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Submitted on 1 Jan 1986
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THE APPLICATION OF EXAFS SPECTROSCOPY TO THE STRUCTURAL DETERMINATION OF FLUORITE TYPE OXYGEN- EXCESS URANIUM
OXIDES AND OF MIXED (U,Ce) OXIDES
D. Jones, J. Roziere, G. Allen, P. Tempest
To cite this version:
D. Jones, J. Roziere, G. Allen, P. Tempest. THE APPLICATION OF EXAFS SPECTROSCOPY TO
THE STRUCTURAL DETERMINATION OF FLUORITE TYPE OXYGEN- EXCESS URANIUM
OXIDES AND OF MIXED (U,Ce) OXIDES. Journal de Physique Colloques, 1986, 47 (C8), pp.C8-
745-C8-748. �10.1051/jphyscol:19868141�. �jpa-00226042�
THE APPLICATION OF EXAFS SPECTROSCOPY TO THE STRUCTURAL DETERMINATION OF FLUORITE TYPE OXYGEN- EXCESS URANIUM OXIDES AND OF MIXED (U,C€?) OXIDES
D.J. JONES, J. ROZIERE, G.C. ALLEN* and P.A. TEMPEST"
Laboratoire des Acides MinBraux, UA 79, Universitg des Sciences et Techniques du. Languedoc, F-34060 Montpellier, France
"Central Electricity Generating Board, Berkeley Nuclear Laboratories, GB-Berkeley GL13 9PB, Glos., Great-Britain
Resume - On a utilis6 llEXAFS pour U02, p-U307 (=U02.33) et U409 (=U02.25) au seuil de lluranium, ainsi qulaux seuils de lluranium et du c6rium pour la s6rie (U, -y ,Cey) 02-x, x=7,20,28% pour caracteri ser 1 I environnement autour de 1 I uranium et de cerium et de determiner la repartition des defauts dans les phases non- stoechiom6triques.
Abstract - EXAFS spectroscopy has been carried out at 77 K at the uranium L I I I edge of U02, 6 -U307 (=U02.33) and U409 (=U02.25), and at both the uranium and cerium edges in the series of mixed oxides (UyCel-y)02-x x=7,20,28%, with the aim of determining the structure of these highly defective phases.
Non-stoichiometric metal oxides have long been of interest to structural chemists, but precise information is difficult to obtain owing to the extended disordering of interstitial atoms and vacancies over a1 1 crystal space. For this reason, classical structural techniques such as X-ray diffraction are not fully adapted to the characterisation of such highly defective materials. EXAFS allows a different approach, being sensitive to short range order around each absorbing atom.
The extent and type of non-stoichiometry depend on the structural type, and the greatest variety is shown by fluorites, which can be anion excess (hyperstoich- iometric) e.g. U02+,, anion deficient (hypostoichiometric) e.g. Ce02-,, and anion deficient on one side of the stoichiometric composition and anion excess on the other e.g. (UyPul-y)02-x. The adaptability of the fluorite structure to tolerate gross deviations from stoichiometry may be attributed to the presence of large vacant interstitial sites in the lattice, allowing anionic disorder, usually anion Frenkel defects.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868141
C8-746 JOURNAL DE PHYSIQUE
Particularly interesting are binary and ternary rare earth and actinide compounds - fluorides and oxides - where the occurance of high concentrations of ionic defects coupled with a high anlon mobility lead to properties a f fast ion conduction.
Whereas hyperstoichiometry is frequently encountered amongst fluorine- containing fluorite structures, amongst the binary oxides this phenomenon is essentially limited to U02. This material, and its oxidised phases U02+x is of industrial and commercial importance in connection with the nuclear fuel cycle.
Some of our recent research has been directed towards the structural invest- igation of p-U307 and a -U409 l a ' Using EXAFS spectroscopy, bond lengths were calculated and used to derive the coordinates of the oxygen interstitials. 1 Experiments were carried out at the synchrotron radiation source at LURE, France, at the U LIII edge at 77 K for stoichiometric U02 (from which the phase and amp1 itude functions were derived) , U409 ( Z U O ~ , ~ ~ ) and d - U307 (=U02.33) . A
difference Fourier technique for U307 was used, essential ly subtracting the EXAFS signal of U02 , and using the working assumption that only oxygens at interstitial sites contribute to the remaining EXAFS. For each oxygen not on a fluorite lattice - site, there is more than one uranium-oxygen distance, all of which are related through symmetry relations in the space group Fm3m. The method was not applied to U409 as its structural relationship to U02 is less straight-forward, complicated by a distinctly non-Gaussian distribution of U-0 distances. The difference R.O.F. for U307 is shown in Figure 1.
Figure 1
U-0 bond lengths derived from this using the Lee-Beni criterion after phase correction and adjustment of Eo are in good agreement with those obtained by a least-squares fit to the U307 data, and have been used to calculate the coordinates of the oxygen interstitials. EXAFS has an advantage for such
0
calculations since between 2-4 A there are three distances, each independently
determined by the scattering effect, which are at our disposal for the calculation
of each set of interstitial oxygen atom coordinates. The analysis of coordination
sites.
For the U-U shell EXAFS demonstrates that the uranium lattice is perturbed by the incorporation of oxygen interstials, both in U409 and U307.
The hypostoichiometric mixed uranium,cerium oxide 'system is expected to show many similarities to (U,Pu)02-,, currently used as fast reactor fuel, but with the great advantage that it does not have the toxicity of the latter. The structure of (U,Ce)02-x is, however, unknown, and even the structural basis for the analysis of thermodynamic data is a matter of contention 4 . Recent diffuse neutron scattering experiments have indicated that the random distribution of cerium in the uranium sub-lattice at low cerium concentrations becomes more ordered as the proportion of cerium is increased 5 .
EXAFS experiments at both metal edges (uranium and cerium) have been performed in order to establish whether Ce replaces U on fluorite type sites in the crystal, and to gain some insight into the nature of the distribution of the cerium atoms, in addition to identifying the oxygen atom arrangement. The Fourier transformed EXAFS spectra of the preliminary experiments on (U,Ce)02, at concentrations (A) 7
%, (B) 20%, (C) 28 % cerium, taken at the cerium absorption edge, are shown in Figure 2.
Figure 2
The two main peaks in the spectra of (B) and (C) may be attributed, in order of increasing distance, to the couples metal-oxygen and metal-metal, and the structure is essentially of a fluorite type. However, the complexity of the spectrum of (A) shows that this arrangement is not retained around the cerium atom at low cerium concentrations. After phase correction and adjustment of Eo,
0
the shortest Ce-0 distances are 2.08, 2.33 and 2.37 A for concentrations A, B and C respectively.
Although the cerium environment is perturbed from that of Ce02, low temperaure
experiments at the uranium edge show that there is relatively less modification to
the uranium environment : the Fourier transformed EXAFS spectra of 7 and 28 %
C8-748 JOURNAL DE PHYSIQUE
samples a r e comparable with t h a t of U02 ( F i g u r e 3 ) .
J 0 2 DOPED ? x i t 0 2 'GC I
Figure 3 REFERENCES
OOPED 2 8 % CEO2
