Article pubs.acs.org/IC
Nonstoichiometry in Strontium Uranium Oxide: Understanding the Rhombohedral−Orthorhombic Transition in SrUO4 Gabriel L. Murphy and Brendan J. Kennedy* School of Chemistry, The University of Sydney, Sydney, New South Wales 2006, Australia
Justin A. Kimpton, Qinfen Gu, and Bernt Johannessen Australian Synchrotron, 800 Blackburn Road, Clayton, Victoria 3168, Australia
George Beridze, Piotr M. Kowalski, and Dirk Bosbach Institute of Energy and Climate Research, IEK-6 Nuclear Waste Management and Reactor Safety, Forschungszentrum Jülich GmbH, Wilhelm-Johnen-Strasse, 52428 Jülich, Germany JARA High-Performance Computing, Schinkelstrasse 2, 52062 Aachen, Germany
Maxim Avdeev and Zhaoming Zhang Australian Nuclear Science and Technology Organisation, Lucas Heights, New South Wales 2234, Australia S Supporting Information *
ABSTRACT: In situ neutron and synchrotron X-ray diffraction studies demonstrate that SrUO4 acts as an oxygen transfer agent, forming oxygen vacancies under both oxidizing and reducing conditions. Two polymorphs of SrUO4 are stable at room temperature, and the transformation between these is observed to be associated with thermally regulated diffusion of oxygen ions, with partial reduction of the U6+ playing a role in both the formation of oxygen deficient α-SrUO4−δ and its subsequent transformation to stoichiometric β-SrUO4. This is supported by ab initio calculations using density functional theory calculations. The oxygen vacancies play a critical role in the first order transition that SrUO4 undergoes near 830 °C. The changes in the oxidation states and U geometry associated with the structural phase transition have been characterized using X-ray absorption spectroscopy, synchrotron X-ray diffraction, and neutron diffraction.
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INTRODUCTION Nuclear fuels are subjected to extreme conditions throughout their operational lifetime as well as during the postoperative decay of the short-lived radionuclides.1−4 These conditions result in the formation of a number of crystalline and amorphous phases, and understanding the transformation between these can be vital in optimizing the usage, and subsequent storage, of nuclear fuels. Phase separation, through the decay of radionuclides in spent nuclear fuels, would provide complex chemical variations on the nanometer length scale. In operando crystallographic studies of such systems remain out of reach, and even ex situ studies are technically challenging. Our current understanding of the crystallographic structural transformations in nuclear fuels is generally obtained through ex situ studies of model compounds, supplemented with high level modeling.5 Experimental studies are vital, given the significant © XXXX American Chemical Society
computational challenges associated with the modeling of nonstoichiometric materials and crystallographic defects that dominate spent nuclear fuels. These results can reveal unexpected complexity that guides future computational studies.6,7 The unique combination of a comparatively large ionic radius, the availability of f orbitals, and the range of stable oxidation states distinguishes the chemistry of uranium and affords it unusual bonding, structural, and catalytic properties.8,9 Strontium is an important daughter of the fission of U for two reasons: first, it has a high fission yield in mixed oxide (MOX) fuels, and second, it is a major constituent of the “gray phase”, the multicomponent oxide precipitate that forms when MOX fuel is used to high burn-up.1−4 SrUO4 can be prepared Received: June 19, 2016
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DOI: 10.1021/acs.inorgchem.6b01391 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Figure 1. Representation of the α-SrUO4 and β-SrUO4 structures and the refinements of the powder neutron diffraction data for these. The closed circles represent the experimental data, the solid black line represents the fit obtained through Rietveld refinement, and the solid red line represents the difference between the experimental data and the refinement. The short vertical lines represent the positions of the space group allowed reflections. In the structural models the U and Sr cations are represented by the blue and green spheres, respectively. Oxygen atoms represented by the red spheres in α-SrUO4 correspond to O(1) with a short U−O(1) bond distance. heating rate of 1 °C/min. The sample was contained in a quartz crucible. X-ray absorption near edge structure (XANES) spectra were collected at the U L3-edge on beamline 12 at the Australian Synchrotron.23 2.7 mg of α-SrUO4 was mixed thoroughly with 4.4 mg of BN, and the uniform mixture was loaded into a 2 mm diameter quartz capillary (∼6 mm in length) and placed into a water cooled Linkam TS1500 heating stage operating in an argon atmosphere.24 The measurements were performed in transmission mode using argonfilled ionization chambers at temperatures between room temperature and 900 °C. The heating rate was set at 30 °C/min, and the sample was given at least 10 min of soak time before each measurement. The beam intensity (I0) was monitored by a flow through ionization chamber located upstream from the sample. XANES spectra were collected by a second ionization chamber after the sample. A third ionization chamber was placed downstream to simultaneously measure a reference spectrum of a natural uraninite sample. Energy steps as small as 0.25 eV were employed near the absorption edges with a counting time of 2 s per step. The absolute energy scale of the monochromator was calibrated using the K-edge of a Zr foil, the first derivative peak was set at 17995.8 eV, and the uraninite reference spectra were used to ensure that no energy shifts occurred during the experiment. Data analysis was carried out using the software package ATHENA.25 The ab initio calculations of α and β phases of SrUO4−δ were carried out using the DFT plane-wave package Quantum-ESPRESSO.26 In order to correctly account for strongly correlated character of f orbitals, we used the DFT+U method with the Hubbard U parameter value derived from first principles using the linear response approach of Cococcioni and de Gironcoli.27 This methodology has been demonstrated to produce good results for thermochemistry of uranium-bearing molecular and solid compounds.28 The PBE29 and PBEsol30 exchange-correlation functionals and the ultrasoft pseudopotentials31 were used to mimic the presence of core electrons. The PBE functional is a variation of DFT commonly used in calculations of energies. The PBEsol functional is a modification of PBE that exactly reproduces the exchange-correlation energy in the slowly varying density limit and thus results in better structures, which is shown to be the case also for the structures considered here, but at a cost of
in either a rhombohedral (α-SrUO4) or orthorhombic (βSrUO4) structure depending on the synthesis conditions.10−13 The transformation between the two forms occurs around 830 °C and is irreversible in air.14,15 While not definitive, previous studies indicate that whereas β-SrUO 4 is stable and stoichiometric, α-SrUO4 is metastable which can exist with oxygen deficiencies and is best described as α-SrUO4−δ,11,15−18 with the precise stoichiometry depending on the synthesis conditions.
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EXPERIMENTAL SECTION
Polycrystalline samples of α- and β-SrUO4 were synthesized using conventional solid state methods as described previously.11 Structural characterization was performed at the powder diffraction beamline at the Australian Synchrotron19 and the high resolution neutron powder diffractometer at ANSTO’s OPAL facility.20 For the former measurements approximately 5 mg of sample was contained in a 0.2 mm diameter quartz capillary, and for the latter ∼2 g of material was housed in a thin walled vanadium can. The structures described here were refined by the Rietveld method as implemented in the program GSAS.21,22 The peak shapes were modeled using a pseudo-Voigt function, and the background was estimated by a 12 term shifted Chebyshev function. Temperature control during the synchrotron X-ray diffraction (SXRD) measurements was achieved using a FMB-Oxford hot air blower. Temperatures were increased at a ramp rate of 5 °C per minute, and the data collection commenced after a 30 s soak at temperature. The data were measured for 5 min at each of the two detector positions. For the neutron powder diffraction (NPD) measurements the sample can was placed in an ILL (Institute Laue Langevin)-type high vacuum furnace employing niobium elements and operating at
