J . Phys. Chem. 1985,89, 1334-1336
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of adsorbatesurface interactions. Further work in this direction for both organic and inorganic electrode reactions, in particular by coupling SERS and rotating disk voltammetry,6d is underway in this laboratory.
Acknowledgment. This work is supported in part by the NSF
Materials Research Laboratory at h r d u e and the Air Force Office of Scientific Research. M.J.W. acknowledges a fellowship from the Alfred P. Sloan Foundation. Registry No. C2H4, 74-85-1; gold, 7440-57-5; chloride, 16887-00-6; bromide, 24959-67-9.
EXAFS: A New Approach to the Structure of Uranium Oxides Geoffrey C. Allen,* Paul A. Tempest, Central Electricity Generating Board, Berkeley Nuclear Laboratories, Berkeley, Glos, GL13 9PB, U.K.
C. David Gamer, Ian Ross, Department of Chemistry, Manchester University, Manchester, M13 9PL, U.K.
and Deborah J. Jones Department of Chemistry, The University, Southampton, SO9 5NH, U.K. (Received: December 6, 1984)
In view of its importance in the chemistry of the solid state, crystal stoichiometry and hence the structure of inorganic compounds have recently attracted a great deal of interest. Anion excesf defect structures, though relatively unusual, are best exemplified by the commercially important system UOZ+*.Not surprisingly, much effort has been directed toward a full characterization of this structure and more than one defect cluster model described. Here, as the initial step in the use of synchrotron radiation to investigate defects in uranium oxides, we report for the first time EXAFS studies of stoichiometric U02, fi-U307,and a-U308.
Introduction The stable fluorite and orthorhombic crystal structures of U 0 2 and a-U308are known to tolerate gross departures from stoichiometry in forming the superstructures U02+*and U308-l.1-5 The high stability of the cation sublattice ensures that the predominant defects are oxygen interstitials in U02+*and oxygen vacancies in U30E1. A number of intermediate phases, such as U8OI9and U205of unknown crystal structure have been reported in the region of composition where neither pure distorted hyperstoichiometric U02nor pure distorted hptoichiometric U308 are easily formed. Large levels of disorder may be established by oxygen defect clustering, and understanding of the complex structure of these nonstoichiometric phases has been considerably improved by diffraction techniques.2v6 However, diffraction techniques provide information averaged over many unit cells, and details of localized defect structures have been sought by other techniques, including X-ray photoelectron spectr~scopy.~*~ Since the association of oxygen defects in clusters leads to localized changes in the length of the metal-oxygen bonds and the coordination number of the metal site@),’ we considered that valuable information could be obtained from the extended X-ray absorption fine structure (EXAFS)+” associated with an absorption edge of the metal. If x ( E ) is the fractional modulation of the total absorption coefficient r ( E ) above an edge
where po(E) is the background absorption coefficient in the absence of backscattering atoms then, in order to relate x ( E ) to structural parameters it is necessary to convert the energy E into the photoelectron wavevector via 112
k = [$(E
- Eo)]
In this expression E is the incident photon energy and Eo the threshold energy of that particular absorption edge. For the plane-wave approximation x ( k ) in k space is then given by
x ( k ) = CNpj(k)F,(k) exp(-2uj2k2) I
X
exp(-2rj/X(k)) sin (2kr1 + @,(k))/kr; (3)
Here Nl is the number of atoms in thejth coordination shell, u is the Debye-Waller factor, and X is the mean free path of the
electron. r, is the distance from the absorber of an atom in the jth coordination shell and aj the phase shift experienced by the photoelectron. F4 is the backscattering amplitude and Sj(k) an amplitude reduction factor.
Results and Discussion
(1) Allen, G. C.; Tempest, P. A. J. Chem. Soc., Dalton Trans. 1982,2169. ( 2 ) Willis, B. T. M.J. Phys. Radium 1964, 25, 431. (3) Catlow, L. R. A. Proc. R. Soc. London, Ser. A 1977, 353, 533. (4) Greaves, C.; Fender, B. E. F. Acra Crystallogr., 1972, 28, 3609. (5) Loopstra, B. 0. Acra Crystallogr. 1964, 17, 651. (6) Belbeoch, B.; Piekarski, C.; Perio, P. Acro Crystallogr. 1%1,14,837. ( 7 ) Allen, G. C.; Tucker, P. M.;Tyler, J. W. J. Chem. Soc., Chem. Commun. 1981, 691. (8) Allen, G. C.; Tucker, P. M.;Tyler, J. W. Vacuum 1982, 32, 8, 481. (9) Sayers, D. E.; Lytle, F. W.; Stern, E. A. Adu. X-Ray Anal. 1970, 13,
Studies of silicate glasses containing uranium have recently been reported by the Argonne EXAFS groupI2 and U-0 distances derived for these amorphous materials. To initiate an EXAFS investigation of nonstoichiometry in crystalline uranium oxides, we have recorded the X-ray absorption spectra for U02,B-U307, and a-U308, three fundamental oxides in the above nonstoichiometric range. Samples were handled under dry nitrogen and incorporated in dry nujol to prevent air oxidation and to avoid problems of operator contamination. Under these conditions changes due to oxidation were most unlikely during the short time scale of the experiment. EXAFS data were recorded on the
(10) Stem, E. A. Sci. Am. 1976, 234, 96. (1 1) ‘EXAFS Spectroscopy: Techniques and Applications”, Teo, B. K., Joy, D. C., Eds., Plenum Press: New York, 1981.
(12) Knapp, G. S.;Veal, B. W.; Lam,D. J.; Paulikas, A. P.; Pan, H. K. Mater. Lett. 1984, 2, 253.
-
7AA .-.
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The Journal of Physical Chemistry, Vol. 89, No. 8, 1985 1335
Letters
169 170 171 172 173 17.6 175 176 177 178 179 18C Energy I keV
Figure 1. The X-ray absorption spectra of the LIll absorption edge of U02, &U30,and a-U30s.
high-energy side of the uranium LIIIedge in two separate experiments: (i) using the facilities of Station 7.1 of the Daresbury Synchrotron Radiation Source operating at an energy of 2.0 GeV with a current of ca. 100 mA for powdered U 0 2 and a-U308 diluted in boron nitride, at room temperature, with a Si (220) monochromator; (ii) at the storage ring DCI, operating at an energy of 1.72 GeV with a current of ca. 100 mA using the EXAFS 1 setup at Laboratoire pour 1'Utiliition de Rayonnement Electromagn6tique (LURE), Orsay for undiluted U 0 2 and W307 at 77 K,with a Si (400) monochromator. The X-ray absorption spectra are presented in Figure 1; each EXAFS and its Fourier transformed spectrum was obtained with either the EXAFS suite (U02, U308) of programs available at the Daresbury Laborat~ry'~ or that at the Centre National Universitaire Sud de Calcul, Montpellier, France14 (U02, &u307). The raw and treated data for U 0 2 recorded at both laboratories were essentially the same. Thus transformed data for all phases are presented together in Figures 2 and 3 without specific identification of the laboratory concerned. As the Fourier transforms have been produced without consideration of the absorber and scatterer phase shifts for the backscattering atoms, the peaks are not expected to correspond exactly to the true interatomic distances. U02and U308display a small preedge feature ca.60 eV below the principal absorption maximum; this feature appears at lower The position of the edge is at a slightly higher energy in U307. energy for a-U308 and @-U307 compared with U02,consistent with the net formal oxidation state of the former oxides (5.3 and 4.7) being greater than that of U 0 2 (4). The amplitude of the EXAFS associated with the uranium LIII absorption edge is generally greater for U02than a-U308or @-u307. Such differences are expected, given the more ordered structure of the
-
6.6 7.2
8.0
Figure 2. EXAFS (X k2) associated with the uranium L-IIIabsorption edge of U02, 8-U3O7,and a-U308.
QI
U
3 +
--
t
U
0,
?
u
0 al
[L
15
(13) Pantos, E. Daresbury Laboratory Preprint, DL/SCI/P346E, 1982. (14) Pascal, J-L.; Potier, J.; Jones, D. J.; RoziSre, J.; Michalowicz, A. Inorg. Chem.. in press.
-
1.6 2.6 3.2 6.0 6.8 5.6 k /!a.u.)-'
30
-
6 5 6 0 7 5 9 0 1 0 5 120
R/au
Figure 3. Fourier transform of the EXAFS associated with the LII, absorption edge of U02, &U30,, and a-U30s.
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J. Phys. Chem. 1985, 89, 1336-1339
former,lS as compared to the latterus Also, it is important to remember that, for a-UsOs, the EXAFS is a summation of that for each of the two types of uranium in this lattice. These EXAFS are interesting, not the least because the backscattering contributions from oxygen and uranium are manifest in different regions of k space, but also due to the very different 2 values of these atoms. Thus, the backscattering from oxygen, essentially those atoms comprising each metal's inner coordination shell, dominates the region k < ca. 3.5 a d , whereas the EXAFS a t higher k is derived almost entirely from backscattering by uranium atoms. The different frequency of the EXAFS oscillations in these two regions of k space correlates with the relative U-0 and U-U separations. Also, the slightly greater separation between the oscillations a t low k in the EXAFS of a-U308,as compared to the corresponding features for UO,, is consistent with the slightly shorter average U-O distance of the inner coordination sphere of a-U30s as compared to that of U02. The Fourier transform of the EXAFS from k space to R space yields a function similar to a radial distribution. Thus the Fourier transform of x ( k ) (Figure 2) yields the spectra of Figure 3, which clearly reveal that the EXAFS is comprised of backscattering contributions from several (13) shells of atoms, up to an (apparent) distance of >8 au (1 au = 0.5291772 A) from uranium. In each case the first peak is attributed to a shell of coordinated oxygen atoms. UOz adopts the fluorite structure,ls and each uranium is coordinated by a cubic array of oxygen atoms with U 4 = 4.48 au. Each of the metal atoms in dJ308 is surrounded by six oxygen atoms a t a distance between 3.91 and 4.21 au; in addition, one uranium has contact with a seventh oxygen at 4.61 au while the seventh oxygen atom of the other uranium is at 5.12 aua5These latter structural data are consistent with the profile of the lowest R feature in the Fourier transform of the a-U308 EXAFS data. Although no single-crystal structure determination has been carried out on p-U307,its neutron powder diffraction pattern corresponds to a tetragonal unit cell of dimensions similar to those of the U 0 2cubic lattice ( a = 10.17 au, c = 10.49 au).I6 The Fourier transform has a distribution of peaks similar to that of U 0 2 but their relative intensities differ. This is interpreted as indicating a modified U02-like structure, with an average U-O distance of 4.48 au and an average U-U distance of 7.36 au. A (15) Asprey, L. B.; Ellinger, F. H.; Fried, S.;Zachariasen, W. H. J . Am. Chem. SOC.1955, 77, 1707. (16) Hoekstra, H. R.; Santoro, A.; Siegel, S.J. Znorg. N u l . Chem. 1961, 18. 166.
further quantitative treatment aimed a t deriving values for the coordination numbers and Debye-Waller factors is presently underway. The interpretation of the backscattering from the uranium atoms is more complicated than that from the oxygen atoms, especially with respect to the profile of the Fourier transforms above k of ca. 6 au. Backscattering from uranium is expected to be complex, in view of the element's high 2 value, and even a single shell of backscattering atoms may not be apparent in the Fourier transform as a single peak with the expected magnitude. Also, backscattering contributions via multiple-scattering pathways, especially involving uranium, are expected to be significant in these systems and such effects would further complicate the interpretation of the backscattering from shells at higher R values. We have been restricted in the simulation of these EXAFS profiles since (i) neither the published ab initio data of Teo and Lee17 nor those calculated by the ab initio approach at the Daresbury Laboratory have been extended to the actinides and (ii) it would be desirable to include relativistic effects in order to obtain accurate phase shifts. Under these conditions, interpretation of the EXAFS spectra must be undertaken by using model compounds. Such quantitative analysis is in progress in our laboratories.I8 However, we observe that the net frequency of the EXAFS oscillations in the region where k > 3.5 au-I is very similar for U 0 2 and U307, and that this is less than that for a-U30s,in agreement with the slightly smaller average U-U separations for UO, as compared to U308. Therefore, this study has demonstrated that EXAFS of a high quality can be obtained for UO,,@-U30,and a-U30s. The information contained therein clearly implies that this technique can provide valuable structural information for localized regions in nonstoichiometric compounds. The U02-U308 system contains many structures with complex anion defects. In this respect a series of EXAFS experiments on a range of oxides of uranium are currently being performed at LURE, Orsay, France.
Acknowledgment. We thank the S.E.R.C. for the award of studentship (to I.R.) and for provision of facilities at the Daresbury Laboratory. We also acknowledge technical support at Orsay. This paper is published with the permission of the Central Electricity Generating Board. Registry No. U02, 1344-57-6; U307, 12037-04-6; U308, 1344-59-8. P. A. J . Am. Chem. SOC.1979, 101, 2815. (18) Allen, G . C.; Jones, D. J.; Tempest, P. A., to be submitted for publication. (17) Teo, B. K.; Lee,
Fluorescence Decay of Jet-Cooled Acetone Odd Anner, Hanna Zuckermann, and Yebuda Haas* Department of Physical Chemistry and The Fritz Haber Center for Molecular Dynamics, The Hebrew University of Jerusalem, Jerusalem 91 904, Israel (Received: December 14, 1984) The fluorescence decay kinetics of acetone and acetone-d6are measured after cooling in a supersonic nozzle beam expansion. At the origin, the decay is nearly exponential, with decay times 1.O ps for acetone-h, and 3.2 ps for acetone-d6. At higher energies, a very fast decay component is observed for CH3COCH3at energies 700 cm-I and over above the origin, and for CD,COCD3 beyond 400 cm-I. This decay component is similar to the one observed at room temperature for any excitation energy. It is assigned to efficient coupling between the initially excited Sl(n?r*) state to another, possibly the Tl(n?r*) state. The onset of this fast radiationless process may be related to the fact that the barrier for internal rotation of CH, torsion is at about 740 cm-I and for C=O out-of-plane inversion at 470 cm-' [ref 61.
Introduction The photochemistry of acetone has been extensively studied, mostly in liquid solutions' or in the high-pressure gas phase.2 (1) J. G . Calvert and J. N. Pitts, "Photochemistry" Wiley, New York, 1966.
0022-3654/8S/2089-1336%01.50/0
Under these conditions, vibrational relaxation is very rapid, possibly masking relatively slow processes that may be observed under collision-free conditions. We have recently reported3 the decay (2) E. K. C. Lee and R. S. Lewis, Adu. Photochem., 12, 1 (1980).
0 1985 American Chemical Society
