Determination of the factorized atomic part of the X-ray absorption cross section in the near-edge spectrum: Application to the analysis of structural changes in β zeolite with an increase in the aluminum content

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A technique for determining the factorized atomic part σ at ( k ) of the X-ray absorption cross section in the near-edge range of small photoelectron wave numbers k , based on the selection of this function from the experimental absorption spectrum of the atom in a compound, has been developed. This technique has been tested using the X-ray absorption near-edge structure (XANES) spectra of aluminum and silicon in a number of model compounds with known structure and applied to investigation of changes in the character of the short-range environment of a silicon atom in β zeolite with a variable Si/Al ratio. Use of the function σ at ( k ) obtained by the developed technique in the ab initio calculation of the XANES spectra of silicon in a model β zeolite with Si/Al = 100 makes it possible to reduce the effect of the errors of the muffin-tin approximation on the fine structure of the calculated spectrum and substantiate the possibility of comparing relative changes in a certain spectral range with a change in the structure of the short-range environment of silicon in β zeolite with an increase in the aluminum content. On the basis of this approach, the formation of local distortions in the oxygen tetrahedron coordinating an ionized silicon atom with an increase in the aluminum content (decrease in the Si/Al ratio from 100 to 12) has been established.