Article pubs.acs.org/IC
Assessment of the U3O7 Crystal Structure by X‑ray and Electron Diffraction Gregory Leinders,*,†,‡ Rémi Delville,† Janne Pakarinen,† Thomas Cardinaels,†,‡ Koen Binnemans,‡ and Marc Verwerft† †
Belgian Nuclear Research Centre (SCK·CEN), Institute for Nuclear Materials Science, Boeretang 200, B-2400 Mol, Belgium Department of Chemistry, KU Leuven, Celestijnenlaan 200F, P.O. Box 2404, B-3001 Heverlee, Belgium
‡
S Supporting Information *
ABSTRACT: Polycrystalline U3O7 powder was synthesized by oxidation of UO2 powder under controlled conditions using in situ thermal analysis, and by heat treatment in a tubular furnace. The O/U ratio of the U3O7 phase was measured as 2.34 ± 0.01. The crystal structure was assessed from X-ray diffraction (XRD) and selected-area electron diffraction (SAED) data. Similar to U4O9−ε (more precisely U64O143), U3O7 exhibits a long-range ordered structure, which is closely related to the fluorite-type arrangement of UO2. Cations remain arranged identical to that in the fluorite structure, and excess anions form distorted cuboctahedral oxygen clusters, which periodically replace the fluorite anion arrangement. The structure can be described in an expanded unit cell containing 15 fluorite-like subcells (U15O35), and spanned by basis vectors A = ap − 2bp, B = −2ap + bp, and C = 3cp (lattice parameters of the subcell are ap = bp = 538.00 ± 0.02 pm and cp = 554.90 ± 0.02 pm; cp/ap = 1.031). The arrangement of cuboctahedra in U3O7 results in a layered structure, which is different from the well-known U4O9−ε crystal structure. text, the notation U4O9−ε, where ε = 0.0625, is used to refer to this compound. When the O/U ratio exceeds 2.234 the symmetry is lowered,24 but the atomic arrangement remains closely related to the fluorite arrangement until U3O8 is formed, which has a different crystal structure.25,26 The compounds intermediate to U4O9−ε and U3O8 have been assigned tentative formulas on the basis of thermogravimetric data (e.g., U3O7, U2O5).27,28 Uranium oxides with an O/U ratio close to 2.333 are commonly referred to as U3O7,29 despite possible variations in composition. Their crystal structure is characterized by a distortion of the fluorite-type cubic structure to tetragonal symmetry. In the absence of detailed crystallographic information, the accepted criterion for identification has been the axial ratio c/a. A few early studies reported the existence of two main polymorphs: α-U3O7 (c/a ≈ 0.986) and β-U3O7 (c/a ≈ 1.031).28,30 However, more recent studies have questioned the existence of α-U3O7, suggesting that early workers failed to differentiate it from the cubic U4O9−ε phase, which is formed at the earlier stage of oxidation.13,31 Variations in axial ratio (1 < c/a ≤ 1.031) are regularly reported, but never exceed c/a ≈ 1.031, that is, the value for β-U3O7.9,13,31−35 In what follows, the notation U3O7 will be used to refer to this state.
1. INTRODUCTION Oxidation of UO2 in dry air at temperatures above about 200 °C results in formation of U3O8 (O/U = 2.667), the thermodynamically more stable oxide of uranium.1 The formation of U3O8 from UO2 is associated with a volume increase of about 36%. This transformation is an important threat for the integrity of storage containers for UO2, especially when considering long-term storage and final repository of irradiated nuclear fuels.1−4 The oxidation behavior of UO2 at ambient to medium temperatures up to 300 °C has been investigated already for many decades, and novel insights continue to be obtained.5−14 A wide variety of intermediate oxides can be formed by oxidation of UO2 under different conditions.1,15 Compounds with an O/U ratio between 2 and 2.5 have structures in which the cation arrangement remains closely related to the original fluorite-type UO2 structure, the most notable change being a deviation from cubic symmetry with increasing oxidation.16 In the broad hyperstoichimetric range of compositions, commonly referred to as UO2+x (O/U < 2.234), the excess oxygen develops randomly distributed defects, and the structure can be described as defective cubic fluorite.17−20 At the composition O/U = 2.234 (“U4O9”), an ordered superstructure develops, which also has cubic symmetry.21,22 The compound thus formed is usually assigned the nominal formula U4O9; however, it should be recognized that this does not correspond exactly with the structural composition U64O143.23 Throughout the © 2016 American Chemical Society
Received: August 10, 2016 Published: September 20, 2016 9923
DOI: 10.1021/acs.inorgchem.6b01941 Inorg. Chem. 2016, 55, 9923−9936
Article
Inorganic Chemistry
further are dominated by strong, fluorite-type parent structure reflections and also display a large number of much weaker reflections corresponding to a commensurate perturbation, similar to what is observed in U4O9−ε.46 In this Article, a comprehensive structural analysis of the long-range ordered U3O7 crystal structure is performed by combining X-ray powder diffraction (XRD) and selected-area electron diffraction (SAED) techniques. A consistent structural model based on the coordination of cuboctahedral oxygen clusters in an expanded unit cell is subsequently derived.
For uranium oxides having 2.234 < O/U ≤ 2.333, the formation of a tetragonal phase with gradually increasing c/a ratio (1 < c/a ≤ 1.031) has been clearly demonstrated by in situ diffraction experiments at 210 °C and 250 °C.11,13 The origin of the c/a variation is often attributed to a deviation from stoichiometry (U3O7‑z),7,13,32,33 but it has also been considered as a lattice strain effect.11 Upon further oxidation (2.333 ≤ O/ U ≤ 2.667), a two-phase system U3O7 + U3O8 is formed, where the lattice parameters of the U3O7 phase remain unchanged, and the fraction of U3O8 increases at the expense of U3O7.11,13,36 Thus, the U3O7 state is found to be the main precursor for U3O8 formation. The crystal structures of the fluorite-type oxidation products which retain cubic symmetry (i.e., UO2+x and U4O9−ε) have been investigated extensively. Oxidation results in a reorganization of the anion sublattice by the incorporation of additional oxygen atoms, while the cation sublattice remains to a large extent undisturbed.18 In UO2+x the excess oxygen atoms occupy sites in the fluorite-type unit cell, which are displaced along the ⟨uu0⟩ and ⟨uuu⟩ directions from the interstitial holes, and some vacancies appear in the fluorite oxygen sublattice.19 These defects, generalized in the concept of Willis’s 2:2:2 cluster, are initially randomly dispersed.20 As the composition approaches the O/U ratio of 2.234, the excess oxygen atoms tend to occupy more of the 12 ⟨uu0⟩-type displaced sites.22,37 These sites form the vertices of a regular cuboctahedron, hence the terminology cuboctahedral oxygen cluster.38,39 In U4O9−ε (U64O143), spatial ordering of these cuboctahedral clusters occurs, which results in a superstructure (I4̅3d) based on the fluorite structure but quadrupled in all three directions: 4ap, 4ap, 4ap (with ap the lattice parameter of the fluorite-like subcell).11,40 The concept of the cuboctahedral oxygen cluster has also been used as a basis for the assessment of the U3O7 crystal structure.11,41−43 Formation of oxygen clusters in U3O7 has been confirmed in recent ab initio studies;44 however, due to limitations in the simulation cell, the cuboctahedral arrangement could not be proven.45 Indications for the long-range ordering of these defects have been reported,11,41,42 but a complete and consistent description of the superstructure has not been obtained yet. McEachern and Taylor have cited unpublished work, which mentioned the existence of a √5a, √5a, 3c superstructure for U3O7.1 Nowicki et al., citing the same unpublished data, studied different stacking possibilities for cuboctahedral oxygen clusters in U3O7 and proposed superstructures of the type 5a, 5a, j3c (j an integer), denoted as Uj75Oj175 (O/U = 2.333).41 In their description, the U3O7 phase exists as a family of polytypes. Desgranges et al. performed a detailed assessment of neutron diffraction data obtained from U3O7 (produced in situ by oxidation of UO2 powder at 210 °C).11 They described the crystal structure as an arrangement of tilted and skewed cuboctahedral oxygen clusters in a 4a, 4a, 4c superstructure (I4̅2d), having a composition O/U = 2.313 (U64O148). Diffraction patterns of crystals having a long-range ordered defect structure typically display a set of strong reflections (“parent structure reflections”) that are characteristic for the underlying parent structure, and much weaker “satellite reflections” corresponding to the perturbations. If the periodicity of the perturbations has only rational components with respect to the parent structure basis vectors, the system is commensurately perturbed, and it is possible to derive an expanded unit cell. The diffraction patterns of U3O7 discussed
2. EXPERIMENTAL SECTION 2.1. Sample Material. Samples were prepared from depleted nuclear grade UO2+x, produced via the Integrated Dry Route (IDR process) and supplied by FBFC International (Dessel, Belgium). The impurity content of this powder was evaluated using inductively coupled plasma mass spectroscopy (ICP-MS, ThermoFisher XSeries2). The presence of in total 50 elements was probed for. A summary is presented in Table 1; the total metallic impurity fraction
Table 1. Impurity Levels Measured via ICP-MS in the AsReceived UO2+x Powder Batcha Li Be B Al Cr Mn Fe Ni Cu a
1 0.1 3 4 0.3 0.3 23 0.5 0.2
± ± ± ± ± ± ± ± ±
1 0.1 3 3 0.3 0.3 24 0.3 0.2
Zn Zr Mo Cd In Sn La Gd Pb
2 0.06 0.2 0.1 0.02 4 0.03 0.006 0.1
± ± ± ± ± ± ± ± ±
2 0.08 0.2 0.1 0.02 8 0.02 0.008 0.1
The values are given in μg g−1 (2σ).
was 47 μg g−1. The powder consists of loosely packed, soft agglomerates of up to 20 μm. Each agglomerate is composed of a large number of small crystals whose size is of the order 100 nm. The specific surface area (BET theory) of the powder was measured using nitrogen gas adsorption (Micromeritics Tristar II 3020), and equaled 2.3 m2 g−1. The stoichiometry of the UO2+x powder batch was measured using in situ thermogravimetric analysis and equaled 2.088 ± 0.002 (2σ); the exact procedure has been described elsewhere.47 According to Rietveld analysis of the X-ray diffraction data measured for this powder, it contains about 1.5 wt % U3O8. 2.2. Thermal Analysis. Simultaneous thermal analysis (STA) using a Netzsch STA 449 F1 Jupiter, coupled to a quadrupole mass spectrometer (403 D Aëolos), was performed to investigate conditions for the formation of U3O7 from UO2 and to evaluate the stoichiometry of the UO2+x powder. All used gases were of high purity (99.9992%) and with no measurable water content (dew point < −80 °C). A constant flow of argon gas (20 mL min−1) was maintained through the balance compartment and leading into the furnace chamber, here referred to as the protective gas flow, resulting in increased balance stability. The flushing (active) gas entered the furnace chamber directly through a secondary inlet with a flow of 80 mL min−1. The total exiting gas flow was therefore equal to 100 mL min−1. The gas supply was controlled via various mass flow controllers (Bronkhorst EL-FLOW), individually calibrated to the type of gas used. Simultaneous thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) was performed with a type S (Pt-10% Rh/Pt) thermocouple, carrying Pt/Rh crucibles with lids (sample mass ≈ 20 mg). Single thermogravimetric analysis was performed with a type B (Pt-30% Rh/Pt-6% Rh) thermocouple, carrying an alumina crucible (sample mass ≈ 2000 mg). In both cases, prior temperature calibration was performed by melting standards (In, Sn, Bi, Zn, Al, Au) and validated with a selection of these metals on a regular basis. Tabulated values for heats of fusion of these high-purity metals were 9924
DOI: 10.1021/acs.inorgchem.6b01941 Inorg. Chem. 2016, 55, 9923−9936
Article
Inorganic Chemistry used to calibrate the calorimeter. Mass change was continuously recorded with an accuracy of ±28 μg (2σ). The absolute mass readout of the balance was calibrated using a reference weight of 2000.00 mg. All STA runs were corrected for drift and buoyancy by subtraction of a blank run under identical conditions. A Carbolite TZF1800 tube furnace was used for performing heat treatments on larger quantities of powder (3−50 g). The sintered alumina tube could be sealed from the lab environment by fitting of specifically designed plugs, allowing for a continuous flow of dry gases (argon or synthetic air, N2/21 vol % O2). Also, the furnace was modified with a retractable type K thermocouple for accurate temperature control. Powders were loaded in dried alumina crucibles, weighed and inserted in the furnace at room temperature. The working tube was then sealed gastight and flushed with dry argon until the dew point of the exiting gas reached −74 °C or less. After completion of the thermal treatment, the alumina crucibles were retrieved and weighed on a laboratory balance at room temperature. 2.3. X-ray Diffraction. X-ray powder diffraction was performed with a Philips X’Pert Pro diffractometer in parafocusing geometry (θ−θ configuration). Validation of zero point calibration was performed against an Al2O3 reference sample (NIST Standard Reference Material 1976b) on a weekly basis. An LFF X-ray tube (Cu Kα1 = 1.5405929 Å)48 was used as radiation source. The beam path consisted of a fixed divergence slit (1/2°) and copper beam mask in combination with 0.02 rad Soller slit assemblies. A position-sensitive 1D detector (PANalytical X’Celerator) was used, with a nickel filter placed in front to avoid Cu Kβ contribution to the diffracted signal. The goniometer was operated with a step size of 0.008° (2θ) through the range 24−121° (2θ). Specimens for powder diffraction were carefully prepared via the back-loading technique. During sample preparation, transfer, and measurement, the specimens were exposed to the normal lab environment (room temperature, relative humidity ≈ 50%). The lattice parameters presented throughout the text are recalculated to their value at 20 °C, using the linear thermal expansion coefficient for UO2 of 9.739 × 10−6 °C−1 near room temperature.49 For U3O7 the same coefficient was assumed, as was also proposed for U4O9−ε by Martin.49,50 Rietveld analysis of the X-ray diffractograms was performed with PANalytical HighScore Plus (v4.1) software. Details on the followed methodology can be found in the Supporting Information. 2.4. Electron Diffraction. Transmission electron microscopy (TEM) was performed with a 300 kV JEOL 3010 microscope equipped with an in-column Gatan 794 MSC CCD camera and a sideentry double tilt specimen holder. A small amount of powder (
