||New opportunities in trace elements structural characterization
New opportunities in trace elements structural characterization: High-energy X-Ray Absorption Near-Edge Structure Spectroscopy.
Modern synchrotron radiation facilities provide indispensable tools for research in a large number of different fields of science like Physics, Chemistry, Biology, as well as in Materials-, Geo- and Environmental Sciences. One of the outstanding techniques associated to synchrotron radiation sources is x-ray absorption spectroscopy (XAS). XAS spectroscopy has been demonstrated to be essential to progress in many scientific fields as it provides, by taking advantage of its highly-flexible experimental capabilities, the knowledge of the local atomic structure of a selected atomic species in the material under study.
Over the last years we have seen the development of the so called third generation sources. One of the main challenges within the new facilities connected to XAS is the extension of the available high-energy range. Indeed, the use of XAS as a dedicated tool to determine local structural environments has previously been confined to absorption edges below 30 keV. Recent works have demonstrated that it is possible to measure XAS spectra with sufficiently high signal to noise ratios in the energy range 30 to 90 keV .
Despite the above advantages there are still only few works dealing with high-energy XAS and most of them still deal with qualitative and fingerprint analyses. The main reason for this lack of experimental work seems to be linked to the assumption that the finite lifetime of the core-hole smears out dramatically the spectral features so as to avoid structural determination, and that this effect becomes more serious for K-absorption edges of heavier elements .
In this work we present the first quantitative analysis made on the x-ray absorption near-edge structure (XANES) region at the Nd K-edge in natural minerals containing Nd in trace concentrations. Among the rock forming minerals, aluminosilicate garnets (X3Y2S3O12, X=Fe2+, Mn2+, Mg, Ca) are intensly studied both due to the complexity of their crystal chemistry and to theirstability over a wide range of physico-chemical parameters . Garnets in lower crustal mafic and ultramafic rocks usually contain rare-earth elements (REE) in trace concentrations. The diffusion coefficients of REE between garnets and the coexisting phases are used to interpret the crystallization and metamorphic history of crustal rocks. An understanding of REE diffusion in garnets cannot be obtained without a characterization of their structural behavior. Despite their geochemical importance, a direct crystal chemistry characterization of REE at trace levels in natural garnets is not available in literature because of the difficulty of obtaining structural information on elements present in so low concentration by means of conventional diffraction methods.
The goal of this work is twofold. On the one hand we provided an exact structural determination about which is the position that Nd enters in the complicated mineral framework. On the other hand we demonstrate the capability of the XANES technique to solve different structural environments, being this capability not affected by the damping and broadening of the signal due to the short lifetime of the excited atomic state.
We have studied the local environment of Nd at trace levels in a series of natural garnets belonging to the pyrope-grossular solid-solution and containing different Nd concentrations (176-1029 ppm). The samples are melanite garnets occurring in carbonatitic rocks and labelled A204, V19 and 89/35. Samples exhibit different Nd concentration, as determined by ionic microprobe analysis, being 1029, 344 and 176 ppm for A204, V19 and 89/35 respectively. The chemical compositions of the A204, V19 and 89/35 samples are respectively:
X:(Ca 2.91 Mg 0.04 Mn 0.02 Na 0.03 )Y:(Mg 0.10 Ti 0.89 Zr 0.04 Al 0.28 Fe 3+0.69 ) Z:(Si 2.34 Fe3+0.66 )O12
(Ca 2.89 Mg 0.05 Mn 0.02 Na 0.04 )(Mg 0.08 Ti 0.83 Zr 0.03 Al 0.20 Fe 3+0.86 )(Si 2.47 Fe 3+0.53 )O12
( Ca 2.97 Mg 0.03 Mn 0.03 Na 0.01 )(Mg 0.09 Ti 0.49 Z r 0.02 Al 0.26
Fe 3+1.14 )(Si 2.66 Fe 3+0.34 )O12
with trace concentrations of Ce of 755, 257 and 159 ppm, respectively.
XAS measurements at the commonly used Nd L3 -edge (~6208 eV) are unattainable in these samples, due to the presence of interfering fluorescence lines from the matrix and by the presence of the Ce L2 -edge (6164 eV) close to the Nd L3 -edge, with a Ce concentration similar to that of Nd. These problems can be overcome by use of the Nd K-edge (~ 43570 eV), since at this energy there are no problems of overlapping of absorption edges and fluorescence lines. This problem illustrate the significant interest of XAS into the possibility of working at high energy.
The Nd K-edge spectra were collected at 77 K in transmission mode on the reference compound Nd(OH)3 , and in fluorescence mode on powdered samples of the three natural garnets at the ESRF GILDA BM8 CRG beamline. The energy resolution was about 5.4 eV at 43.5 keV.
Figure 1 reports the raw Nd K-edge XAS spectra for the three garnets under study. The XAS oscillations are clearly visible above the edge. Therefore, the Nd K-edge XANES spectra can be used to perform a quantitative determination of the local environment of trace Nd in these garnets. The comparison of the XANES region indicates also that the structural environment of Nd is the same for the three garnets.
Figure 1. Detailed comparison of the Nd K-edge
XANES region for the 3 garnets under study.
The computation of the Nd K-edge absorption cross-section was carried out using the multiple-scattering CONTINUUM according to standard FMS methods . The Coulomb part of each atomic potential was generated using charge densities for neutral atoms. The atomic orbitals were chosen to be neutral for the ground state potential, whereas we follow the Z+1 rule to build the excited state potential. Finally, an appropriate exchange and correlation (ECP) potential was added to the Coulomb part of the input potential.
Initially, we have considered two different possibilities for the position that trace Nd enters in the X3Y2Si 3O12 garnet structure. Hence, we have built-up two class of clusters assuming that Nd enters the dodecahedral X-site and the octahedral Y-site, respectively. The calculations performed by using a real Hedin-Lundqvist ECP spectra reproduce all the features present in the experimental spectra, their relative intensity and their relative energy separation. After taking into account the effect of the core-hole lifetime (17.3 eV) and the experimental resolution, the agreement between the computation and the experimental XANES is remarkable. Figure 2 shows the comparison between the Nd K-edge XANES spectrum of the garnet with a Nd content 1029 ppm (A204) and the theoretical computations performed by assuming that Nd enters the octahedral and dodecahedral sites. It should be noted that the energy scale of each computation is referred to its own muffin-tin potential so that calculation provides an unique energy scale to align both X- and Y-site theoretical spectra. The results of the computation strongly suggest that trace Nd enters the dodecahedral X-site. The same comparison made after convolution of the theoretical spectra with the full core-hole width not only validates this result, but also shows convincely that the structural determination is not affected by the damping and broadening of the signal due to the short lifetime of the excited atomic state.
Figure 2. Comparison between the experimental Nd K-edge XANES signal
of A204 and those calculated on the basis of the FMS theory assuming
Nd in the dodecahedral (X-site) or in the octahedral (Y-site) sites
after taking into account the full Nd K-edge core-hole width.
Summarising, the study of the high-energy K-edge x-ray absorption of Nd occurring in trace concentrations in natural garnets demonstrates that Nd enters a structural garnet site and does not occupy matrix defects. Indeed, Nd enters the dodecahedral X-site of the X3Y2S3O12 garnet structure and does not substitute Al at the octahedral Y-site. Therefore we provide a direct crystal chemistry characterization of rare-earths elements at trace levels in natural garnets not available in literature. Moreover, the capability of high-energy XANES as powerful structural tool providing structural details unattainable by using standard methods has been demonstrated.
J. Chaboy, E. Cotallo, S. Quartieri and F. Boscherini, J. Synchrotron Rad. 2002, 9 ,86.
This work was partially supported by the Arag\'on DGA P0004/2001, Spanish DGICYT MAT99-0667-C04-04 grants.
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