficient in increasing cationic mixing. 3.3. MgAl2O4 Spinel Irradiation with Sulfur Ions. Compared to krypton, sulfur has a lower weight (36S, 5 × 1026 neutrons/m2 and

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Figure 6. Repartition of aluminum in the different environments, [6]Al(1), [6]Al(2), [4]Al, and [5]Al, for pristine MgAl2O4 or the pure powder part irradiated by sulfur ions (a) and krypton ions (b). Black circles and asterisks represent the ratios of intensities I([4]Al)/I([6]Al(2)) and I([4]Al)/I([6]Al(1)).

En > 0.1 MeV). These results are very similar to our own NMR results at 1013 Kr ions/cm2 on a MgAl2O4 powder sample. E. A. Cooper et al. have examined by 27Al MAS NMR the same neutron-irradiated MgAl2O4 single crystals.15 By reason of a lack of resolution, at low field and low spinning speed, and of the complexity of spectral modelization, these authors obtained very high values for the inversion parameters, resulting in a nearly inverse structure, inconsistent with previous neutron diffraction determination. R. L. Millard et al. have demonstrated the deciding part of different NMR acquisition parameters, especially the sample spinning speed, for the disorder calculation.14 Similarly, measurements of the inversion parameter have been made from X-ray diffraction and Rietveld analysis in the case of the ZnAl2O4 spinel. The authors obtained 0.075 for unirradiated samples and 0.39 for irradiated samples in quite similar irradiation conditions by Kr ions at high fluence.12 These results are comparable with our own values obtained from NMR studies. In the present study, NMR experiments are performed at a very high field (17.6 T), producing a clear improvement of resolution and sensitivity as compared to previous works and allowing good agreement of the inversion parameter values with results from the usual techniques and analysis of the spinel structure. This good agreement validates our NMR spectral interpretation. 27Al NMR spectra show that irradiation induces broadening of the lines, in agreement with an increment of the chemical environment distribution, indicated by the increase of the isotropic chemical shift and electric field gradient distributions.

Pellerin et al. NMR line broadening is generally consistent with amorphization. The apparition of a pentacoordinated environment for aluminum confirms this evolution. The substitution of aluminum ions by magnesium (or zinc) causes formation of cation vacancies necessary to maintain the charge neutrality.39,40 As these vacancies, the aluminum 5-fold-coordinated with oxygen contributes to the maintenance of charge neutrality. Some precedent results have before reported amorphization induced by heavy ions: by TEM and electron diffraction for the MgAl2O4 spinel irradiated by 72 MeV I ions (fluence of 1016 I ions/cm2) 22 and by X-ray diffraction study.12 Moreover, as the heat treatment, irradiation leads to a decrease of the lattice parameters of the MgAl2O4 spinel according to X-ray diffraction measurements.40 That involves a variation of the cation-oxygen bond lengths. According to the ionic radii of Al and Mg for tetrahedrally or octahedrally coordinated ions (obtained by Shannon and Prewitt), Mg-Al exchange involves a contraction of the tetrahedral site and an expansion of the octahedral site. In the case of ZnAl2O4, H. St. C. O’Neill et al. have observed an increase of the lattice parameters with the inversion parameter according to powder X-ray diffraction structural refinements.41 Transformations of the spinel material during irradiation are also involved in the atomic displacements and the lattice distortions introduced by ions implanted into the spinel structure.21 NMR is sensitive to such local changes of the Al environment. Addition of these different and complex processes contributes to a distribution of NMR parameters and to the observed line broadening. Comparing spinel samples in the same conditions, the pristine or irradiated ZnAl2O4 spinel always has a more reduced cationic disorder than MgAl2O4 (Table 3). This ascertainment can be interpreted as a function of the preference for the tetrahedral or octahedral site in the spinel structure. Navrotsky et al. have shown that Zn2+ has a high preference for the tetrahedral site, Al3+ has a pronounced preference for the octahedral site, and Mg2+ shows no preference.13 According to their thermodynamic analysis, if the difference between the site preference energies of the two cations is large, a completely normal or a completely inverse structure is predicted, which is verified here by the inversion parameter obtained for pristine spinels. According to Table 3, the increment of the inversion parameter between pristine and irradiated samples at 1013 Kr ions/cm2 is about 0.5 for both MgAl2O4 and ZnAl2O4 spinels. Nevertheless, the Se parameter is higher in ZnAl2O4 than in MgAl2O4 (Table 1). In another part, at 1013 Kr ions/cm2, the fraction of pentacoordinated aluminum in the pure irradiated sample part is 4% in MgAl2O4 and only 1% in ZnAl2O4. The tendency for amorphization is then more significant in the case of MgAl2O4, staying however rather low. The ZnAl2O4 spinel samples seem then to be more stable than MgAl2O4 spinels when the local disorder and amorphization tendency are considered. A more extensive study would be necessary to confirm these observations. With the local probe point of view, the NMR results are in agreement with the hypothesis that irradiation induces an orderdisorder phase evolution. The broadening of the 27Al lines, the increase of the tetrahedral environment, and the appearance of the pentahedral environment indicate an increase of local disorder. According to TEM and X-ray diffraction diagram interpretation, a second hypothesis has been proposed by some authors: a phase change with a modification of the space group and the unit cell parameters.7,25 D. Simeone et al. have shown the difficulty of interpretation of X-ray diffraction diagrams since the X-ray atomic form factors associated with Mg and Al

Cation Sublattice Disorder Induced by Heavy Ions atoms are quite similar,12 which can lead to a false determination of the space group of MgAl2O4. These authors have also validated the space group of the irradiated MgAl2O4 spinel by Raman spectroscopy. The irradiation conditions implemented in our experiments on MgAl2O4 and ZnAl2O4 spinel ceramics allow comparison of the impact of fluence and electronic stopping power parameters on cationic disorder, which indicates irradiation damage. According to 27Al NMR, the histogram of Figure 6 presents the relative amount of aluminum in the different environments in the pure irradiated powder part compared to the pristine powder: [6]Al(1) and [6]Al(2) for the octahedral sites, [4]Al for the tetrahedral sites, and [5]Al for the pentacoordinated units. According to Table 3 and Figure 6, cationic exchange increases with the electronic stopping power and with the fluence. However, for this studied range of values, the Se parameter is largely dominant: at the same fluence, krypton irradiation induces a cation exchange much more important than that of sulfur. The fluence increment gives rise to an increase of aluminum in the tetrahedral site essentially detrimental to the [6]Al(2) component, rather than [6]Al(1). This behavior is very pronounced for krypton irradiation at high fluence, whereas for sulfur irradiation the results are consistent with a possible reordering of the octahedral site, the [6]Al(1) line intensity increasing slightly, in agreement with calculated inversion parameters. These results show that a damaging threshold for fluence must certainly be considered. Many authors have studied the variation of the inversion parameter with temperature for a thermodynamic modeling of Mg-Al disordering. A compilation is presented by G. B. Andreozzi et al. for the MgAl2O4 spinel.42 They measure a variation of the inversion parameter from 0.18 to 0.29 between 600 and 1100 °C by X-ray diffraction on a single crystal. A maximal value of ∼0.35 at 1500 °C has been obtained by H. Maekawa et al. by in situ high-temperature 27Al NMR.17 Considering these measurments, we can conclude that no heat treatment of the spinel (up to 1500 °C) could generate such an important cationic mixing, as observed here with krypton irradiation. N. Kashii et al. have analyzed by 27Al NMR heat-treated spinel samples between 700 and 1100 °C for different periods of time. They have simulated the spectra with two components for the octahedral aluminum environment signal and obtained a rather different behavior, compared to that of the irradiated samples. They observe an increase of both [6]Al(2) and [4]Al components when the duration of heat treatment increases and a ratio of [6]Al(2) to [4]Al around 5-6.10 For irradiated samples, we note here that the intensity for the [6]Al(2) environment decreases when [4]Al increases, the [6]Al(1) component being slightly larger at low Se parameter and lightly reduced for krypton irradiation at the maximum fluence. Disordering mechanisms in irradiation conditions and heat treatment are then quite different. In the case of MgAl2O4 irradiated with krypton at the maximum studied fluence, the presence of aluminum pentacoordinated is certainly responsible for the low fraction of the [6]Al(2) component, which corresponds to the more distorted octahedral units. These new aluminum environments can characterize a transition layer which could change toward an amorphous layer, increasing the electronic stopping power and/ or the fluence, as observed by transmission electron microscopy or deduced by microdiffraction results.7,25 A thermal treatment of the irradiated spinel ceramic could perhaps be efficient to restore the local structure. This phenomenon has been observed in the case of MgAl2O4 single crystals15 which had been

J. Phys. Chem. B, Vol. 111, No. 44, 2007 12713 irradiated with neutrons to high doses at various temperatures: the specimen for 137 dpa at 750 °C has a lower inversion than the 137 dpa at 400 °C sample, due to thermal reordering, which partially restores some of the disordering obtained by irradiation. 5. Conclusion 27Al NMR spectroscopy has been used to analyze the structural transformations of spinel ceramics MgAl2O4 and ZnAl2O4 after irradiation with swift heavy ions. Compared to pristine samples, irradiation of these spinel ceramics gives rise to an increase of cation mixing, an evolution of the aluminum octahedral units’ distribution, and the formation of a small amount of aluminum in 5-fold coordination with oxygen (4% of the aluminum in MgAl2O4). These [5]Al units, generally observed in amorphous spinel ceramics, characterize an evolution toward amorphization. The inversion parameter has been determined: the largest value (x ) 0.77) is obtained for the MgAl2O4 spinel irradiated with krypton ions under a fluence of 1013 Kr ions/cm2. Inversion parameter values obtained here from NMR spectral interpretation are in agreement with the values calculated from more usual techniques such as X-ray diffraction. NMR results show clearly the preponderance of the electronic stopping power parameter compared to the fluence parameter according to local structure evolution, which testifies to irradiation damages. Thanks to cation exchange aptitude, the local structure of MgAl2O4 and ZnAl2O4 spinel ceramics appears highly resistant to irradiation by swift heavy ions and develops little amorphization.

Acknowledgment. We acknowledge financial support from CNRS UPR4212, FR2950, MIAT, and the Re´gion Centre, and we are grateful to Drs. F. Fayon and P. Florian for stimulating discussions. References and Notes (1) Clinard, F. W.; Hurley, G. F., Jr.; Hobbs, L. W. J. Nucl. Mater. 1982, 108, 109, 655. (2) Sickafus, K. E.; Larson, A. C.; Yu, N.; Nastasi, M.; Hollenberg, G. W.; Garner, F. A.; Bradt, R. C. J. Nucl. Mater. 1995, 219, 128. (3) Redfern, S. A. T.; Harrison, R. J.; O’Neill, H. St. C.; Wood, D. R. R. Am. Mineral. 1999, 84, 299. (4) Satoh, Y.; Kinoshita, C.; Nakai, K. J. Nucl. Mater. 1991, 179181, 399. (5) Summers, G. P.; White, G. S.; Lee, K. H.; Crawford, J. H., Jr. Phys. ReV. B 1980, 21, 2578. (6) Zinkle, S. J.; Skuratov, V. A. Nucl. Instrum. Methods Phys. Res., B 1998, 141, 737. (7) Yu, N.; Sickafus, K. E.; Nastasi, M. Philos. Mag. Lett. 1994, 70, 235. (8) Schmocker, U.; Waldner, F. J. Phys. C: Solid State Phys. 1976, 9, L235. (9) Wood, B. J.; Kirkpatrick, R. J.; Montez, B. Am. Mineral. 1986, 71, 999. (10) Kashii, N.; Maekawa, H.; Hinatsu, Y. J. Am. Ceram. Soc. 1999, 82, 1844. (11) Peterson, R. C.; Lager, G. A.; Hitterman, R. L. Am. Mineral. 1991, 76, 1455. (12) Simeone, D.; Dodane-Thiriet, C.; Gosset, D. J. Nucl. Mater. 2002, 2, 3, 151. (13) Navrotsky, A.; Kleppa, O. J. J. Inorg. Nucl. Chem. 1967, 29, 2701. (14) Millard, R. L.; Peterson, R. C.; Hunter, B. K. Am. Mineral. 1992, 77, 44. (15) Cooper, E. A.; Hughes, C. D.; Earl, W. L.; Sickafus, K. E.; Hollenberg, G. W.; Garner, F. A.; Bradt, R. C. Mater. Res. Soc. Symp. Proc. 1995, 373, 413. (16) Gobbi, G. C.; Christoffersen, R.; Otten, M. T.; Miner, B.; Buseck, P. R.; Kennedy, G. J.; Fyfe, C. A. Chem. Lett. 1985, 771. (17) Maekawa, H.; Kato, S.; Kawamura, K.; Yokokawa, T. Am. Mineral. 1997, 82, 1125. (18) Szenes, G. Nucl. Instrum. Methods Phys. Res., B 1996, 116, 141. (19) Fleischer, R. L.; Price, P. B.; Walker, R. M. J. Appl. Phys. 1965, 36, 3645.

12714 J. Phys. Chem. B, Vol. 111, No. 44, 2007 (20) Itoh, N.; Tanimura, K. Radiat. Effects 1986, 98, 269. (21) Afanasyev-Charkin, I. V.; Dickerson, R. M.; Cooke, D. W.; Bennett, B. L.; Gritsyna, V. T.; Sickafus, K. E. J. Nucl. Mater. 2001, 289, 110. (22) Zinkle, S. J.; Matzke, H. J.; Skuratov, V. A. Mater. Res. Soc. Symp. Proc. 1999, 540, 299. (23) Gentils, A. Ph.D. Thesis, University of Paris-Sud XI, 2003. (24) Devanathan, R.; Sickafus, K. E.; Yu, N.; Nastasi, M. Philos. Mag. Lett. 1995, 72, 155. (25) Ishimaru, M.; Afanasyev-Charkin, I. V.; Sickafus, K. E. Appl. Phys. Lett. 2000, 76, 2556. (26) Massiot, D.; Cote, B.; Taulelle, F.; Coutures, J.-P. 27Al MAS NMR of crystalline and Amorphous Materials. In Application of NMR Spectroscopy to Cement Science; Colombet, P., Grimmer, A. R., Eds.; Gordon and Breach: New York, 1994; pp 153-69. (27) Samoson, A.; Lippmaa, E. Phys. ReV. B: Condens. Matter 1983, 28, 6567. (28) Alemany, L. B.; Massiot, D.; Sherrif, B. L.; Smith, M. E.; Taulelle, F. Chem. Phys. Lett. 1991, 177, 301. (29) Amoureux, J.-P.; Fernandez, C.; Steuernagel, S. J. Magn. Reson., Ser. A 1996, 123, 116. (30) Massiot, D. J. Magn. Reson., Ser. A 1996, 122, 240. (31) Montouillout, V.; Massiot, D.; Douy, A.; Coutures, J.-P. J. Am. Ceram. Soc. 1999, 82, 3299.

Pellerin et al. (32) Frydman, L.; Harwood, J. S. J. Am. Chem. Soc. 1995, 117, 5367. (33) Massiot, D.; Fayon, F.; Capron, M.; King, I.; Le Calve´, S.; Alonso, B.; Durand, J.-O.; Bujoli, B.; Gan, Z.; Hoatson, G. Magn. Reson. Chem. 2002, 40, 70. (34) Le Caı¨r, G.; Brand, R. A. J. Phys.: Condens. Matter 1998, 10, 10715. (35) Neuville, D. R.; Cormier, L.; Massiot, D. Geochim. Cosmochim. Acta 2004, 68, 5071. (36) Cooley, R. F.; Reed, J. S. J. Am. Ceram. Soc. 1972, 55, 395. (37) Dauce´, R.; Le Floch, M.; Sangleboeuf, J.-C.; Verdier, P. J. NonCryst. Solids 2007, 353, 390. (38) Mathur, S.; Veith, M.; Haas, M.; Shen, H.; Lecerf, N.; Huch, V.; Hu¨fner, S.; Haberkorn, R.; Beck, H. P.; Jilavi, M. J. Am. Ceram. Soc. 2001, 84, 1921. (39) Ibarra, A.; Vila, R.; Jimenez de Castro, M. Philos. Mag. Lett. 1991, 64, 45. (40) Skvortsova, V.; Mironova-Ulmane, N.; Ulmanis, U. Nucl. Instrum. Methods Phys. Res., B 2002, 191, 256. (41) O’Neill, H. St. C.; Dollase, W. A. Phys. Chem. Minerals 1994, 20, 541. (42) Andreozzi, G. B.; Princivalle, F.; Skogby, H.; Della Giusta, A. Am. Miner. 2000, 85, 1164.