Possibilities and limitations of synchrotron X-ray powder diffraction with double crystal and double multilayer monochromators for microscopic speciation studies

Spectrochimica Acta Part B 64 (2009) 775–781

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Spectrochimica Acta Part B j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s a b

Possibilities and limitations of synchrotron X-ray powder diffraction with double crystal and double multilayer monochromators for microscopic speciation studies☆ Wout De Nolf a,⁎, Jakub Jaroszewicz a, Roberto Terzano b, Ole Christian Lind c, Brit Salbu c, Bart Vekemans d,1, Koen Janssens a,2, Gerald Falkenberg e a

Department of Chemistry, University of Antwerp, Universiteitsplein 1, B-2610, Antwerpen (Wilrijk), Belgium Dipartimento di Biologia e Chimica Agro-forestale ed Ambientale, Via Amendola 165/A, I-70126, University of Bari, Bari, Italy Isotope Laboratory, Norwegian University of Life Sciences, PO Box 5003, N-1432 Ås, Norway d Department of Analytical Chemistry, Ghent University, Krijgslaan 281 S12, B-9000 Gent, Belgium e HASYLAB at DESY, Beamline L, Notkestraat 85, D-22603, Hamburg, Germany b c

a r t i c l e

i n f o

Article history: Received 17 March 2008 Accepted 3 June 2009 Available online 11 June 2009 Keywords: X-ray powder diffraction Multilayer optics Scanning microscopy Synchrotron sources X-ray optics

a b s t r a c t The performance of a combined microbeam X-ray fluorescence/X-ray powder diffraction (XRF/XRPD) measurement station at Hamburger Synchrotronstrahlungslabor (HASYLAB) Beamline L is discussed in comparison to that at European Synchrotron Radiation Facility (ESRF) ID18F/ID22. The angular resolution in the X-ray diffractograms is documented when different combinations of X-ray source, optics and X-ray diffraction detectors are employed. Typical angular resolution values in the range 0.3–0.5° are obtained at the bending magnet source when a ‘pink’ beam form of excitation is employed. A similar setup at European Synchrotron Radiation Facility beamlines ID18F and ID22 allows to reach angular resolution values of 0.1– 0.15°. In order to document the possibilities and limitations for speciation of metals in environmental materials by means of Hamburger Synchrotronstrahlungslabor Beamline L X-ray fluorescence/X-ray powder diffraction setup, two case studies are discussed, one involved in the identification of the crystal phases in which heavy metals such as chromium, iron, barium and lead are present in polluted soils of an industrial site (Val Basento, Italy) and another involved in the speciation of uranium in depleted uranium particles (Ceja Mountains, Kosovo). In the former case, the angular resolution is sufficient to allow identification of most crystalline phases present while in the latter case, it is necessary to dispose of an angular resolution of ca. 0.2° to distinguish between different forms of oxidized uranium. © 2009 Elsevier B.V. All rights reserved.

1. Introduction X-ray powder diffraction (XRPD) is a well established method of phase identification that finds its application in numerous research fields where structural investigation of materials is relevant, such as material science, condensed matter physics and protein crystallography. The method can be employed using (high performance) laboratory X-ray sources or at synchrotron end stations. The interest for the use of XRPD as an analytical tool on the micro scale is growing, and an increasing number of X-ray imaging beamlines combine it with X-ray microscopic and spectroscopic techniques [1,2]. At these facilities, an X-ray beam of microscopic dimensions is used to locally

☆ This paper was presented at the 19th qInternational Congress on X-ray Optics and Microanalysisq (ICXOM-19) held in Kyoto (Japan), 16–21 September 2007, and is published in the Special Issue of Spectrochim. Acta Part B, dedicated to that conference. ⁎ Corresponding author. Tel.: +32 3 2652359; fax: +32 3 2652376. E-mail address: [email protected] (W. De Nolf). 1 Tel.: +32 9 264 48 56; fax: +32 9 264 49 60. 2 Tel.: +32 3 2652373; fax: +32 3 2652376. 0584-8547/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2009.06.003

excite the sample material while X-ray fluorescence, X-ray absorption and/or X-ray diffraction signals are collected. Recently, the infrastructure at the X-ray fluorescence and spectroscopy Beamline L at the Hamburger Synchrotronstrahlungslabor (HASYLAB, Hamburg, Germany) has been augmented with an area detector for diffraction measurements. In what follows, the achievable XRPD angular resolution in monochromatic excitation mode [during which a Silicon (111) double crystal monochromator (DCM) is used for energy selection] and in ‘pink’ beam excitation mode [where instead of the DCM a double multilayer monochromator (DMM) is employed] is considered together with the consequences this has for phase identification in materials typically encountered during metalpollution studies. Since DMMs generally feature a higher throughput than silicon monochromators, shorter exposure times can be achieved, a crucial point in time consuming scanning experiments, where extended series of diffraction patterns are collected from regions-of-interest on a sample. In a first part, this article describes the implementation of the combined scanning micro X-ray fluorescence and micro X-ray

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diffraction setup at Beamline L and compares it with its analog at ESRF Beamline ID18f, one of the X-ray imaging beamlines at the European Synchrotron Radiation Facility (Grenoble, France). Secondly, in order to illustrate the practical usefulness as well as some of the limitations of the scanning µ-XRF/µ-XRD facility at HASYLAB Beamline L, the speciation of heavy metals in polluted soil samples and radioactive particles is discussed.

2. Techniques and instrumentation During experiments involving diffraction, the primary radiation generated in the bending magnet source of HASYLAB Beamline L is focused down to micrometer dimensions (beam diameter of 10– 15 µm) by means of a single-bounce elliptical capillary (SBC) [3,11]. The divergence of the SBC-focused beam is estimated between 2 and 2.5 mrad for an energy in the 25–30 keV range. A double crystal Si (111) monochromator upstream from the optic produces monochromatic radiation with an energy bandwidth ΔE/E of about 0.02% [4,12]. In pink beam mode, the two silicon crystals are exchanged for two multilayers, each composed of alternating layers of molybdenum and silicon or of nickel and carbon, producing a ‘pink’ (i.e., partially monochromatic) beam with a bandwidth of about 1% for the Mo/Si and about 2% for the Ni/C double multilayer [5]. The total efficiency of a multilayer depends on its reflectivity and energy band selection. The 200 period Mo/Si multilayer with a mean layer thickness of 2.98 nm has a theoretical reflectivity of 80 to 85% (calculated for a single perfect multilayer) when used in the 25–30 keV energy range. The 100 period Ni/C multilayer with 3.34 nm layer thickness has a calculated reflectivity between 94% and 96% in the same energy range. This range is very suitable for performing µ-XRPD experiments. The resulting flux from the Mo/Si DMM was measured to be ca. 20 times higher than that yielded by the Si(111) monochromator; in case of the Ni/C multilayer a flux 30 times higher is obtained [5]. In order to collect two dimensional X-ray diffraction patterns in a transmission geometry, a 1 k × 1 k Bruker SMART1000 camera (Karlsruhe, Germany) with 60 µm pixel to pixel resolution or a 2 k × 2 k Mar CCD 165 area detector (165 mm diameter, MARResearch, CA, US) with 80 µm pixel size, is positioned behind the samples under investigation. AVORTEX Silicon Drift Detector (SDD, SII Nanotechnology, CA, US) with 50 mm2 active area at 90° relative to the primary X-ray beam, as shown in Fig. 1, simultaneously registers X-ray fluorescence data. At ESRF beamline ID18f, the radiation produced by an undulator is monochromatized using a Si(111) monochromator with energy bandwidth ΔE/E ≈ 0.02%. A 3 to 5 µm spot size is obtained by focusing the monochromatic radiation by means of parabolic compound refractive lenses (CRLs) made in aluminum. For combined µ-XRF/ XRPD experiments, a Si(Li) detector (Gresham Scientific Instruments

Ltd., Buckinghamshire, England UK) with a 30 mm2 active area and a MarCCD area detector is used. A detailed description of the beamline components at ID18f can be found elsewhere [6,13]. 3. Results and discussion 3.1. Factors determining XRPD angular resolution An important figure-of-merit of XRPD setups is the angular resolution in the 2θ-profiles that can be obtained by azimuthal integration of the two dimensional diffractograms. It can be expressed as the full-width-at-half-maximum (FWHM) of the diffraction peaks. Several sources contribute to peak broadening in XRPD patterns. (a) The finite crystallite size or coherently diffracting domain size, strain effects within the crystal lattice and extended defects are the most important sources of peak broadening that are related to sample properties. They give rise to an increasing peak FWHM with scattering angle [7]. (b) The finite energy bandwidth ΔE of the primary X-rays also results in increasing peak widths as a function of rising 2θ. This energy bandwidth of a double multilayer is proportional to the mean transmitted energy E, i.e. ΔE/E is constant. Accordingly, via Bragg's law, it is possible to write the resulting angular bandwidth Δ(2θ) of the diffracted beam as: Δð2θÞ = 2

ΔE tan θ: E

Therefore, the broadening caused by the finite energy-width of the primary beam does not depend on the energy itself, but is proportional to the relative energy resolution ΔE/E, which is a constant for a given multilayer system. (c) Finally the X-ray beam divergence and the X-ray detection system spread make up most of the instrumental broadening. The resolution of a CCD area detector depends on its point spread function (PSF) [14] and on the number of image pixels covering the measured 2θ range, which is determined by the combination of pixel size, number of pixels and the distance between sample and detector. The SMART1000 camera used at HASYLAB BL-L captures a maximum scattering angle of 40° (the maximal opening angle of a diffraction cone fully intersected by the square detector as shown in Fig. 2b) when positioned at 3.7 cm from the sample. The MarCCD has a larger pixel size (80 µm instead of 60 µm) but four times more pixels and would have to be positioned at 9.7 cm in order to cover the same angular range. In such a situation, twice as many pixels are used, resulting in (slightly) narrower diffraction peaks being recorded. This is schematically illustrated in Fig. 2a. Also, peaks at higher scattering angles are sampled using more detector pixels than peaks at lower

Fig. 1. Experimental setup for combined µ-XRPD/µ-XRF at HASYLAB Beamline L, with a single-bounce capillary focusing the monochromatic or pink primary X-ray beam coming from the right onto the sample, mounted on a motorized stage. A CCD camera is positioned behind the sample and a silicon drift detector at 90° to the primary beam.

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Fig. 2. (a) Schematic side view of the projection of diffracted radiation on the top-half part of two differently sized area detectors, positioned to capture the same maximum scattering angle; the corresponding 2θ-axes of their one-dimensional radial profiles are shown. The effect of azimuthal integration on one projected Gaussian peak on the left-most CCD camera is shown. (b) A two dimensional diffraction pattern obtained with the SMART1000 detector, illustrating incomplete Debye ring registration.

scattering angles, resulting in a slight peak narrowing with increasing 2θ. The peak distortion and widening effect due to projection on the detector plane is eliminated by a geometrical correction applied in the azimuthal integration process to transform planar area detector space into diffraction angle space. The angular resolution obtained by means of different combinations of monochromators and diffraction cameras at Beamline L is compared to the resolution achievable at ESRF ID18f in Figs. 3 and 4. Standard LaB6 and Si (NIST SRM640c) powders were measured under the experimental conditions summarized in Table 1. The standards were measured for calibration purposes during different experimental sessions. Depending on the needs of the experiment, different primary energies, monochromators, camera types and sample-camera distances were employed. In Fig. 3, the average FWHM of the diffraction peaks of the NIST SRM640c silicon standard is plotted for the ESRF ID18f and the various HASYLAB BL-L µ-XRPD configurations. Each measurement was repeated several times for different positions on the powder and the

average peak width ± 1 s standard deviation over 10 to 40 measurements is shown for each reflection. At the ID18f station, where monochromatic undulator radiation is employed and focused by an Al CRL lens, FWHM values in the range 0.08–0.15° are recorded for scattering angle between 5 and 35°. It can be readily seen that by employing Configuration 3 at BL-L (◊), on average the peak FWHM is a factor 5 larger compared to ESRF ID18f. This increase in FWHM compared to ID18f is reduced to factors 2–3 when configurations 2 (Δ) and 5 (□) are used, mainly by using a CCD camera with two times more pixels than the SMART1000 in each dimension, positioned at 19 instead of 6 cm behind the sample. When additionally a primary beam with a smaller energy bandwidth is employed (Configuration 2, Mo/Si multilayer) an average FWHM of 0.22° is obtained. Note that the increasing spread of the FWHM values with 2θ for Configuration 3 can be explained by the square geometry of the SMART1000 camera images. Because of the X-ray beam energy and sample-detector distance, Debye rings which are only visible in the corners of the CCD image (as illustrated in Fig. 2b) were included in

Fig. 3. Diffraction peak FWHM versus 2θ-position for Si (NIST SRM640c) reflections, obtained under different experimental conditions: ◊ = Configuration 3; Δ = Configuration 2; □ = Configuration 5; × = Configuration 7. Dashed lines only intended to guide the eye.

Fig. 4. Diffraction peak FWHM versus scattering angle 2θ for the reflections of Lanthanum Hexaboride (LaB6), measured under different conditions: ◊ = Configuration 1; Δ = Configuration 4; □ = Configuration 6; × = Configuration 5. Error bars indicate 1 s standard deviation on mean values.

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Table 1 Instrumental XRF/XRPD configurations employed at Beamlines L (HASYLAB) and ID18f (ESRF). Configuration X-ray source

1 2 3 4 5 6 7

BMa BM BM BM BM BM Undulator

Monochromator Energy Optics (keV)

XRD Camera

Camerasample distance (cm)

Ni/C MLb Ni/C ML Mo/Si ML Mo/Si ML Mo/Si ML Si(111) Si(111)

SMART1000 MarCCD165 SMART1000 SMART1000 MarCCD165 SMART1000 MarCCD165

12.7 19.1 6.4 6.5 19.6 7.0 13.1

24.0 29.2 21.4 22.0 20.1 25.1 28.0

SBCc SBC SBC SBC SBC SBC Al-CRLd

bounce capillary as microfocusing optic, allows to compare the influence of the use of either double multilayer or double crystal monochromators on the XRPD peak widths. While the combination of the SMART1000 CCD camera and the Ni/C multilayer monochromator only allows to reach an FWHM of 0.42°, slightly narrower diffraction peaks can be obtained if the Mo/Si DMM is used (FWHM ~ ca. 0.37°) or when the Si(111) DCM is employed (FWHM ~ ca. 0.32°). However, the major factor determining the XRPD peak widths is clearly not the energy bandwidth of the primary beam, but rather the number of pixels and the PSF of the CCD camera employed for diffractogram recording, as is evidenced by the FWHM values obtained by means of Configuration 5, situated between 0.15–0.25°.

a

Bending Magnet. Multilayer. c Single-bounce capillary. d Aluminum compound refractive lenses. b

order to cover the same 2θ range as for the other settings. (The same effect can be seen in the data points of Configuration 4 in Fig. 4.) Since for Configurations 1 and 6, only full Debye rings were taken into account, the uncertainty does not increase with scattering angle for these configurations, as can be seen in Fig.4. For configurations employing the MarCCD camera on the other hand, the internal collimator of the MarCCD masks off the corners of the diffraction pattern so that only full Debye rings are visible when the primary beam is positioned at the center of this camera. The FWHM data in Fig. 4, obtained from a LaB6 powder standard in various configurations realized at HASYLAB BL-L by using a single-

3.2. Minimum angular resolution required: two case studies The minimum angular resolution required to allow identification of the species that are present in a material under study, depends to a large extent on the complexity of the mixture of crystalline phases that are present and on the proximity in diffraction angle between the reflexes of species among which one would like to distinguish. To demonstrate on the one hand that even with a relatively coarse angular resolution of ca. 0.4°, it is possible to identify crystal phases in a relatively complex matrix by using a combination of microscopic XRF and XRD data, in what follows we discuss a study about the distribution of heavy metals in polluted industrial soils (case I). Case II, on the other hand, discusses the identification of structurally similar uranium compounds in depleted uranium (DU) as an example of a situation where an angular resolution of at least 0.2° is required to arrive at meaningful results.

Fig. 5. (a) Barite and (b) chromium oxide phase distributions obtained from XRPD data from a 200 × 300 µm2 area of a polluted soil sample from Val Basento, Italy. The 2θ-plots show individual diffraction patterns and corresponding ICDD PDF-2 database entries at the pixel locations indicated by the arrows; (c) optical microscope image of the soil section (d) binary phase composition map; (e) elemental distributions (dashed border) obtained from XRF intensity data and binary elemental map. Darker tones indicate higher intensity. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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3.3. Case I: metal speciation in polluted soil samples As part of a larger study on the origin and fate of heavy metals in polluted soil in the Val Basento district along the Basento River (Basilicata, Italy), soil thin sections of 20 to 60 µm thickness were studied at HASYLAB Beamline L [8]. For risk assessment purposes, the chemical form in which the heavy metals are present in the polluted soil needs to be known in order to infer information about their solubility and stability. Next to X-ray absorption spectroscopic investigations performed at the same beamline, the previously described scanning micro X-ray fluorescence and diffraction setup was used to visualize phase and elemental distribution of selected areas of the thin sections. The XRPD distribution maps were obtained by means of the software package XRDUA [10]. Apart from performing the necessary CCD image corrections and calibration of the instrumental parameters, it focuses mainly on the automatic processing of two dimensional XRD patterns, gathering several types of integrated signals versus the scan coordinates of a mapping experiment. Intensities in regions-ofinterest (squares, circles or arcs) in the two dimensional diffraction patterns and in the one-dimensional azimuthally integrated diffractograms can be used. Structureless modeling of diffractograms, individual profile fitting or whole-powder-pattern decomposition [9], allows to create maps of individual peak parameters (position, area, FWHM and other shape parameters) and global peak parameters (unit cell parameters, intensity scaling factors, etc.). Fig. 5 shows such µ-XRD and µ-XRF distributions for a 200 × 300 µm area of a soil thin section, mapped using a 20 µm step size. The Ni/C multilayer is used as monochromator, the Bruker SMART1000 camera as area detector and a single-bounce capillary focuses the beam to approximately 20 × 20 µm2 on the sample (Configuration 1; ◊ in Fig. 4). Two phases are identified using the ICDD PDF-2 database: chromium oxide (Cr2O3, 074-0326) and Barite (Strontian, Ba0.75Sr0.25SO4, 0391469). The intensity scaling factors of both phases are taken as a measure for their distribution, preserving the relative intensities of the individual reflection as given by the PDF-2 database for each phase. A one-dimensional diffractogram is shown for each compound with addition of the powder diffraction database information. From the XRF elemental distribution on the right hand side of Fig. 5, a binarized distribution for barium/strontium (red) and chromium (green) is constructed, which clearly correlates well with its XRD analog (top-

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right) showing the distribution of Barite (red) and chromium oxide (green). Additionally, in the calcium-rich grain (lower-left of scanned area), a few single crystal diffraction spots of calcite are visible on the CCD image, as opposed to the full Debye rings for Barite and chromium oxide indicating the presence of a polycrystalline powder with random orientation. A larger area of 580 × 440 µm2 of the same soil sample, showing barium and lead as main constituents, is investigated in Fig. 6, using the Si(111) monochromator instead of the Ni/C multilayer in the primary beam (Configuration 6, □ in Fig. 4). The most common phases are identified as Barite (BaSO4, 076-0214) and Minium (Pb3O4, 0761799). Since the experimentally observed XRD peaks are situated between the positions indicated in the ICDD database for Barite (0760214) and for Strontian (Ba0.75Sr0.25SO4, 039-1469, Fig. 5) while the Sr and Ba XRF maps show a very high degree of similarity, we assume that in the Ba-rich phase, barium is substituted for strontium for less than 25%. The phase correlated to iron is hematite (Fe2O3, 079-0007) while calcite (CaCO3) is identified as the calcium rich phase. Again the phase scaling factors are taken as representatives for spatial distribution imaging. The smaller strontium containing grain in the strontium map of Fig. 6, left of the large Ba-rich area, is likely to be indicative of strontium-substituted CaCO3, since its composition from XRD is found to be calcite. No substantial peak shifts are found however suggesting that only a minor fraction of Ca has been replaced by Sr. This is consistent with the relative Ca and Sr XRF intensities observed. The remaining XRF maps show that transition metals such as Cr, Ni, Cu and Zn are present, most likely sorbed at the surface or substituting principal elements in the identified mineral phases. Summarizing, we can say that the moderate level of angular resolution employed in this study (0.4°) in almost all cases was sufficient to identify the crystalline phases consistent with the elemental maps available. Only in the case of the Barite–Strontian like phase, the investigation might have benefited from a higher resolution in the XRPD patterns. 3.4. Case II: U speciation in depleted uranium particles To illustrate further the importance of XRPD resolution in certain environmental speciation studies, two specific XRF/XRPD measurement sets that form part of a study [15] of depleted uranium (DU) particles are considered below. These particles originate from DU ammunition used in the 1999 Balkan conflict and were encountered in

Fig. 6. Elemental (dashed border) and crystal phase maps (full border) obtained by simultaneous XRF and XRPD mapping of an 580 × 440 µm2 area (optical microscope image leftbottom) of polluted soil sample from Val Basento, Italy. Darker tones indicate higher XRF- or XRPD-intensity.

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Fig. 7. (a) Electron micrograph of a DU particle; a series of XRF/XRPD measurements were performed along the horizontal line, 60 µm long. (b) X-ray diffractograms from the middle of the particle using the ID22 (full line) and BL-L setups (dotted line, data rescaled to match the 2θ-scale of the ID22 experiment). Inset: detail of the (400), (331) and (420) reflections from UO2 and UO2.34. Additionally UC is present. (c) Relative phase intensity distribution of the three U-species along the line shown in (a).

soil samples collected at the Ceja Mountains (Kosovo). In Fig. 7a, a scanning electron micrograph of one of the DU particles in shown. In order to estimate in a more objective manner the current and future hazards the uranium presents to the local environment, it is relevant to determine the phases and oxidation state in which U is present. Some of the particles were analyzed both at HASYLAB and at ESRF using a combination of microbeam XRF and XRPD. The first experiment was performed at HASYLAB BL-L using Configuration 3 (Mo/Si DMM, SBC as focusing optic, SMART1000 area detector, ◊ in Fig. 3), corresponding to an XRPD peak width of ~ 0.5°. A second series of measurements was performed at ESRF beamline ID22 where a Photonic Science CCD camera with 55 µm pixel size and 1270 × 1160 frame size was positioned at 7.2 cm behind the sample. A KB mirror system focuses the 29.5 keV radiation produced by a Si(111) monochromator. The FWHM of the XRPD peaks of Silicon (SRM640c) on average was ca. 0.15°, i.e. three times more narrow than those obtained in the corresponding experiment at BL-L. The angular resolution at ID22 is comparable with that at ID18f (Configuration 7; × in Fig. 3). For the XRF/XRPD analysis of DU particles like the one shown in Fig. 7a, this difference in angular resolution turned out to be significant. Comparison of diffractograms acquired at ESRF ID22 with the ICDD PDF-2 database reveals the presence of Uraninite (UO2, 41-1422), uranium oxide (UO2.34, 75-0456) and uranium carbide (UC, 73-1709). Uraninite and UO2.34 have the same face-centered-cubic Fm-3m symmetry, but different unit cell dimensions (a = 5.467 Å and a = 5.410 Å respectively). The presence of higher oxidized UO2 was reported before in DU particles in soil samples [16]. The lattice parameter difference between UO2 and UO2.34 gives rise to a maximal peak position difference in the given 2θ range for the ESRF ID22 experiment ( Fig. 7b, full line) of about 0.23° for the (420) reflection. The FWHM of this peak for the two oxides is on average 0.15°. A threefold increase in peak width for HASYLAB BL-L, results in a FWHM twice the peak position difference at the (420) reflection of both phases. Thus the two uranium oxide phases could not be resolved in the diffractogram from BL-L, as can be seen in Fig. 7b (dotted line). Both the ID22 and the BL-L data revealed the presence of UO2 and UC. However, the higher angular resolution at ID22 permitted us to make the distinction between UO2 and UO2.34, which revealed additional oxidation of UO2 taking place in the DU particles. Oxidation of UO2 results in higher oxides, depending on temperature and oxygen pressure. Several compounds have been observed

[17,18]: hyperstoichiometric UO2 + x (face-centered cubic; S.G. = 225), U4O9 and its non-stoichiometric forms (body-centered cubic, fourfold superstructure of the UO2-structure, S.G. = 220 [18]), several U3O7 polymorphs (mostly tetragonal) and U3O8 (orthorhombic). The refinement of the data collected at ESRF ID22 results in two significantly different lattice parameters: a = 5.470(1) Å for UO2 and a = 5.408(1) Å for UO2 + x, both with spacegroup Fm-3m. In Fig. 7c, it can clearly be seen that the UO2.34 phase has a different distribution from that of UO2. Note: The more oxidized phase can also be modeled by using spacegroup I-43d (i.e., the fourfold U4O9 superstructure of the regular uranium oxide lattice cell [19]) with a lattice parameter of 21.636(1) Å. Although we cannot make the distinction between the FCC and BCC structure on the basis of the XRPD data, this lattice parameter value indicates an O:U atomic ratio higher than 2.25 [20]. This phase could therefore be denoted as U4O9 + y. From the currently accepted U–O phase diagram [20], it can be seen that in this range, U3O7 is formed by oxidation at temperatures below ±500 °C or precipitates during cooling, unless the cooling rate is high (i.e. during quenching events) [20]. The absence of the tetragonal U3O7 phase at this O:U ratio, suggests that the U4O9 + y phase is formed at high temperatures, most probably generated during the impact of the DU ammunition and not afterwards due to oxidation in the environment. From the available data, it is not possible to establish whether the third crystalline U-containing phase is UO2 + x or U4O9 + y. Whatever the exact nature of this phase may be, what is relevant in the context of this paper, is to underline that the angular resolution of ca. 0.2° in this case is of significant value, as it allows to demonstrate the presence (and absence) of other crystalline species of oxidized uranium, all with different dissolution characteristics, in the DU particles examined. 4. Conclusion The performance of a combined microbeam X-ray fluorescence/Xray powder diffraction (XRF/XRPD) measurement station at HASYLAB Beamline L has been discussed. Typical angular resolution values in the range 0.3–0.5° are obtained at the bending magnet source when a ‘pink’ beam form of excitation is employed. Especially the characteristics of the camera employed for recording XRPD data determine the angular resolution, although decreasing the energy spread of the primary beam also helps to improve the angular resolution. A similar

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setup at ESRF beamlines ID18F and ID22 allows to reach angular resolution values of 0.1–0.15°. In order to document the possibilities and limitations for speciation of metals in environmental materials by means of HASYLAB BL-L XRF/XRPD setup, two case studies were discussed, one involved in the identification of the crystal phases in which heavy metals such as Cr, Fe, Ba and Pb are present in polluted soils of an industrial site (Val Basento, Italy) and another involved in the speciation of uranium in depleted uranium particles (Ceja Mountains, Kosovo). In the former case, the angular resolution is sufficient to allow identification of most crystalline phases present, except for one in which a change in the lattice constant of the mineral Barite is suspected due to the substitution of Ba by Sr. In the case of the speciation of uranium in depleted uranium particles from Kosovo, it was necessary to dispose of an angular resolution of at least 0.2° to distinguish between the different forms of oxidized uranium. Acknowledgments This research was supported by the Interuniversity Attraction Poles Programme — Belgian Science Policy (IUAP VI/16). The text also presents results of GOA “Atom” (Research Fund University of Antwerp, Belgium) and of FWO (Brussels, Belgium) projects no. G.0177.03, G.0103.04 and G.0689.06. We would like to thank A. Somogyi, M. Drakoupolos and R. Barret for their assistance during the ESRF measurements. References [1] A. Manceau, C. Tommaseo, S. Rihs, N. Geoffroy, D. Chateigner, M. Schlegel, D. Tisserand, M.A. Marcus, N. Tamura, Z.S. Chen, Natural speciation of Mn, Ni, and Zn at the micrometer scale in a clayey paddy soil using X-ray fluorescence, absorption, and diffraction, Geochim. Cosmochim. Acta 69 (2005) 4007–4034. [2] T.A. Kirpichtchikova, A. Manceau, L. Spadini, F. Panfili, M.A. Marcus, T. Jacquet, Speciation and solubility of heavy metals in contaminated soil using X-ray microfluorescence, EXAFS spectroscopy, chemical extraction, and thermodynamic modeling, Geochim. Cosmochim. Acta 70 (2006) 2163–2190.

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