Single Crystal Synthesis Methods Dedicated to Structural

Phone: +33 (0)4 66 79 16 03. ... The structure of dihydrated actinide oxalates, AnIV(C2O4)2·2H2O, was determined for An .... particularly allowing the...
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Single Crystal Synthesis Methods Dedicated to Structural Investigations of Very Low Solubility Mixed-Actinide Oxalate Coordination Polymers Christelle Tamain,† Bénédicte Arab-Chapelet,† Murielle Rivenet,‡ Francis Abraham,‡ and Stéphane Grandjean*,† †

CEA, Nuclear Energy Division, Marcoule, RadioChemistry & Process Department, Separation Process Chemistry Service, Actinide Chemistry and Conversion Laboratory, F-30207 Bagnols sur Cèze, France ‡ Université Lille Nord de France, Unité de Catalyse et de Chimie du Solide, UCCS, UMR CNRS 8181, ENSCL-USTL, BP 90108, 59652 Villeneuve d’Ascq Cedex, France S Supporting Information *

ABSTRACT: Two crystal growth methods dedicated to very low solubility actinide coordination polymers have been developed and applied to the synthesis of mixed actinide(IV)−actinide(IV) or actinide(IV)−actinide(III) oxalate single crystals of a size (typically 100−300 μm) suitable for isolating them and examining their crystal structure. These methods have been optimized on mixed systems composed of U(IV) and lanthanide (surrogate of trivalent actinides) and then assessed on U(IV)−Am(III), Pu(IV)−Am(III), and U(IV)−Pu(IV) mixtures. Three types of single crystals characterized by different structures have been obtained according to the synthesis and the chemical conditions. This is the first time that these well-known or recently discovered key compounds are formed by crystal growth methods specifically developed for actinide crystal handling (i.e., in glove boxes), thus enabling direct structural studies on transuranium element systems and acquisition of basic data. Characterization by X-ray diffraction, UV−visible solid spectroscopy, thermal ionization mass spectroscopy (TIMS), energy-dispersive X-ray spectroscopy (EDS), and inductively coupled plasma atomic emission spectroscopy (ICP-AES) demonstrates the potentialities and complementarity of the two crystal growth methods for obtaining the targeted mixed oxalates (actinide oxidation state and presence of both metallic ions in the crystal). More generally, this development opens broad prospects for single crystal synthesis of novel actinide organic frameworks and their structural description.

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INTRODUCTION Oxalic acid is a well-known reagent used to recover actinides for immobilization and/or recycling thanks to the very low solubility of An(IV) and/or An(III) oxalates and their ability to form oxides after heat treatment without generating problematic secondary wastes. In the framework of next generation nuclear fuel cycles, mixed actinide oxalates have recently become key precursors of mixed actinide oxides1−3 used for the fabrication of advanced nuclear fuels. Oxalic coprecipitation has emerged as a convenient process for actinide co-conversion into oxalate solid solutions, precursors of actinide oxide solid solutions with homogeneous actinide distribution at molecular scale. The microstructure and properties of the mixed oxides largely depend on the oxalate microstructure and thus on its crystalline structure.2,4 Moreover the structure of the oxalate phase is influenced by the specificity of each actinide and its particular oxidation state. It is thus essential to perform studies directly on the considered actinide system instead of isomorphic or non-isomorphic systems based on surrogates such as lanthanide (or the substitution of lighter actinides such as thorium or uranium for heavier ones).5 Despite some © 2012 American Chemical Society

powder X-ray diffraction studies limited to determining unit cell parameters, few compounds have been fully studied by powder or, better, single crystal X-ray diffraction to determine their structure. Today there is renewed interest concerning crystallographic studies in order to establish structural databases to better characterize these candidate starting materials for production of nuclear fuels or transmutation targets for generation IV systems. Until recently, many studies were carried out on lanthanide elements, which are often considered as good crystallographic surrogates of transuranium actinides(IV or III) according to their ionic radii.6 Despite these investigations, only limited knowledge about transuranic-based solids is available and it is difficult to fully understand their chemistry from sometimes poorly analogous lanthanide or light actinide compounds. There is a huge impact of the An(III)/ Ln(III) and An(IV) nature on the structure of the solid compounds.1,5 Received: July 23, 2012 Revised: September 17, 2012 Published: September 18, 2012 5447

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21−23 for An(IV)31−33) that precipitate generally as small crystals strongly aggregated due to exacerbated nucleation rates and present some chemical particularities (hydrolysis, redox chemistry) that make most of the existing single crystal syntheses unsuitable or adaptable only to a few specific systems. For example, hydrothermal routes or synthesis using temperature to modify the solubility are contradictory with the stabilization of lower actinide oxidation state. From a practical point of view, considering the difficulty of recovering the crystals from gel growth synthesis, this method is difficult in glove boxes. It is thus necessary to devise new efficient and general crystal growth methods adaptable to specific manipulation conditions and to actinide chemistry, particularly allowing the synthesis of interesting mixed An1(IV)-An2(III) oxalate single crystals. These routes are based on kinetic concepts, the idea being to delay the contact between the actinides and the oxalate ions using physical barriers (synthesis by diffusion: method 1) and/ or chemically controlled transformation of dedicated precursors (synthesis by anionic complexes: method 2). This controlled mixing between species allows limited local oversaturation, thus favoring crystal growth of only a few nuclei. These new synthesis routes were defined and developed on U(IV) and U(IV)−Ln(III) before being applied to mixed actinide systems containing transuranium elements such as U(IV)−Am(III), Pu(IV)−Am(III), and U(IV)−Pu(IV).

Moreover, even if more and more studies are dedicated to uranium or thorium oxalates, few structural characterizations by single-crystal X-ray diffraction of heavier actinide oxalates have been carried out to date. The structure of dihydrated actinide oxalates, AnIV(C2O4)2·2H2O, was determined for An = Th, U.7 The hexahydrates AnIV(C2O4)2·6H2O were first described for An = Np8 and subsequently for An = U.9 Despite its industrial production in the PUREX process for treatment and recycling of spent nuclear fuels, this structure has not yet been solved for plutonium because of single crystal synthesis difficulties. Another An(IV) oxalate AnIV(C2O4)2(H2O)2·2H2O has been obtained by in situ oxalate synthesis under hydrothermal conditions only for An(IV) = Th(IV).10 Plutonium(III) oxalate [Pu2(C2O4)3(H2O)6]·3H2O, isomorphic with the lanthanide oxalate decahydrates [Ln2(C2O4)3(H2O)6].4H2O,11 has just been synthesized as single crystals and described by Runde et al.12 The latter plutonium oxalate is an industrially known product, but more than 50 years were necessary between its identification and its possible synthesis as a single crystal. It is worth noting that 40 years separate the determination of the structure of Ln(III) and Pu(III). These last two points highlight the difficulties involved in crystal growth and structural determination of transuranium compounds, particularly for very low-solubility solids which aggregate at the microscopic scale. More complicated An(IV)-oxalate structures contain some single-charged cations located in the free space within the coordination polymer. The first structural description of the socalled hexagonal phase in this paper was given in a subcell with P3 symmetry (a = 11.001(2), c = 6.332(2) Å) for the neptunium oxalate (H3O)2Np2(C2O4)5·nH2O.13 Better resolution was then obtained for the uranium oxalate (NH4)2U2(C2O4)5·nH2O14 in a larger cell (P63/mmc symmetry, a = 19.177(3), c = 12.728(4) Å). With potassium, two different structures were described, K4AnIV(C2O4)4·4H2O (with AnIV = U15 or Th16) and a hydroxyl oxalate KPu(C2O4)2(OH)·2H2O.12 Only one polymeric oxalate containing molecular cations, {C(NH2)3}4[AnIV(C2O4)4·2H2O], has been described for An = Th, U, Pu, and Np:17 the structure is built of AnIV-oxalate chains similar to that found in K4AnIV(C2O4)4·4H2O compounds. Concerning mixed valence (IV, III) oxalates, three mixed U(IV)/Ln(III) oxalates, M2+xUIV2−xLnIIIx(C2O4)5·nH2O (hexagonal phase) for the first one and M1−x[LnIII1−xUIVx(C2O4)2·H2O]·nH2O (tetragonal and triclinic phases) for the last two, were recently synthesized and described.14,18,19 In these oxalate compounds, a mixedcrystallographic site which accommodates both elements in spite of their different charges has been evidenced, the charge compensation being ensured by the single-charged M cations. This property is of interest as it ensures a complete homogeneity of the materials which is particularly important for precursors of nuclear fuels. Synthesis routes dedicated to the growth of high-quality single crystals of transuranic oxalate compounds are extremely rare. Many oxalate single crystal synthesis options exist for lanthanide or light actinide (Th, U) elements based on different concepts: cooling11,20 or evaporation21 of solutions, hydrothermal conditions,22 modification of solvent,15,16,23 diffusion in gel,8,14,18,19,24 and in situ oxalic ion synthesis from precursors (in solution,25 in silica gel 26 or under hydrothermal conditions10,27). Structural studies of actinide oxalate single crystals are more difficult because trivalent or tetravalent actinides form very poorly soluble oxalate compounds (the pKs are in the range 25−31 for Ln/An(III)28−30 and in the range

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EXPERIMENTAL SECTION

Reagents. Because of the radioactive nature of some actinides, especially plutonium and americium, the experiments involving these elements were carried out in glove boxes with very restrictive protocols. Actinide(IV) and actinide(III) solutions were prepared using specific procedures, either from purified monometallic solutions or by dissolving monometallic oxides. U(IV) nitrate solution is prepared by catalytic reduction of U(VI) nitrate by H2 on a Pt/Si backing. Pu(IV) and Am(III) solutions were prepared by dissolving the corresponding oxide, PuO2 and AmO2, with concentrated HNO3 in a glove box. Hydrazinium nitrate (N2H5+, NO3−) was used as an anti-nitrous agent to stabilize the lowest oxidation states, U(IV). Pu(IV) and Am(III) are stable in nitric acid solutions. The concentration, purity, and oxidation state were essentially determined by UV−vis spectroscopy. Lanthanide nitrates (Aldrich, 99.9% Reagent grade Ln(NO3)3·6H2O) were used to prepare Ln(III) solutions to simulate An(III) solutions in pseudoactive experiments, considering some similarities between Ln(III) and An(III) ions.34 Ammonium, sodium, or potassium nitrates were added when appropriate (Aldrich, 99.9% reagent grade). Synthesis by Diffusion (1). In synthesis method 1 the contact between reagents is delayed by physical constraints so that the diffusion of the different species is controlled, leading to gradual contact between oxalic acid and actinide/lanthanide. Although this principle has already been reported with gel growth methods, the experimental adaptation described in this paper is new, with diffusion slowed down by membranes. The specially designed diffusion cell is composed of three compartments of the same volume separated by membranes (Figure 1). The diffusion rate of the reagents depends on the nature and properties (porosity, thickness, etc.) of the membranes. Among the different existing membranes (pseudoliquid membranes, functionalized polymer membranes, and ceramic membranes), compressed glass fiber membranes (Whatman GF/F) were chosen as they are nonselective with no preliminary synthesis and highly stable in acidic media. The waterproofness is ensured by silicon grease and Teflon ruber or parafilm. The cohesion of the cell is ensured by metallic clip brooch. Large crystals of oxalic acid at an equivalent concentration of 0.2 mol·L−1 in solution are placed in compartment A (progressive dissolution of the solid, instead of using a prepared solution, 5448

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angular detector. The scan step was 0.02° with a counting time of 0.5 s/step between 5° to 60°. Silicon was added to all samples as an internal standard to calibrate the angular positions of the observed XRD lines. Kα2 radiation stripping according the Rachinger method as well as baseline and angle shift corrections were performed by empirical calculations and DIFFRACplus EVA software.36 The XRD patterns were compared with the M+-U(IV)-Ln(III) oxalate structure database recently built from XRD patterns calculated from single crystal structure determination results.14,18,19 ICP-AES. The concentrations of neodymium and uranium in solution were measured to study the diffusion rate by inductively coupled plasma-atomic emission spectrometry (ICP-AES) with a Perkin-Elmer Optima 3000 DL system. Uv−visible Spectroscopic Analyses. Actinide and lanthanide concentrations in solution were determined by UV−vis spectroscopy using a GBC Cintra 10e UV spectrophotometer between 350 and 900 nm. UV−vis spectra of the solid products were acquired between 400 and 800 nm with a HITACHI U-3000 analyzer equipped with an integration sphere. The reference spectra of solid compounds formed by direct oxalic precipitation were previously registered. TIMS Analyses. Thermal ionization mass spectrometry (TIMS) analyses were performed on an average of a dozen single crystals dissolved in 125 μL of nitric acid solution (8 mol·L−1) and diluted to 1 mol·L−1 before analysis. A VG-54 magnetic sector mass spectrometer was used. An internal standard of known isotopic composition and concentration was added to the sample. Finally, the actinide content could be determined from the measured final sample concentration and the known internal standard concentration.

Figure 1. Scheme and photos of the three-compartment diffusion cell. participates in controlling the diffusion of oxalic acid in the cell). A nitric solution with one or two actinides ([An1] + [An2 or Ln] = 0.04 mol·L−1) is introduced in the opposite compartment C. The intermediate section B is filled with 1.5 mol·L−1 of nitric acid containing 1.0 mol·L−1 of single-charged nitrate cation (NH4NO3, N2H5NO3 or NaNO3). Nitric acid and single-charged cation concentrations are identical in the three compartments in order not to disturb the diffusion process of the main reagents by other concentration gradients. The presence of single-charged cations is necessary to allow the formation of mixed oxalate compounds. The mounted cell is allowed to stand for two or three days to grow single crystals. Method 1 is quite convenient as it allows a large number of different physical-chemical conditions for a dedicated membrane. The principle of this crystal growth method is similar to that of gel growth synthesis without the difficulty of crystal recovery, the main limitation of gel growth in glove boxes. Synthesis by Anionic Complexes (2). Crystallization is induced by equilibrium displacement of a solution of soluble anionic complexes to form the insoluble neutral complex. This synthesis is more suitable for actinide(IV) than actinide(III) which does not form anionic complexes with oxalate ligands. The following equilibria are taken into account:

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RESULTS AND DISCUSSION

Synthesis by Diffusion (1). To minimize manipulation of highly radioactive transuranium elements, the crystal growth method 1 was first developed on the U(IV)−Nd(III) system and further applied to An(IV)−Am(III) systems for An(IV) = U(IV) and Pu(IV). The U(IV)−Nd(III) system was studied in the presence of ammonium ions with a neodymium atomic percentage, x = 100Nd/(Nd + U), ranging from 10 to 90%. For all studied domains, the recovered oxalate solids contained two types of single crystals, green hexagonal tubes and purple-green square plates, assumed to belong to the hexagonal, 14 M 2 + x U I V 2 − x Nd I I I x (C 2 O 4 ) 5 · nH 2 O, an d t et rago nal, 1 9 M1−x[NdIII1−xUIVx(C2O4)2·H2O]·nH2O, series derived from M2UIVx(C2O4)5·nH2O and M[NdIII(C2O4)2·H2O]·nH2O compounds with UIV → NdIII + MI and NdIII + MI → UIV substitutions, respectively (Figure 2). The signatures of the hexagonal (2θ = 9°, 14° and doublet at 16°) and tetragonal (2θ = 11°, doublet centered on 14.5° and 17.5°) phases clearly appear on the powder XRD patterns in Figure 3 (analysis of the whole solid filtered from the diffusion cell). In addition, the broad peak at 17.5° and the split of the

acid − base reaction: H 2C2O4 + 2H 2O → C2O4 2 − + 2H3O+ complexation reaction: Anx (C2O4 )y + 12 − → Anx (C2O4 )y + C2O4 2 − The equilibrium shift is caused by slow acidification of the anionic complex solution. Acidifying the solution, the oxalate ligands change into oxalic acid and are therefore less available for complexation, leading to the promotion of the neutral complex. As the drop by drop addition of acid is too fast to control local oversaturation (even with dilute solution),35 a slow and homogeneous diffusion of nitric acid toward the anionic complex solution is considered with the use of the diffusion cell (with the same membranes) described above. A nitric acid solution (3 mol·L−1) placed initially in compartment A slowly diffuses toward the actinide anionic complex solution in compartment C. This solution is prepared according to the optimal physical and chemical conditions previously determined,35 [U(IV)] = 0.02 mol·L−1, [H2C2O4] = 0.4 mol·L−1, [HNO3] = 0.1 mol·L−1, [H2C2O4]/ [An(IV)] = 20. The solution pH is then increased and adjusted to 2.3−2.5 by adding concentrated ammonia, potassium hydroxide, or sodium hydroxide (depending on the single-charged cation studied) until complete dissolution of the initial precipitate. The intermediate compartment contains 0.01 mol·L−1 nitric acid solution. The concentration of single-charged cation (NH4+, K+, or Na+) introduced as nitrates is identical in the three compartments (1 mol·L−1) in order not to disturb the diffusion of the main reagents by other concentration gradients. Crystals appear after two or three days. Characterization. EDS. Chemical composition analysis was performed by energy dispersive spectroscopy (EDS) on a JSM 5300 scanning electron microscope (SEM) equipped with a PGT X-ray microanalysis system (IMIX). Powder X-ray Diffraction. X-ray diffraction data (XRD) for all mixed oxalate powders were obtained with a Bruker AXS D8 diffractometer (curved position-sensitive detector) using Cu radiation (Kα1 = 1.5406 Å, Kα2 = 1.5444 Å) and equipped with a LynxEye

Figure 2. Single crystals formed by method 1 applied to the U(IV)− Nd(III) system for x = 50%. 5449

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Therefore each type of single crystals conforms to a mixed oxalate containing U(IV) and Nd(III). It appears that the tetragonal phase integrates more trivalent cations than the hexagonal phase. The structures of the single crystals were also checked by single-crystal X-ray diffraction with data allowing a complete resolution already done by Chapelet-Arab et al.14,19 It is worth noting that the two types of single crystals grow at different places inside the diffusion cell. Square plates are mainly formed in compartments A and B, whereas the hexagonal tubes are principally located in compartments B and C. To better understand the diffusion mechanism, the diffusion kinetics were studied by ICP-AES with and without oxalic acid to determine whether complexation by oxalate ions has an impact on the diffusion of the different species. The uranium and neodymium concentrations were continually measured in the three compartments of the diffusion cell for three days (Table 1). Without oxalic acid, uranium and neodymium behave identically and simultaneously migrate to reach equilibrium at 33% of the initial concentration in each compartment. In the presence of oxalic acid, the concentrations in compartments B and C decrease faster following gradual crystallization. Moreover, the solubility of uranium being lower than that of neodymium in these particular physical-chemical conditions, its concentration drop is more significant. It appears that the two metallic cations have the same behavior and the same diffusion kinetics (not taking into account their solubility), which appear not to be modified by oxalate ion complexation. Therefore, the inhomogeneous distribution of

Figure 3. Powder XRD patterns of solids formed by method 1 applied to the U(IV)−Nd(III) system (bottom) compared to calculated XRD patterns of identified oxalate phases.

peak at 11° indicate the presence of UIV(C2O4)2·6H2O9 powder not discernible as single crystals. The presence of both metallic cations assumed by the crystal color was checked by elementary EDS analyses on both types of crystal with 60−80% and 20−30% (atomic percentages of the actinides composition) neodymium for the crystals belonging to the tetragonal, M1−x[NdIII1−xUIVx(C2O4)2·H2O]·nH2O, and the hexagonal, M2+xUIV2−xNdIIIx(C2O4)5·nH2O, series, respectively, regardless of the initial atomic ratio of trivalent cation in solution, x.

Table 1. Diffusion Kinetics Studied by ICP-AES for U(IV)−Nd(III) System with and without Oxalic Acid

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The “sea-urchin” crystals and striations visible on the needle surface already observed in previous studies44 corroborate this model (Figure 6). Nevertheless, even if this model seems to be the most suitable, it is difficult to conclude because of insufficient data.

single crystals is not related to a diffusion mechanism but can be explained by two hypotheses. The physical−chemical conditions evolved along the diffusion cell because of the actinide and above all the oxalic acid concentration gradients. The variation of the ratio between the oxalic acid and actinide concentrations could explain the preferential growth of one phase compared to the other. (a) The high oxalic excess or the low actinide concentration could impact the kinetics of nucleation and crystal growth, accelerating the formation of the tetragonal phase in compartments A and B. (b) The high oxalic excess could thermodynamically favor the formation of the tetragonal phase in compartments A and B. The tubular morphology of the hexagonal crystals, with sharp edges, is quite unexpected, although hollow crystals have already been described in the literature for inorganic crystals such as Se and Te,37 ZnSe and ZnS,38 ZnO,39 or rutile-type GeO2,40 metal organic framework crystals,41 and organic crystals such as pharmaceutical compounds.42,43 Secondary electron images of U(IV)-oxalate crystals (Figure 4) indicate

Figure 6. Heaps of “sea-urchin” crystals and growth striations visible on the surface.

Method 1 was then applied to U(IV)−Am(III) and Pu(IV)− Am(III) systems, candidate material precursors incorporating minor actinides for the fabrication of transmutation fuel or targets.4,46 The initial chemical conditions for synthesis method 1 are identical to those of the surrogate U(IV)−Nd(III) system. Only two initial americium percentages, 10 and 30%, were studied for both systems according to the expected fuel compositions for minor actinides recycling.46 As for the U(IV)−Nd(III) system, the application of diffusion method 1 to the two An(IV)−Am(III) systems (An = U, Pu) led to the formation of two types of single crystals, square plates corresponding to a tetragonal phase,19 M1−x[AmIII1−xAnIVx(C2O4)2·H2O]·nH2O, principally located in compartments A and B, and hexagonal tubes characteristic of the hexagonal structure,14 M2+xAnIV2−xAmIIIx(C2O4)5·nH2O, formed principally in compartment C (Figure 7). The square plates were formed only with the higher Am/An ratio. In both systems, the presence of both actinide metallic cations is attested by TIMS analyses performed separately on all single crystals. The square plate and the hexagonal tube crystals contain 43% and 32% of trivalent cation, respectively, for the U(IV)−Am(III) system and 40% and 9% for the Pu(IV)−Am(III) system. It is interesting to note that, whatever the system, the percentage of trivalent cation is higher for the square plates than for the hexagonal tubes. The square plates assumed to belong to the tetragonal series incorporate more trivalent cations. The variability of the percentage of trivalent cation according to the system highlights the specificity of each system and the need to pursue individual structural studies for all of them. UV−vis analysis on the solid confirms conservation of the actinide oxidation state during crystallization. For each system,

Figure 4. Secondary electron images of uranium(IV) oxalate showing hexagonal tubular single crystals.

that the hole appears all along the needles. The morphology is reproducible even though the diameter of the holes is different according to the synthesis method and the studied system. Various mechanisms have been proposed for tubular crystal growth including an aggregation model,44 a diffusion limitation model,43 and a model based on the temperature gradient.45 In this last study, a small amount of powder is warmed by a Kofler bench above the melting temperature; the sublimated compound is transported inside the hollow crystal by convection phenomena (temperature gradient), and when the matter reaches the tip of the needle where the temperature is lower, it feeds the growth. Adapted to our synthesis, the model is no longer based on the temperature gradient but on concentration gradients through the pores of the membranes. The ionic reagents flow through the membrane pores due to diffusion gradients, leading to the formation of some crystals on the pores at the surface of the membrane. One of the reagents is transported inside the crystal by the concentration gradient and encounters the other reagent at the top where crystal growth occurs (Figure 5).

Figure 5. Scheme of the crystal growth model adapted from Martins.45 5451

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devices under development to minimize the delay between crystal recovery and single crystal analysis. Synthesis by Anionic Complexes (2). Method 2 cannot a priori be adapted to trivalent metallic cations as they do not form anionic complexes with oxalate ligands. Therefore the synthesis was first tested on U(IV). Preliminary results indicate that the presence of single-charged cation is necessary to promote crystal growth. It is important to note that the solution of anionic complexes does not diffuse in the cell and all the solid accumulates in compartment C. Regardless of the nature of the single-charged cation, ammonium, sodium, or potassium (pH adjustment with NH3, NaOH, and KOH, respectively), only hexagonal tube-shaped single crystals containing uranium are formed with different sizes according to the nature of the single-charged cation (Figure 9).

Figure 7. Oxalate single crystals formed by method 1 applied to An(IV)−Am(III) systems, An(IV) = U(IV) and Pu(IV).

the analysis was carried out on the whole solid without differentiation of the different single crystals. The spectra of the crystallized solids are compared to those of precipitated americium(III) oxalate, AmIII2(C2O4)3·10H2O, plutonium(IV) oxalate, PuIV(C2O4)2·6H2O, and of uranium(IV) oxalate, UIV(C2O4)2·6H2O (Figure 8). The spectra show several

Figure 8. UV−visible spectra of both single crystal types formed by application of method 1 for An(IV)−Am(III) systems, An(IV) = U(IV) and Pu(IV) compared with Pu(IV)3 or U(IV) oxalate hexahydrates and Am(III) oxalate decahydrate.

Figure 9. Single crystals of uranium oxalate with ammonium (top), sodium (middle), and potassium (bottom) cations formed by application of method 2 on U(IV)-M+ systems.

different Laporte-forbidden f-f transition bands in the visible region 400−800 nm. The bands observed on the mixed oxalate spectra are in agreement with the reference spectra, with a sharp intense band at 509 nm characteristic of americium for both systems. For the U(IV)−Am(III) spectra, the broad bands at 480, 550, and 660 nm attest the presence of the uranium(IV) and the conservation of the oxidation state. In the same way, the band at 480 nm and the valley at 590 nm characteristics of the plutonium(IV) UV−vis signature is recognized on the Pu(IV)−Am(III) spectrum. These crystals have not yet been analyzed by single-crystal Xray diffraction. Radiation damage due to the high activity of Am quickly degrades the crystals and requires specific laboratory

For single-charged potassium or sodium cations, powder Xray diffraction (Figure 10) on the whole solid indicates that the solid is single-phased and is characterized by an orthorhombic structure already reported in the Na−U−Nd system and similar to the hexagonal structure and thus included in the so-called hexagonal series.14 But with ammonium cation, the powder XRD pattern of the solid is identical to the theoretical hexagonal pattern. These results confirm that the monovalent cation nature has an influence on the structure of the solid. It should be noted that the hexagonal crystals seem less hollow than the ones formed by method 1, showing that the crystal growth mechanism is different. The crystal growth time also has an impact on the structure and shape of the crystals. Indeed, the same synthesis method but for a duration of three 5452

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Figure 10. Powder XRD patterns of solids formed by application of method 2 for different U(IV)-M+ systems (M+ = NH4+, Na+, K+) compared to calculated XRD patterns of oxalate phases.

Figure 12. UV−visible spectrum of both single crystal types formed by application of method 2 to the U(IV)−Nd(III) system.

probably due to the coordination of the metallic actinide by oxalate ions. Apparently the neodymium cations do not have time to reach compartment C before crystallization. The powder X-ray diffraction pattern for the whole solid confirms the presence of both phases belonging to the hexagonal13 (2θ = 9°, 14° and doublet at 16°) and tetragonal8 (2θ = 11°, doublet centered on 14.5° and 17.5°) series (Figure 13). So t he formulas of the crystals are thus M1−x[NdIII1−xUIVx(C2O4)2·H2O]·nH2O and M2UIV2(C2O4)5·nH2O.

weeks leads to the formation of another type of single crystal appearing as rectangular cuboids (Figure S1, Supporting Information). The XRD pattern of the single-phase solid presents the recognizable signature of the monoclinic phase UIV(C2O4)2·6H2O9 (Figure S2 Supporting Information). Even if method 2 is a priori not adaptable to trivalent cations, application of this slightly modified method to the U(IV)− Nd(III) system with NH4+ as the monovalent cation allowed us to produce mixed oxalate compounds with tetravalent and trivalent metallic cations. For this synthesis, a nitric acid solution of Nd(III) (0.02 mol·L−1) is introduced in compartment A and was allowed to diffuse toward the actinide(IV) anionic complex solution placed in compartment C. The physical and chemical conditions are identical to the ones optimized on U(IV)-M+ systems. Two different types of single crystals are formed with a real spatial segregation, green hexagonal sticks corresponding to the hexagonal series,14 M2+xUIV2−xNdIIIx(C2O4)5·nH2O, only in compartment C and purple square plates characteristic of the tetragonal series,19 M1−x[NdIII1−xUIVx(C2O4)2·H2O]·nH2O, exclusively in compartments A and B (Figure 11).

Figure 13. Powder XRD patterns of oxalate solid formed by application of method 2 for U(IV)−Nd(III) system compared to calculated XRD patterns of oxalate phases.

Synthesis method 2 was finally applied to the U(IV)−Pu(IV) system with NH4+ as monovalent cation starting from U(IV)/ Pu(IV) = 1. This system is a bit particular and presents a great scientific interest as it is a priori unstable in nitric acid solution because of a redox reaction leading to the formation of U(VI) and Pu(III). The mixture of U(IV) and Pu(IV) can coexist adding complexing agent in order to bring closer or to cross the two redox potentials and induce kinetic or thermodynamic stabilization.35 Previous studies showed that complexation by oxalate ligands until the formation of anionic complexes prevents the redox reaction and allows the stabilization of the oxidation state (IV) of plutonium for several days.35 Therefore it is coherent to test this unusual U(IV)/Pu(IV) mixture stabilized by complexation with the synthesis route based on the same An(IV) anionic complexes. Two types of single crystals were formed in compartment C: rectangular cuboids, similar to those obtained for the U(IV) system after 3 weeks, and long hexagonal sticks (Figure 14).

Figure 11. Purple square plates (left) and green hexagonal sticks (right) oxalate single crystals formed by modified method 2 applied on U(IV)−Nd(III) system.

Both single crystals were separately analyzed by UV−vis spectroscopy (Figure 12) and compared to reference spectra of U(IV) oxalate and neodymium(III) oxalate previously prepared (by direct oxalic precipitation) and recorded. The analyses reveal that the square plates are a mixed phase containing U(IV) and Nd(III) with the specific bands of Nd(III) at 520, 580, and 740 nm and the specific bands of U(IV) at 480, 550, and 660 nm, whereas the green hexagonal sticks are composed only of U(IV) (broad bands at 480, 550, and 660 nm). The small shift of the peaks to higher wavelengths, the broadening of the major peaks, and the slight change in the peak shapes are 5453

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Figure 14. Oxalate single crystals formed by method 2 applied to the U(IV)−Pu(IV) system.

The TIMS analyses realized separately on both single crystals give a percentage of plutonium of 62−63% for the two types of single crystals. This percentage is surprisingly higher than the initial rate (50%) probably due to the partial oxidation of U(IV) during experiment to form U(VI) remaining in solution due to a greater solubility of U(VI) oxalate compared to the U(IV) oxalate. To confirm the oxidation state of the actinides in the different crystals, a UV−vis analysis was carried out on the whole solid. The spectrum is compared in Figure 15 with the

Figure 16. Powder XRD pattern of oxalate solid formed by application of method 2 to the U(IV)−Pu(IV) system compared to calculated XRD patterns of known oxalate compounds.

Because of the chemical instability of the crystals (redox reaction in the solid), single-crystal X-ray diffraction analyses require the same laboratory devices under development to avoid radiolytic degradation.

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CONCLUSION Two new single crystal synthesis methods adapted to transuranium elements and glove box manipulation conditions are presented. These crystal growth routes are efficient and can be adapted to a large number of systems. The redox chemistry is controlled particularly for the lower oxidation states which until now had no dedicated single crystal synthesis routes. The crystal growth media are composed only of reagents without other chemical components, contrary to other known synthesis routes such as gel growth. These two crystal growth routes are complementary and increase the number of systems that can be studied. The application of these methods to mixed actinide systems allows the formation of several single crystals with various formulas and structures. Because of radiation damage or solid redox reactions, however, the full elucidation of the structure of these transuranium elements-based crystals will be completed in a subsequent step of development. The structural studies on the prepared single-crystals will be published in a forthcoming paper.

Figure 15. Comparison of UV−visible spectra of the solid formed by application of method 2 to the U(IV)−Pu(IV) system, with U(IV) and Pu(IV) oxalate hexahydrates and U(IV)−Pu(III) oxalates.

spectra of U(IV) and Pu(IV) oxalate hexahydrates and several U(IV)−Pu(III) oxalate solids with various percentage of Pu(III) all previously prepared by direct oxalic precipitation. The signature of plutonium(IV) with the characteristic band at 480 nm is highlighted as well as the distinctive band of the uranium(IV) at 660 nm and the width of the band at 480 nm. By comparison with U(IV)−Pu(III) oxalate spectra, the percentage of Pu(III) in the crystals is estimated to be below 7%. The presence of Pu(III) is probably due to a partial redox reaction between U(IV) and Pu(IV). Moreover a UV−vis study on a seven-day-old solid reveals that this percentage increases to 50%. The XRD pattern of the old solid is similar to that of AnIV(C2O4)2·6H2O, indicating the degradation of the mixed U(IV)−Pu(IV) hexagonal phase. The powder X-ray diffraction of the whole solid (Figure 16) shows the presence of both a hexagonal phase (2θ = 9°, 14° and doublet at 16°) and a (UIV,PuIV)(C2O4)2.6H2O monoclinic phase (broad peak at 17.5° and doublet at 14°), however the presence of a small quantity of tetragonal phase cannot be ruled out. These results indicate that the hexagonal sticks correspond to a mixed oxalate M2(UIV, PuIV)2(C2O4)5·nH2O belonging to the hexagonal series14 and the rectangular cuboids to (UIV, PuIV)(C2O4)2·6H2O.9

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ASSOCIATED CONTENT

S Supporting Information *

The characterizations of the single crystals formed by application of synthesis method 1 with a long reaction time. This information 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]. Phone: +33 (0)4 66 79 16 03. Fax: +33(0)4 66 79 69 80. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work benefited from the financial support of the Agence Nationale de la Recherche (ANR-08-BLAN-0216). 5454

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