224
J. Phys. Chem. 1982, 86, 224-228
Oxidation of Uranium Dloxlde at 298 K Studled by Using X-ray Photoelectron Spectroscopy G. C. Allen,' P. M. Tucker, and J. W. Tyler Central Ektrlcity Generating Bead, Berkeley Nuclear Laboratories. Berkeley, Gloucestershire GL 13 9P8, Unlted Klngdom (Received: August 5, 198 1; I n Flnal Form: September 30, 198 1)
The oxidation of U02 at 298 K has been studied by using X-ray photoelectron spectroscopy with a view to the characterization of U 4f shake-up satellites as a possible monitor of surface composition. On oxidation a U 4f shake-up satellite positioned at 8.2 eV to the high binding energy side of the U 4f5/2,7/2 peaks is observed to grow progressively. It is proposed that this satellite is associated with the presence of anion defect clusters in UOz+x.
Introduction A study of the chemical environment experienced by U in nuclear fuels using X-ray photoelectron spectroscopy (XPS) can identify the mechanisms involved in reactions between the fuel and ita coolant and containers. To this end various well-characterized U compounds have been studied at this laboratory. These include U metal,'f binary oxide^,^ some ternary oxides, and uranyl compound^.^ In an earlier study of a series of oxides of different stoichiometryranging from U02 to UO,V binding energies for the U 4f core lines were recorded. The magnitude of the chemical shifts observed within the series U02, U02.1, U02.2,U40g,U307,U308proved to be small. However, for UOzoolweak structures were noted on the high binding side of each of the U 4f core lines at 6.9 eV and attributed to shake-up satellites.6 In an XPS study of commercially available U02 powder, Pireaux et al.7 observed three satellites situated at 548.2, and 16.0 eV to the high binding energy side of the U 4f512peak. However, surface oxidation of UOz.oois known to occur in air at room temperature8 and among the products of oxidation are U409and interstitial oxides of type U02+, where 1c lies between 0 and 0.25. Thus, when investigating U02.00using XPS, it is important to ensure that stoichiometric U02.00is free from surface contamination by UOZ+% phases. Recently we reported high-resolution spectra of core and valence electrons in U and U02.001where two different techniques were used to prepare stoichiometric UOZm The first involved reacting O2 with a clean U surface and allowing the surface film of U02 formed to equilibrate with the bulk U. The second involved the preparation and analysis of a sample in pellet form as described previ~usly.~ In each case a single satellite at 6.8 and 16.1 eV was observed associated with the U 4f612,712 core lines to the high binding energy side of these peaks. The weak satellite at 16.1 eV was assigned as an energy loss peak. The satellite (1) G. C. Allen and P. M. Tucker, J. Chem. Soc., Dalton Trans., 470 (1973). ~ - - -,. . (2) G. C. Allen, I. R. Trickle, and P. M. Tucker, Philos. Mag., [Part] B,, 43, 689 (1981). (3) G. C. Allen, J. A. Crofts, M. T. Curtis, P. M. Tucker, D. Chadwick, and P. J. Hampson, J. Chem. Soc., Dalton Trans., 1296 (1974). (4) G. C. Allen, A. J. Griftitha, and B. J. Lee, Transition Met. Chem. (Weinheim, Ger.), 3, 229 (1978). (5)G. C. Allen, J. A. Crofta, M. T. Curtis, and P. M. Tucker, CEGB Report RD/B/N2348, 1972. (6) G. C. Allen and P. M. Tucker, Chem. Phys. Lett., 43, 254 (1976). (7) J. J. Pireaux, J. Riga, E. Thibaut, C. TenretNdl, R. Caudano, and J. J. Verbist, Chem. Phys., 22, 113 (1977). (8) E. H. P. Cordfunke, "The Chemistry of Uranium", Elsevier, Amsterdam. 1969.
QQ22-3654/02/2Q06-Q224$0 1.2510
at 6.8 eV was assigned to a shake-up transition involving the predominantly 0 2p and U 5f molecular orbitals. Baer and Schoenesgand Beatham, Orchard, and Thornton'O also reported a single satellite associated with U 4f peaks at 7.1 and 6.7 eV, respectively. When a stoichiometric U02.00 surface was exposed to O2 (>1X torr) at room temperature, significant alterations in the satellite positioned at 6.8 eV were observed-notably the generation of a second satellite in the 8-eV region.2 This study investigates the surface oxidation of U02.00 in situ by monitoring the U 4f core level binding energies, their associated satellites, and the valence region, the aim being to characterize the U 4f satellites as a possible monitor of surface composition.
Experimental Section The spectrometer used in this work was the Kratos ES 300 electron spectrometer described previously." X-ray photoelectron spectra were produced by using A1 and Mg Ka radiation, the anode being routinely operated at 300 W with source and collector slit widths set to 1.8 and 3.0 mm, respectively. The spectrometer was operated in the fixed analyzer transmission scanning mode with a pass energy of 65 eV. The spectrometer energy scale was internally calibrated by using the known 233.0-eV difference in the photon energies for A1 and Mg Ka radiation.12 Natural U metal was cut to shape for mounting (26 X 10 X 2 mm) and electropolished in a solution of one part each of H20,H$04, and.40H & ' The foil was washed with deionized HzO and stored under dry CH,OH before transfer to the spectrometer. The U surface was cleaned with an Ion-Tech B22 wide beam ion source operating typically at 4 kV with an ion current of 0.04 mA in a dynamic pressure of 1 X torr of Ar (British Oxygen, Research Grade) for 5 h with thermal cycling to 673 K. The spectrometer base pressure (-1 X torr) preserved the U surface in an essentially uncontaminated state for 6-7 h. A surface film of stoichiometric U02.00on the U metal was produced in situ in the spectrometer following the procedure of Allen et alS2The clean U metal was exposed to O2 (British Oxygen, Research Grade) (3 X torr of dynamic pressure, 393 K, 30 min) and the surface film of (9) Y. Baer and J. Schoenes, Solid State Commun., 33, 885 (1980). (10) N. Beatham, A. F. Orchard, and G. Thornton, J. Electron Spectrosc. Relat. Phenom., 19, 205 (1980).
(11) G. C. Allen, P. M. Tucker, and R. K. Wild, Surf. Sci., 68, 469 (1977). (12) G. C. Allen, P. M. Tucker, and R. K. Wild, J. Chem. Soc., Faraday Trans. Z,, 74, 1128 (1978).
0 1982 American Chemical Society
The Journal of Physical Chemistry, Vol. 86, No. 2, 1982 225
Oxidation of Uranium Dioxide at 298 K
TABLE I: Binding Energies of U 4fS,Z,7,, and 0 1 s Core Levels in U , UO,, and UO,,, binding energy, eV
U 4f,,,
sample U U02.W
+ 1 6 h a t -5
x torr of 0,, 298 K UO, t 32 h at -5 X
UO,
lo-)
torr of 0,, 298 K UO, + 120 h at -5 X lo-’ torr of 0 2 ,298 K UO, + 120 h at - 5 X torr of 0, t 16 h at several tdrr of 0,, 298 K sample warmed (393 K) -5 x torr of 0,. 30 min
U
( t O . 1 eV)
fwhm
satellite positions
(+0.1eV)
fwhm
388.2 391.2 391.5
2.2 2.6 2.7 (5)
13.7 6.7, 16.1 6.4, 8.2, 16.1
377.3 380.3 380.6
2.2 2.6 2.7 (5)
530.3 530.0
1.8 1.8 (5)
391.6
2.9
6.3, 8.1, 16.4
380.7
2.9
530.0
1.8 (5)
391.5
2.9
6.4, 8.2, 19.2
380.6
2.9
1.8 (5)
391.6
2.9
6.3, 8.2, 18.6
380.7
2.9
530.0, 532.8 (small shoulder) 530.0, 532.9 (small shoulder)
391.2
2.6 (5)
7.0, 18.7
380.4
2.6 (5)
0 1s
fwhm
530.3, 533.0 (small shoulder)
1.8 (5) 2.0 (5)
Satellites of the U 4f7,, peak were distorted or appear beneath the U 4f,,, peak. TABLE 11: U 4f7,, Binding Energies for a Series of Uranium Oxides oxide stoichiometry
*n
binding energy, eV
.........
Allen et al.3 this work Allen et a1.l 377.1 380.0 380.6 380.7 380.8 380.8 381.1 381.9
377.3 380.3
377.2 380.2
U02 formed was left for 12 h at -1 X torr to equilibrate with the bulk U metal a t room temperature. The stoichiometric film of UOzo0was subsequently exposed to higher partial pressures of O2 (16,32,120h at -5 X torr, and 16 h at several torr of 0,)at room temperature in the spectrometer preparation chamber. Spectra of the U, 4f5/2,7/2 peaks with associated satellites and the valence band region were recorded. In order to improve the signal-to-noise ratio and the resolution, many spectra were subjected to computer accumulation and averaging using a Varian 620/L minicomputer.” Stoichiometric U02 is a moderately good conductor at room temperature,8.14and charging effects were found to be absent in U02.00samples by Veal and Lam15 and Allen et aL2 No evidence for surface charging was found in the present work. U40, was prepared following the procedure of Crofta and Swan.16 A weighed sample of U02+zwas reduced to U02.0 by heating in CO for 16 h at 623 K and cooling in the resultant CO/CO2atmosphere (now in the ratio 9:l). The apparatus was evacuated and the required amount of oxygen admitted to produce stoichiometric U409 The oxide was then equilibrated by heating to 573 K for 16 h to ensure homogeneous oxidation.
Results and Discussion Binding Energies of U 4f Core Level Peaks in U, u02.00, and U02+,. Binding energies measured for U 4f core electrons in clean U and stoichiometric and nonstoichiometric UOz are presented in Table 1. The binding ~~
(13) G.A. Swallow and P. M. Tucker, CEGB Report RD/B/N4331, 1978. (14) P. Nagels, J. Devreese, and M. Denayer, J. Appl. Phys., 35,1175 (1964). (15) B. W. Veal and D. J. Lam,Phys. Rev. B, IO, 4902 (1974). (16) J. A. Crofts and T. Swan, CEGB Report RD/B/N2170, 1971.
uoztx
1
L, e.....
0.8
I
I
374 I
414.7
BINDING ENERGY I eV Figure 1. U 4f photoelectron region for U02 and U02+,.
energies reported for clean U and UOz.ooare in excellent agreement with those reported previously2%which are reproduced in Table 11. Representative spectra of the U 4f peaks for U02,,,,, and UOz+, (-5 X torr of 02,298 K, 120 h) are shown in Figure 1. During surface oxidation of U02.00,the U 4f peaks exhibit a small shift to higher binding energies and also broaden slightly. As noted earlier: the binding energy of the U 4f peaks increases with increasing formal oxidation number and the full width at half-maximum (fwhm) increases along the series U02.0, UOZ1,and UO2.* The structures of various U oxides have been reviewed by Cordfunke8 and Eyring.” U02.00possesses a fluorite structure, and the U02+, phases may be viewed as being formed by the addition of oxygen rather than the removal of U. The U sublattice remains undisturbed by the rearrangement of the atoms in the oxygen sublattice. Binding energies and line widths for the 0 1s peak are also presented in Table I. A chemical shift of 0.3 eV to lower binding energy occurs on exposing the U02.00surface to 02.The position of the 0 1s peak remained constant on subsequent exposures. However, when the sample was heated in O2 (5 X 10” torr, 393 K, 30 min), the binding energy of the 0 1s peak returned to the U02.00value. The (17) L. Eyring in ‘High Temperature Oxides”,Part 11, Allen M. Alper, Ed., Academic Press, New York, 1970, pp 41-97.
228
The Journal of Physical Chemistry, Vol. 86, No. 2, 1982 ~
m
s
Allen et ai.
m
h
In
4.-
C
3 >,
e .-
L
c
ea
Y
W
2 a I-
z
3
8
I
1
1
613 6
J
I4 6
I
I
I
I
3%
BINDING ENERGY l e v Flgure 2. U 4f satellites from U02 and U02+x
U 4f peaks also displayed binding energies characteristic of the UOeaoo surface. While there was no indication of surface contamination during the development of the satellite through the oxidation process, a small shoulder on the high binding energy side of the 0 1s peak was observed after the UO,.,,, sample had been exposed to O2 in the spectrometer preparation chamber for several days. The peak at -532.9 eV is characteristic of hydroxide formation and is due to contamination from residual H20 adsorbed on the walls of the stainless-steel chamber. When the sample was exposed to several torr of 0,for 16 h at 298 K, no significant change was observed in the U 4f binding energies or the associated satellite structure. By comparing the U 4f512,712 binding energies with those recorded during an earlier study3 (see Table 11), one can estimate x. Following exposure of the stoichiometric UOZw surface to 0,(-5 X 298 K, 120 h), the U 4f5127,2peaks exhibited a chemical shift of 0.3 eV, which corresponds to x = 0.07 f 0.02. Satellites in the XPS Spectrum of U02and U02+z.The U02.00spectrum shown in Figure 1 shows two satellites associated with the U 4f peaks. The weak peak at 16.1 eV has been attributed to an energy loss process and that at 6.7 eV to shake-up excitation of an electron from the 0 2p-U bonding band to partially occupied or unoccupied localized metal 5f level^.^^^^' Recent cluster calculations performed by Weber and Gubanovls on (UOs)12-,which is representative of UO,, have shown that 0 2p to metal 5f excitation is the only feasible shake-up process occurring in the final state following U 4f core ionization. The U 4f photoelectron spectrum recorded from U02+* is also shown in Figure 1. Here two satellites are observed to the high binding energy side of the 4f core levels. These are expanded for greater clarity in Figure 2. The satellite a t -6.7 eV corresponds to that observed in UOZmbut its precise position was sensitive to the extent of surface oxidation. Initially a small shiftto lower binding energy was observed with increasing O2 dosage until its position stabilized at -6.4 eV to the high binding energy side of the U 4f peaks. The satellite at 8.2 eV, on the other hand, appeared following exposure of the U02.00surface to O2and intensified steadily as oxidation proceeded. The (18) J. Weber and V. A. Gubanov, J. Znorg. Nucl. Chem. 41, 693 (1979).
BINDING
ENERGV I eV
I
373 6
Figure 3. U 4f region recorded from U,Op after (a) insertion Into the spectrometer and (b) exposure to vacuum for 4 days.
origin of this second satellite in the spectrum of oxidized UO, is uncertain, but a number of pmibilities exist. Since XPS is a surfacesensitive technique providing information from the 1-10 atom layer region near the surface, the oxidation of UO,.,,, could by limited to the surface region. Thus, a new oxide such as U40g, U3O7, U308,or U03 could be formed as a separate phase above the underlying U02, but measurements of the shift in position of the U peak indicate a relatively low degree of oxidation corresponding to the formula U02.07. Although these results could derive from a new oxide formed as a very thin or incomplete layer on the UO, surface, measurements from freshly prepared and characterized samples of the higher oxides reveal distinctly different peak profiies and satellite structures in the U 4f region compared with these reported here (see the spectrum recorded from U409in Figure 3, for example). Moreover, inhomogeneous oxide mixtures would be expected to give broad U 4f peaks and ill-defined satellites. Alternatively U02could undergo oxidation at the surface to form a mixture of the oxides UO,, U02+,, or U40g identified with neutron diffraction by W i l l i ~ . ' ~The ~~~ satellites may then be assigned to the U02.00and U02+, or u4og structures, respectively. In this respect the U 4f X-ray photoelectron spectrum recorded from U409and reproduced in Figure 3 shows particularly interesting behavior. When the sample is left in the spectrometer for 4 days at room temperature and 1 X lo4 torr, the surface apparently changes to give a spectrum very similar to that of U02+% in Figure 1. Oxidation of U02 is known to take place through the incorporation of oxygen at interstitial sites, and as a consequence a neighboring lattice oxygen is forced from its original site creating a composite center or " ~ l u s t e r " . ~Above ~ 9 ~ -400 OC the phase diagram of the uranium-oxygen system shows that the product of such oxygen dissolution is hyperstoichiometric UO,,,. At the low oxygen partial pressures and temperatures of this experiment though thermodynamic data indicate that UO,, disproportionates into a less oxidized form of UO,,, and into U409 or, more exactly, into nonstoichiometric U,O, the latter having a structure very similar to U02+, and y is very sma11.21i22 (19) B.T.M.Willis, Nature (London), 197, 755 (1963). (20) B. T.M. Willis, J. Phye. Radium, 25, 431 (1964). (21) P. 0.Perron, AECL Report 3072, Chalk River Nuclear Laboratories, Chalk River, Ontario, Canada, 1968.
The Joumal of Physical Chemistry, Val. 66. No. 2, 1962 221
OxMatiin of Uranium DioxMe at 298 K
[;lo1
ursmvm atom Normal oxygen atom
@ Figure 4. Structure of U O ,
bl*l.tl,lll
0 atom
I"telst,t,dl
0 atom
showing the 2 2 2 clustw.
Experimental evidence therefore supports the notion that a U02, + U,O* solid solution is the oxidation product, but a t first sight it is puzzling that such closely related structures should produce distinct satellites in the XPS spectrum. An explanation may be found in the fact, mentioned above, that the structures of UOz+, phases may be viewed as defect clusters formed by interstitial addition of oxygen rather than removal of uranium. The fluorite structure of U 0 2 may be described as a simple cubic packing of Oz- ions in which alternate positions of cubic coordination are occupied by metal ions. For UOz+,9 though, the precise location of the interstitial oxygen ions is still not settled despite two neutron diffraction studi e ~ . ' ~Various . ~ ~ models have been proposed for the type of cluster formed. KingeryZ4suggested the existence of molecular oxygen, but Blank and RonchiS using arguments based on bond length data reasoned against this structure. The same arguments may be extended to the views of Kroger,= who regarded the defect center as a peroxide ion, OZ2-. The Willis model shown in Figure 4 (after Saitoz7) has two kinds of interstitial sites occupied by oxygen atoms identified as 0' and 0". These correspond to displacementa from the center of an octahedral hole in (100) and (111)directions, respectively, and may be considered to be accommodated within the initially 8-coordinate structure unoccupied by cations. Here upon oxidation the ex(22) M. H. Rand, R. J. Aelrermann. F. Gronvold. F. L. Oetting, and A. Pattoret, Reu. Int. Haute8 Temp. Refract., IS 355 (1978). (23) N. Masaki and K. h i , Acta Crystnlfogr.,Sect. B, 28,785 (1972). (24) W. D. Kingery, Commisaarist a I%nqie Atomique (CW) Report CEA-S5, 1965. (25) H. Blank and C. Ronehi, Acto Crystalfogr., Sect. A, 24, 657
,.""",. ,,ocIII
(26) F.A. Kr6ger. Z. Phys. Chem., 49, 178 (1966). (27) Y. Saito, J. Nucf.Mater., 51, 112 (1974).
cem oxygen atom 0' enters the fluorite lattice at a position 0.86 A along the (110) direction from the center of the unoccupied cubic hole. Its presence displaces the two nearest-neighbor oxygen atoms, 0", from their normal lattice sites to positions 1.05 b, along ( l l l ) , leaving normal oxygen vacancies, denoted V,, in the oxygen sublattice. It is now possible to insert a second 0' atom along (110) without further displacement of the normal cubic oxide ion array to form a rhombic interstitial aggregate of oxygen atoms, in which the average 0-0 separation is -2.34 A compared with the normal 2.74 b, 0-0 distance in UO,. Figure 4 shows such a cluster, in which the ratio of numbers of V,:O':O" is 2 2 2 , the model favored by Willis in his most recent consideration of hyperstoichiometric U02.= We conclude that the development of a satellite a t 8.2 eV could be related to the formation of a diphasic oxide containing such "Willis-type" clusters in which an individual oxygen atom has a total negative charge less than in the normal isolated oxide ion. The new feature in the XPS spectrum may therefore be assigned to a shake-up (charge transfer) of an electron from the oxygen cluster to metal 5f or 6d levels following photoionization, and the small shift in the position of the original satellite from 6.7 to 6.4 may then also be understood to result from charge equilibration following stabilization of the new lattice. Hence, satellite structure may be a delicate monitor of surface stoichiometry for it is noteworthy that a similar feature was seen between 9.4 and 10.5 eV to the high binding energy side of the U 4f5,2line in metal uranates." This satellite, which occurred in all of the uranate spectra, agreed in position with those recorded from UO,, U308, and U205by Pireaux et al.7 and were considered by these authors to be characteristic of the oxidation state U(V1). To this point, only the U 4f photoelectron spectrum has been considered, but the 0 1s region would also be expected to reflect these changes. In accord with previous observations,3 the 0 1s peak was observed to shift from 530.3 to 530.0 eV on oxidation consistent with a less covalent oxygen coordination when additional atoms are incorporated within the lattice. The charge state of the interstitial in the anion excess is, of course, unknown, but experimental data accord best with a model for the anion excess phase of doubly charged interstitials compensated by oxidation of lattice cations to the pentavalent ~ t a t e . 2 ~ An increase in cation charge, it could be argued, would lead to an increase in covalency of the metal-oxygen bond. In a study of some alkaline earth metal uranate compounds in which the uranium atom has mainly oxidation state VI, 0 1s binding energies spanned the 529.5-533.4-eV range and certain general trends could be related to the structural chemistry of these systems! Moreover, among nonstoichiometric compounds only Fel.,O has been as thoroughly studied as UOz+,, and it is interesting to note, therefore, that in an XPS study of Fe,,O Oku and K i m kawa= considered that the relatively large width of the 0 1s peak implied vacancy clustering. Here also an increase in the half-height peak width was observed but only to the extent of -3%. Valence Region. Valence band spectra for UOz and the same surface following exposure to -5 x io-, torr of O2 at 298 K for 120 h are shown together in Figure 5. The spectrum recorded from U02 agrees well with the previous s t ~ d i e s . 2 J ~The ~ ' ~sharp ~ ~ ~and ~ ~intense ~ band near the ~~
~
(28) B. T. M. Willis. Acta Cryatollogr., Sect. A, 34, 88 (1978). (29) J. S.Anderaon, D. N. Fdingtnn, L. E. J. Roberts. and E. Wait, J. Chem. Soc.. 3324 (1954). (30) M. Oku and K. Hirokawa, J. Appl. Phys., 50, 6303 (1979). (31) B. W. Veal and D. J. Lam. Phys. Lett. A, 49,466 (1974). (32) S. Evans, J. Chem. Soe., Faraday Tram. 2.9, 1341 (1977).
228
Allen et al.
The Journal of Physical Chemistty, Vol. 86, No. 2, 1982 17 6
9 L 7 6 6.0 L l
1L I
1
BINDING ENERGY I eV
p-type semiconductor at the surface. Our spectra also show alterations within the fine structure of the 0 2 p U bonding band on oxidizing U02.00. In agreement with the calculations of Gubanov, W n , and Ellis,”the XPS valence-band spectrum in Figure 5 depicts the 0 2p (bonding-band) as two weakly resolved maxima, and it is the lower binding energy one of these which intensifies on oxidation. That this should be the region in which interstitial oxygen is evidenced in the photoelectron spectrum is given further credence by the sputter etching work of Veal and Lam,15 whose results showed a corresponding diminution in intensity in the same region. Whereas our results may be taken to indicate the incorporation of interstitial oxygen, theirs demonstrated its removal, since, although it is likely that oxygen is equally readily removed from either site, the difference in strain energy is such that an oxygen ion vacancy on a lattice site would be quickly filled by a nearby interstitial oxygen atom.
Fermi level (EF)at 1.5 eV is attributed to U 5f electrons, the broader band a t -6.0 eV is assigned as the 0 2p-U “bonding band” formed from overlapping oxygen 2p and metal orbitals, and the peak at 18.0 eV stems from the U 6p3/2 level. The spectrum recorded by Verbist et al.33 showed a marked reduction in the U 5f/0 2p (bonding band) intensity ratio compared with those reported previously and that reproduced in Figure 5. Beatham et al.’O have commented that this is indicative of contamination of the surface by U02+%phases. Veal and Lam15P1preferentially removed oxygen from U308and U02+xby bombardment with argon ions. They noted a systematic increase in the intensity of the U 5f peak and a small shift in its position away from EF. Moreover, as U308was systematically converted to U02 through several intermediate concentrations, the U 5 f / 0 2p (bonding band) intensity ratio was observed to increase steadily, indicating a transfer of the metal-oxygen bonding electrons back to the localized 5f-electron states as the oxygen is removed from the lattice. These experiments essentially reverse the process reported here. When our U02.00surface was oxidized, a significant reduction in the U 5f peak intensity was observed together with a small shift, -0.1 eV, toward EF. Thus,for small increases in the O/U ratio above 2.00, fairly large changes occur in the occupation and the character of the U 5f levels. Moreover, the shift toward EF is consistent with the interpretation that the diphasic oxide film U02+z U40,, described above, forms as a heavily doped
Conclusions Not surprisingly our understanding of the X-ray photoelectron spectroscopy of the uranium/uranium oxide surface has markedly improved during the past 10 years. Originally it was one of our intentions to develop the technique as an analytical method for the determination of stoichiometry in the U02+zsystem. However, the total chemical shift in the uranium 4f core lines observed acrow the oxide series U02 to U308is relatively small so that for values of x < 0.1 X-ray diffraction provides a more accurate determination of stoichiometry. Notwithstanding this limitation, for small values of x , photoelectron spectroscopy can accurately determine changes in oxidation state and can give fairly precise qualitative information about compositional changes. More specific information can be obtained from detailed observation of fine structure in the spectra. Thus,the preceding results show that the satellite structure observed between 6 and 9 eV below the uranium 4f core levels can be explained on the basis of shake-up transitions involving electronic excitation from the mainly 0 2p band into the U 5f levels. This fine structure in the photoelectron spectra has also revealed other interesting surface features which may be attributed to the presence of discrete clusters or extended defects in two or three dimensions. Moreover, the measurements would appear to permit the identification of the U02+xsurface phase as U02+x+ U40, which under the conditions of our experiment persists throughout the range of stoichiometry U 0 2 U409. Acknowledgment. This paper is published by permission of the Central Electricity Generating Board.
(33) J. Verbist, J. Riga, J. J. Pireaux, and R. Caudano, J.Electron Spectrosc. Relat. Phenom., 5, 193 (1974).
(34) V. A. Gubanov, A. Roeln, and D. E. Ellis,J.Inorg. Nucl. Chem., 41,975 (1979).
Flgure 5.
Valence band changes following oxldatlon of U02to UO,+, .
+
-
