A J Kropf

D T Reed

S B Aase

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X-ray Absorption Near-Edge Spectroscopy of Plutonium Solid Species
A.J. Kropf, D.T. Reed, and S.B. Aase
Argonne National Laboratory
Chemical Technology Division
9700 South Cass Avenue
Argonne, IL 60439
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To be presented at
10* International Conference on X-Ray Absorption Fine Structure
Illinois Institute of Technology (IIT)
Chicago, Illinois
August 10-14, 1998
This work is supported by the U.S. Department of Energy, Office of Environmental
Restoration and Waste Management, under Contract W-3 1-109-ENG-58.
This work not performed at Argonne National Laboratory.
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X-ray Absorption Near-Edge Spectroscopy of Plutonium Solid Specks
A. J. Kropf, D. T. Reed, and S. B. Aase
Chemical Technology Division
Argonne National Laboratory
9700 South Cass Avenue
Argonne, Illinois 60439 USA
Submitted to Journal of SynchrotronsRadiation
Proceedings of the Tenth International Conference on XAFS
Abstract
We present XANES at the L1lledge for four plutonium solid phases: Pu(III)Fs, PU(IV)02,
NaPu(V)02C03, and B@U(v~06. These correspond to the four important oxidation
states in the process chemistry and environmental chemistry of plutonium. By a fitting
method that uses an arc tangent function and gaussian curves, it was possible to
reproducibly determine the edge energy and distinguish among the four oxidation states.
These data demonstrate a 1.85&0.20 eV shift per oxidation state.
Keywords: XANES, plutonium, actinide, oxidation state
Background
The environmental and process chemistry of plutonium remains an important area
of research as DOE continues to address problems related to the management of nuclear
waste (Banaszak et al., 1998; Lieser, 1995; Silva and Nitsche, 1995). The chemistry of
plutonium is diverse because it can exist as Pu(III, IV, V, and VI) under environmentally
relevant conditions. A key to understanding its environmental chemistry is, for this
reason, its oxidation state. In this context, the application of synchrotron-based
techniques to identify the oxidation state in complex environmental samples has great
utility.
X-ray absorption near edge spectroscopy (XANES) measurements were made at
the Pu L edges of four plutonium phases, PU(XII)F3,PU(IV)02, NaPu(V)02C03, and
&i&U(v~06. Solid phases may be expected to show stronger geometric effects in the
near-edge structure. These effects may show up as changed “white line” intensities,
shifted edge positions, and changed pre-edge features. If XANES is to be used to
determine the Pu oxidation state, it must be determined how reliably these edge features
may be measured and identified. The edge energy is an attractive means of measuring
the oxidation state because of its intrinsic simplicity. However, the use of edge energy to
identify oxidation state is only meaningful if the distortions introduced by differing
environments may be modeled and eliminated. To this end, we are studying Pu
compounds in different oxidation states, as well as compounds with the same oxidation
state but different local environments (not reported here).
Any method of measuring the edge position that results in a unique determination
of the oxidation state is valid; this is a phenomenological approach rather than a
theoretical model of the edge features. Herein, several methods are described which
attempt to define such a metric. Coupling the quantitative measurement with an
evaluation of the spectral features should result in an unambiguous assignment of the
oxidation state when primarily one oxidation state exists in unknown systems.
Sample Preparation
The plutonium samples analyzed by XANES were all fine powders dispersed on
Kapton tape and encapsulated in a styrene matrix to contain the radioactivity during the
analysis. Two layers of Kapton film further encapsulated the sample. Since the actinide
L edges were all at a fairly high energy (>15 keV), even relatively large amounts of
plastic did not significantly reduce the x-ray flux or measured signal. In addition, only
small quantities (c1 pg Pu/mm2) were necessary to obtain a good signal.
The four types of plutonium solids were synthesized using standard methods with
Pu-242 (50% purity by activity, >99% purity by isotope). The PuF3 was synthesized by
reducing Pu to Pu(III), taking the product to dryness in hydrobromic acid, adding fluoride
after dissolution in water, and precipitating. The plutonium oxide was made from high
purity oxide stock, which was provided by Oak Ridge National Laboratory. This was
high-fired to -1 OOO”C. The NaPuOzCOJ was precipitated from Pu(V) stock [made by
electrolytic reduction of Pu(VI)] by titrating with sodium carbonate solution to pH -7.
The BaJpUOG was used as received from stock previously synthesized at Argonne
National Laboratory.
Analysis Methods
Several methods of measuring the edge position were studied to determine which
procedure would produce the most reliable measurement and which would produce the
largest difference among oxidation states. The most common method, finding the
second-derivative zero crossing, is sensitive to noise in the data and is therefore not
adequately accurate. All of the following fitting methods use a larger portion of the data.
Noise in the data can be reduced significantly by performing a polynomial fit of
the edge data (fourth order generally works well), and then finding the second-derivative
zero crossing of the fit function. As shown in the second column of Table 1, this edge
measurement is accurate but does not produce a signature energy for each oxidation state.
In addition, this method is somewhat influenced by the exact choice of end points. Noise
in the data is not of much concern when evaluating these standard compounds, but for
unknowns in which the Pu concentration may be in the 1 ppm range, noise would be an
important consideration.
Using the same polynomial as for the above method, and then calculating the
energy at 25fZ0of the edge step, produces smaller errors (not shown here). After a pre-
edge fit line is subtracted, the data are normalized by the standard techniques employed
in x-ray absorption fine-structure (XAFS) spectroscopy; a fit through the data from about
50 to 300 eV above the edge is extrapolated to the edge. The measured edge position is
fairly insensitive to both the normalization and the end points of the polynomial fit.
Unfortunately, this method also results in an ambiguous oxidation-state determination
due to shape effects.
The method of fitting the edge to an arc-tangent step function and a number of
gaussian curves results in a correlation between the measured edge energy and the
oxidation state. The errors are somewhat larger than the previously described methods.
But as in the previous method, the exact data range is relatively unimportant since a large
amount of data is used to determine the edge energy.
First the background slope is removed using a line to fit the pre-edge region.
Then, the data are fit to the following equation, which is based on the function presented
by Conradson et al. (1998a):
[[ l]E-Fn 2#( E)= A+ Z3xtan-’(Cx(E-Eo ))+~Dnxexp - ~ +Hx(E– Eo)x@-Eo)n=l * )
(1)
A line is added to the post-edge region to account for the slope remaining after the
pre-edge background removal. The edge position, Etj, is defined as the inflection point of
the arc-tangent component. N is the number of gaussian curves. The variables A-D and
F-H are the parameters in the fit, while E is the incident x-ray energy. Since this is a
.
nonlinear least-squares fit, careful attention must be paid to avoid false minima in the
final fit parameters. An example using Eq. (1) to fit the I%(W) data is shown in Fig. 1.
There are other criteria that must be applied to the analysis of the fit results. First,
the arc-tangent step function must accurately reproduce the edge step, neither under- nor
over-shooting the actual step. The physical basis for this requirement is that we are using
the arc tangent as a crude model for the atomic absorption; the EXAFS oscillations are
plus or minus this function. Second, the data range was taken to be from about -30 eV to
+450 eV relative to EO. Since the EXAFS oscillations are not modeled by the fit, and so
far an adequate method of de-emphasizing this region has not been found, one must
qualitatively evaluate the fit. In practice, this means that the pre-edge portion from -30
eV to the edge is more important than the EXAFS region and must be fit well, even at the
expense of the quantitative best fit.
Taken together, these criteria mean that one must find the arc-tangent inflection
point that has the lowest energy resulting in a good fit. A fit that passes quantitative
inspection can easily have a higher energy edge position than the actual edge, but a fit
that has too low an edge energy will result in neither a qualitatively nor a quantitatively
good fit.
Results
The edge positions from both the second-derivative method and arc-
tangentigaussian method are shown in Table 1. The change in the edge position with
oxidation state is much clearer in the arc-tangent fit. A line drawn through the LIIIedge
results (Fig. 2) gives a change in the edge energy of 1.85 &0.20 eV per unit change in
formal oxidation state. However, there is no a priori reason to expect that the change be
linear.
An important difference is evident in the aqueous vs. the solid phase I% data for
the Pu(V) and Pu(VI) oxidation states. In the aqueous data, the white line intensity is
reduced and a pronounced shoulder appears above the edge (Conradson et al., 1998a).
This has been attributed to the two short-distance apical oxygen atoms of the plutonyl
species. Inthesolid compounds (Fig. 3), the Pu(V)does have the ‘-yl’group, but the
associated features are not as distinct. In fact, the solid I%(V) data above the edge is
qualitatively more similar to Pu(IV) than Pu(VI).
In addition, small differences in the measured edge position are observed in the
fluorescence and transmission spectra, which cannot be accounted for by self-absorption
in the fluorescence spectra. This aspect of the XANES data will be investigated further
to determine the extent of the systematic effect.
Conclusions
The results of this study and the other studies referenced show that the edge
energy may be used as an oxidation-state identifier for Pu materials, demonstrating the
utility of synchrotron-based techniques for the study of Pu chemistry. As is conventional
with x-ray spectroscopes, a sample with mixed species (oxidation state and/or phase)
will remain difficult to unequivocally identify. However, other techniques which rely
more on the spectral shape maybe brought to bear on this problem (Conradson, 1998b).
Acknowledgments
This work was supported by Argonne National Laboratory Directed Research and
Development funds to investigate the application of synchrotron-based techniques to
actinide systems. Use of the Advanced Photon Source was supported by the U.S.
Department of Energy, Basic Energy Sciences, Office of Energy Research (DOE-BES-
OER), under Contract No. W-31- 109-Eng-38. The MRCAT beamlines are supported by
the member institutions and the U.S. DOE-BES-OER under Contracts DE-FG02-
94ER45525 and DE-FG02-96ER45589.
References
Banaszak, J. E., Reed, D. T. & Rittmann, B. E. (accepted 1998), Journal of
Radioanalytical and Nuclear Chemistry.
Conradson, S. D., Al Mahamid, I., Clark, D. L., Hess, E. A., Hudson, M. P., Neu, P. D.,
Palmer, P. D., Runde, W. H. & Tait, C. D. (1998a), Polyhedron 17,599.
Conradson, S. D. (1998b), Applied Spectroscopy 52(7), 252A-279A.
Lieser, K. H. (1995), Radiochimica Acts 70/71, 355.
Silva, R. J. & Nitsche, H. (1995), Radiochimica Acts 70/71, 377.
Table 1. Table of fit results for the I% L1lledges. The second-derivative zero crossing
was determined by fitting the edge data to a polynomial as described in the text, with the
end points taken at 20% and 90% of the white line height. The remaining columns detail
the results for the arc-tangent/gaussian function. Errors are shown in parentheses.
Figure 1. Example fit of L1llXANES data for I.%(IV)sample using Eq. (1) with gaussian
curves (N=2). Data (crosses) are shown with the fit (solid line) and the fit components
(dashed lines).
Figure 2. XANES spectra for the Llll edges of the four I% oxidation states (offset for
clarity).
Figure 3. Edge energy from the arc-tangentigaussian fit vs. formal oxidation state for
four Pu solids.
Arctangcnt
2ti derivative
Species (eyt & (ev) C (Wdrh)
I 18059.2(0.3) I 18054.9(0.4) 0.19(0.01)
I Iw’v)oz I 18062.2(0.3) I 18057.6(0.3) 0.17(0.02)
I NaPu(V)OZCQ I 18062.8(0.3, I 18059.0(0.4, 0.19(0.01)
I Ba&u(VI)OG I 18065.0(0.3) I 18060.6(0.4) 0.20(0.01)
F, (n=l peak) G, (n=l width) F*(n=2 peak) G (n=2 width)
18063.6(0.4) 5.5(0.7) 18068(5) 7.5(2.0)
18067.3(2.5) 5.9(3.0) 18071(3) 7.3(1.3)
18067.0(0.2) 5.2(0.5) 18072(3) 8.4(2.0)
18068.8(0.5) 6.0(0.5) 18075(2) 9.ql .5)
Table 1.
2.0
1.6
s
0.4
I
I I0.0‘
-40 -20 0 20 40 60 80 100 120 140
X-ray Energy, Relative to Edge (eV)
Figure 1.
.
18062
18061
-al
~18059
o
‘%18058
z
018057
18055
T
18054
2 3 4 5 6 7
OxidationState
Figure 2.
,
2.4
2.2
2.0
s 1.8
0.--
E
z
z
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
PU(HI)F3
PU(IV)02
NaPu(V)02CO~
J
Ba3Pu(Vl)06
I t I s I a t a 9 I I I
-50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80
X-ray Energy (E -18060 eV)
Figure 3.
Experimental Details
W%erewas the sample measured:
Materials research – collaborative access team (MRCAT) insertion device line at the Advanced Photon
Source (APS)
Ring current and energy:
7 GeV, 50-100 mA
Magnet used (ifundulator, tapered or scanned):
Undulator A, tapered and scanned
Type of monochromator (cut, type, fried exit?):
Double Si(l 11) crystal non-fixed exit, conventional design with thin web first crystal. At these energies
the beam position was essentially fixed.
Details of harmonic rejection:
Rh coated mirror
Detectors used and details:
b filled with nitrogen gas (first run) or mixture of 15%Ar/85%N (later runs)
ITfilled with 15/85 mixture
IFfilled with argon gas (first run) or krypton gas (later runs)
IR.ffilled with argon
Energy resolution and how it was determined:
Resolution -4 eV at 18 keV. Resolution estimated from a measurement of the rocking curve width and
the vertical slit size. Beam size 0.4x0.4 mm spot and smaller masked down from full beam.
Sample preparation (particle size, checks on phase purity, unformity, etc.):
Finely ground powders (no check of size). Particles that were electrostatically attracted to the side of the
vial were used.
Irrelevant, sample thickness p and Apx (fluorescence, thicWdilute,
A/.tr,0.01-0.25
thin/concentrated):
.
*
.
.
Data Analysis
How was background removed and data normalized:
Pre-edge background line removed. Normalized, when required, to data from 50-300 eV above the
edge. A smoothing fi.mctionwas applied to the data to reduce the effect of the chosen lower-energy end
point of the norrmdization fit range.
Identifi k=O location:
Energy reference was a Zr foil with the first inflection point defined as 17999.3 eV.
Describe window function in k- and r-space:
N/A
Window ranges:
NIA
Reference compounds used and crystallographic data (N, R):
NIA
For theoretical amplitudes an phases, describe structure and give references:
NIA
Other relevant information:
Step size at the edge was -0.4 eV.


X-ray Absorption Near-Edge Spectroscopy of Plutonium Solid Species

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X-ray Absorption Near-Edge Spectroscopy of Plutonium Solid Species

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