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3.3 b. Systematics in Si-K ELNES spectra of mixed-Si coordination compounds (B.T. Poe, F. Seifert and T.G. Sharp, in collaboration with Z. Wu/Paris)

ELNES (Electron Loss Near Edge Structure) spectroscopy provides element-specific short range structural information with the potential to determine such properties as coordination number, oxidation state and site geometry on a sub-micron spatial scale. This makes ELNES spectroscopy especially useful for the characterization of high pressure phases, which often have very small grain sizes and are contained within multiphase assemblages. Our efforts have focused on the collection and evaluation of Si-K and Si-L edge data for compounds having mixed Si coordination with various x=SiIV/SiIV+SiVI ratios. These include wadeite-type K2Si4O9 (x=0.75), Mg3Al2(SiO4)3 majorite garnet (x=0.75), NaMg0.5Si2.5O6 (x=0.8) high pressure pyroxene phase, and the CaSi2O5 phase with pentacoordinate Si (see 3.3a). These data are compared with those of compounds having Si exclusively in tetrahedral (quartz, pyrope, jadeite) or octahedral (stishovite) coordination.

Reduction of the data to its structure-dependent components proves to be the most challenging part of these studies. Raw ELNES spectra are characterized by a significant exponentially decaying background function which must first be removed. This is followed by removal of intensity resulting from plural scattering of electrons which is dependent upon both energy and sample thickness. Finally, the spectra are fitted to simple symmetric line functions and a single arctan step function. The step of the arctan occurs at an energy at which an electron may undergo transition to an unbound state. Its width is similar to the resolution (˜ 1 eV) of the measurement technique.

In Fig. 3.3-3, we compare Si-K ELNES spectra in which background, plural scattering and arctan components have been removed. Each spectrum is shifted in energy such that its strongest peak is located at 0 eV in order to more easily distinguish differences between them. All spectra except that of stishovite have a second, less intense, but broad feature near 20 eV. The absence of this band in the stishovite spectrum likely indicates that it is representative of Si in tetrahedral coordination. In Fig. 3.3-4, we show a simple correlation between the ratio of the integrated intensity of the peak near 20 eV (integrated over the range 10 to 30 eV) over

Fig. 3.3-3: Comparison of Si K edge ELNES spectra after removal of exponential background, plural scattering intensity, and arctan components.
Fig. 3.3-4. Correlation between integrated intensity ratio (+10 to +30 eV)/(-10 to +30 eV) from Si K edge ELNES spectra (see Fig. 3.3-3) and nominal fraction of octahedral Si to total Si.

integrated intensity of both peaks (-10 to 30 eV) and the fraction of octahedral Si, given as SiVI/(SiIV+SiVI).

The correlation is not excellent, but clearly illustrates a semi-quantitative relationship between tetrahedral silicon and the peak near 20 eV. Additionally, the main peaks associated with compounds containing octahedral silicon (stishovite, wadeite, majorite, and CaSi2O5 in Fig. 3.3-3) appear asymmetric, with a shoulder near 5 eV. (We note that the position of the main peak in the stishovite spectrum is approximately 2 eV higher in absolute energy relative to the main peak position of the quartz spectrum.)

In contrast, both quartz and pyrope have a symmetric main peak. This suggests that a SiVI dependent feature is present near 5 eV. A better correlation results if we "fit" the spectra of the mixed coordination compounds using the function y=x(tet)+(1-x)(oct), where (tet) is the spectrum of quartz or another phase with x=1.0 (e.g. pyrope in comparison to majorite), and (oct) is the spectrum of stishovite. In this treatment, we have also allowed the energy scales of (tet) and (oct) to vary up to ±2 eV. The fitting routine thus has three adjustable parameters. Results are shown for majorite in Fig. 3.3-5.

Fig. 3.3-5: Majorite Si K edge ELNES spectrum fitted by the combination pyrope and stishovite Si K edge ELNES spectra, optimized to x=0.76, the fraction of the spectrum pyrope, and (1-x=0.24) is the fraction of the stishovite spectrum. For majorite, x is nominally equal to 0.75. Spectrum of majorite and starting spectra of pyrope and stishovite were normalized to integrated intensity over the energy range shown. Both the pyrope and stishovite spectra were also allowed to shift along the energy axis by up to ± 2 eV.

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