Article pubs.acs.org/cm
Cerium Substitution in Yttrium Iron Garnet: Valence State, Structure, and Energetics Xiaofeng Guo,† Amir H. Tavakoli,† Steve Sutton,‡,§ Ravi K. Kukkadapu,∥ Liang Qi,⊥ Antonio Lanzirotti,‡ Matt Newville,‡ Mark Asta,⊥ and Alexandra Navrotsky*,† †
Peter A. Rock Thermochemistry Laboratory and NEAT ORU, University of California, Davis, California 95616, United States Center for Advanced Radiation Sources, University of Chicago, Chicago, Illinois 60637, United States § Department of Geophysical Sciences, University of Chicago, Chicago, Illinois 60637, United States ∥ Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352, United States ⊥ Department of Materials Science and Engineering, University of California, Berkeley, California 94720, United States ‡
S Supporting Information *
ABSTRACT: The garnet structure is a promising nuclear waste form because it can accommodate various actinide elements. Yttrium iron garnet, Y3Fe5O12 (YIG), is a model composition for such substitutions. Since cerium (Ce) can be considered an analogue of actinide elements such as thorium (Th), plutonium (Pu), and uranium (U), studying the local structure and thermodynamic stability of Ce-substituted YIG (Ce:YIG) can provide insights into the structural and energetic aspects of large ion substitution in garnets. Single phases of YIG with Ce substitution up to 20 mol % (Y3−xCexFe5O12 with 0 ≤ x ≤ 0.2) were synthesized through a citrate−nitrate combustion method. The oxidation state of Ce was examined by X-ray absorption near edge structure spectroscopy (XANES); the oxidation state and site occupancy of iron (Fe) as a function of Ce loading also was monitored by 57Fe− Mö ssbauer spectroscopy. These measurements establish that Ce is predominantly in the trivalent state at low substitution levels, while a mixture of trivalent and tetravalent states is observed at higher concentrations. Fe was predominately trivalent and exists in multiple environments. High temperature oxide melt solution calorimetry was used to determine the enthalpy of formation of these Cesubstituted YIGs. The thermodynamic analysis demonstrated that, although there is an entropic driving force for the substitution of Ce for Y, the substitution reaction is enthalpically unfavorable. The experimental results are complemented by electronic structure calculations performed within the framework of density functional theory (DFT) with Hubbard-U corrections, which reproduce the observed increase in the tendency for tetravalent Ce to be present with a higher loading of Ce. The DFT+U results suggest that the energetics underlying the formation of tetravalent Ce involve a competition between an unfavorable energy to oxidize Ce and reduce Fe and a favorable contribution due to strain-energy reduction. The structural and thermodynamic findings suggest a strategy to design thermodynamically favorable substitutions of actinides in the garnet system. KEYWORDS: cerium, yttrium iron garnet, nuclear waste form, calorimetry, X-ray absorption spectroscopy, Mössbauer spectroscopy, density functional theory
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INTRODUCTION The radiotoxicity and long half-lives associated with high level waste (HLW) generated from nuclear reactor fuels has driven the development of solidification/stabilization technologies that can potentially immobilize the nuclear waste as a more durable form for long-term storage and/or geologic disposal. Although vitrification, the incorporation of actinides in a mostly glassy matrix,1,2 is the method of choice in the United States at present, more stable matrices consisting of crystalline phases have been proposed as the basis for alternative waste forms.3 Since the early 1970s, various tailored ceramic waste forms containing mineral phases such as monazite, pyrochlore, zircon, zirconolite, and defect fluorite4 have been proposed. Garnet as a new storage form for radioactive waste5−7 has been of interest © XXXX American Chemical Society
recently due to its large loading of actinides and chemical flexibility. The long-term stability of natural U-bearing garnet,8 one sample containing ∼27 wt % U that was discovered in the Northern Caucasus, Russia, suggests that garnet can be an excellent host for actinides. Various uranium-bearing garnets have been synthesized,5,6,9−11 and to date, a maximum uranium content of ∼30 wt % in synthetic iron garnet based ceramics6,12 has been reported. Investigations of radiation tolerance of garnets indicate that their average amorphization dose is Received: October 18, 2013 Revised: December 11, 2013
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comparable to that of zircon.11,13,14 In addition, the radiation response of garnet was found to be topologically constrained and less related to chemical composition,14 implying that a variety of garnet compositions as nuclear waste hosts will have predictable radiation tolerance. Despite this interest, the thermodynamics of actinide substitution in garnets has not been studied experimentally. In this work, we selected yttrium iron garnet (YIG) as the host for calorimetric studies of the substitution of cerium (an actinide analogue) because of its simple end-member composition and large lattice parameter, which potentially favors the incorporation of large ions. The structure of YIG (Figure 1), Y3cFe2aFe3dO12 (Ia3d, Z = 8), has three types of
by oxide melt solution calorimetry, enabling calculation of the energetics of cerium substitution. The enthalpy plus the calculated configurational entropies of substitution enable calculation of the free energy, which defines the thermodynamic stability of cerium substitution in these garnets.
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EXPERIMENTAL METHODS
The garnet samples were synthesized using a citrate−nitrate combustion method.22−24 Stoichiometric mixtures of Y(NO3)3·6H2O (Alfa Aesar, 99.9%), Fe(NO3)3·9H2O (Sigma-Aldrich, 99.99%), and (NH4)2Ce(NO3)6 (Alfa Aesar, 99%) were dissolved in an aqueous solution of citric acid monohydrate (Alfa Aesar, 99.9%), where the ratio of citric acid to nitrate was kept equal to 0.75.23 The solutions were stirred with a magnetic bar (to ensure homogeneity) while being evaporated by heating at ∼90 °C until viscous gels formed. The gels were heated to 350 °C in about 2 h for drying. Brownish-black aggregates of loose powders were obtained through self-propagating combustion24 of the dried powders. Finally, the obtained burnt powders were calcined in the air at 1300 °C for 24 h. Chemical composition and homogeneity were determined by electron microprobe analysis (EMPA; wavelength dispersive spectroscopy, WDS) using a Camera SX-100 electron microprobe (15 kV accelerating voltage, 10 nA beam current, and a spot size of 1 μm). The analytical standards are yttrium aluminum garnet (YAG), hematite (Fe2O3), and cerianite (CeO2). Phase purity was examined by powder X-ray diffraction (XRD) using a Bruker D8 diffractometer (Cu Kα radiation, 40 kV, 30 mA). The XRD patterns were further analyzed for the determination of lattice parameters using a whole profile fitting procedure (Jade v6.11, 2002, Materials Data Inc., Livermore, CA). To evaluate the oxidation state of Ce, XANES spectroscopy was performed at the GSECARS X-ray microprobe beamline (13-ID-E) at the Advanced Photon Source (APS), Argonne National Laboratory (Argonne, IL USA). Detailed information on procedures can be found in the literature.25 XANES spectra were collected in fluorescence mode with a four-element, silicon-drift-diode, solid-state, X-ray fluorescence detector (Vortex-ME4, Hitachi High-Technologies Science America, Inc.) or in transmission mode with He-(I0) and N2-(I1) filled ion chambers. XANES spectra were obtained by scanning the monochromator through the Ce LIII absorption edge (∼5725 eV) and recording both the Ce Lα fluorescence and total absorption. The energy step sizes were 5 eV from 5623 to 5713 eV, 0.25 eV from 5713 to 5748 eV, and 1.0 eV from 5748 to 6100 eV. Dwell time at each energy step was 1 s, and up to eight spectra were collected and summed to improve signal-to-noise. Energy calibration was obtained using either an Fe metal foil (first derivative peak defined to be 7110.75 eV) or a Ti metal foil (first derivative peak defined to be 4966 eV). Four valence standards were measured: Ce(III)PO4, Ce(IV)O2, Ce(III)-nitrate (Ce(NO3)3·6H2O), and Ce(IV)-nitrate ((NH4)2·Ce(NO3)6). The samples and standards were prepared as thin powder layers mounted between Scotch tape. The X-ray absorption spectroscopy data processing software Athena26 was used for analysis. Mössbauer spectra were collected using a 50-mCi (initial strength) 57 Co/Rh source. The velocity transducer MVT-1000 (WissEL) was operated in a constant acceleration mode (23 Hz, ± 12 mm/s). An Ar−Kr proportional counter was used to detect the radiation transmitted through the holder, and the counts were stored in a multichannel scalar (MCS) as a function of energy (transducer velocity) using a 1024 channel analyzer. Data were folded to 512 channels to give a flat background and a zero-velocity position corresponding to the center shift (CS or d) of a metal iron foil at room temperature (RT). Calibration spectra were obtained with a 25-μmthick α-Fe(m) foil (Amersham, England) placed in the same position as the samples to minimize errors due to geometry changes. A closedcycle cryostat (ARS, Allentown, PA) was employed below RT measurements. Sample preparation is similar to the previously reported procedures.27 High-temperature oxide melt solution calorimetry was applied to the obtained garnet samples to obtain enthalpies of drop-solution from
Figure 1. Schematic figure of garnet structure. The translucent polyhedra are c sites. The dashed octahedra are a cites, and the gray tetrahedra are d cites.
polyhedra. FeO4 tetrahedra and FeO6 octahedra are joined alternately by sharing corners, and both together share edges with YO8 dodecahedra to form a three-dimensional framework.15−17 The 24c eight-coordinated dodecahedral sites can be occupied by large divalent, trivalent, and tetravalent cations, and 16a six-coordinated octahedral sites can be occupied by trivalent and tetravalent cations.16,18 Thus, both of these sites can potentially incorporate actinides (or lanthanides) depending on the charge balance,16,19 while the 8-coordinated site is preferred over the octahedral site to accommodate larger tetravalent ions due to its larger dimensions. Substitution of uranium in garnet is complex because uranium can occur in different valence states, from tetravalent to hexavalent, each with distinct bond lengths and preferences in site occupancy. This has led to assigning a charge-unbalanced crystal-chemical formula8 for natural uranium garnet. Controversy also exists regarding the valence states of iron in uranium-substituted garnets.20,21 Straightforward substitution of U for Y in YIG requires U cations to be tetravalent, which is hard to control, since oxidation can occur easily. Therefore, substitution of other cations such as Ce and Th, which are analogous to tetravalent U in terms of valence state and size, is a useful approach to probe tetravalent ion substitution in garnet. In this work, we focus on the structure and thermodynamic stability of Cesubstituted YIG (Ce:YIG). Ce:YIG garnet samples were prepared by citrate−nitrate combustion synthesis and then characterized by X-ray absorption near edge structure spectroscopy (XANES) and 57Fe Mössbauer spectroscopy to determine the oxidation state of Ce and nature of Fe in the structure, respectively. Enthalpies of formation were measured B
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which enthalpies of formation could be calculated. For each measurement, a ∼5 mg pellet of sample was dropped from RT into molten sodium molybdate (3NaO·4MoO3) solvent at 702 °C in a custom built Tian-Calvet twin microcalorimeter.28,29 Multiple drops (6−10 per composition) were made to obtain statistics. Oxidizing condition was maintained by bubbling oxygen gas through the solvent at 5 mL/min to prevent local saturation and facilitate dissolution of samples. In addition, oxygen was continuously flushed through the calorimeter glassware assembly at 52 mL/min to maintain a constant composition for the head space gas above the solvent and remove any evolved gases.30 The equipment, calibration, and experimental method have been described in detail elsewhere28,29 and have been applied previously to materials containing Ce, Y, and/or Fe.31−33
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RESULTS All synthesized samples were first analyzed using XRD. The formation of a phase with cubic garnet structure (Ia3d space group) was confirmed (Figure 2), and no secondary phases Figure 3. Lattice constant obtained from whole profile fitting procedure versus cerium concentration in the Ce:YIG samples.
XANES spectra of the four standards shown in Figure 4 demonstrate the major contribution of a single characteristic XANES peak for trivalent Ce at ∼5727 eV, originating from the electron transition 2p3/2 → (4f1)5d. In contrast, tetravalent cerium spectra are shifted to higher energy and show a distinct doublet (5730 and 5738 eV) of roughly equal intensity, which are due to the 2p3/2 → (4 fL)5d, and 2p3/2 → (4f0)5d transitions, respectively (L denotes relaxation transition of an electron from oxygen 2p to cerium 4f orbital).34 Figure 5 shows the XANES spectra of the substituted garnets with 10 and 20 mol % cerium. Both spectra are dominated by the singularly intense Ce3+ characteristic peak. The spectrum of the 0.2Ce:YIG has a lower intensity of this peak but enhanced intensity in the higher energy (5738 eV) peak, indicating a higher abundance of Ce4+ in this sample. The fitting results obtained using a linear combination of all standard spectra are summarized in Table 2. They show average Ce valences for 0.1Ce:YIG and 0.2Ce:YIG of 3.00 and 3.11, respectively. The corresponding spectral fits are shown in Figure 6a and b. The fit for the 0.1Ce:YIG sample (Figure 6a) deviates from the measured spectrum in several respects (being less intense at the 5727 eV peak, more intense at the 5737 eV peak, and somewhat mismatching the higher energy oscillations). Nonetheless, the absence of significant Ce4+ is reasonable based on the fact that the intensity of the Ce4+ sensitive peak at 5738 eV is already slightly overestimated by the peak from the Ce(III) nitrate standard. The fit for the 0.2Ce:YIG sample (Figure 6b) is satisfactory, showing only a slight mismatch at high energy. These fitting deviations most likely indicate that the standards do not exactly represent the local environment of Ce in the garnet structure. A second fitting approach was also used for the 0.2Ce:YIG sample. The 0.1Ce:YIG spectrum was used as the sole Ce3+ standard based on the apparent domination of the 0.2Ce:YIG garnet by Ce3+ and the expectation that the local environment of Ce3+ in the 0.2Ce:YIG garnet is best represented by that of the 0.1Ce:YIG structure. This much better fit is shown in Figure 7, where it can be seen that the only significant deviation is a slight underestimation of the intensity at the 5738 eV peak. The energies of all the features in the 0.2Ce:YIG spectrum are well fit, including those at high energy. The resulting valence from this fit was 3.24 (Table 2), i.e., slightly more oxidized than the 3.11 valence result using all four independent standards.
Figure 2. Powder X-ray diffraction patterns of cerium substituted garnet. All of the patterns are indexed based on cubic yttrium iron garnet structure. JCPDS no. 77-1998.
were detected for Y3−xCexFe5O12 with Ce concentration (x) from x = 0 to 0.2. The lattice constants were calculated by using a whole profile fitting procedure, and an approximately linear expansion of the lattice constant was observed with increasing cerium content (Figure 3). This may be due to larger Ce3+ (1.14 Å) substituting for Y3+ (1.02 Å), and/or the net effect of smaller Ce4+ (0.97 Å) and larger Fe2+ (0.63 Å) generated from Fe3+ (0.49 Å) in tetrahedral sites. However, the lattice parameter stops expanding with cerium content beyond 23 mol %, indicating the solubility limit near this concentration. Indeed beyond this point, excess CeO2 is detected in the samples (x > 0.23). The chemical compositions measured by EMPA are based on oxygen stoichiometry (Table 1). A small amount of Fe (
