Texture effect on polarized EXAFS amplitude

Abstract

Application of polarized extended X-ray absorption fine structure (P-EXAFS) spectroscopy to thin films of fine-grained minerals is emerging as a powerful method to investigate the in-plane and out-of-plane local structure of phyllosilicates. Mineral platelets have no preferential orientation in the plane of the film, and their c* axes are oriented essentially along the film normal. The angular dependence of the EXAFS amplitude critically depends on the orientation distribution of c* axes due to mosaic spread. The effect of film texture on EXAFS amplitude has been calculated as a function of the mosaic spread, the orientation of the electric field vector, and the crystallographic orientation of the atomic pair. Calculations show that the reduction in amplitude for partially ordered films is more important when the electric field vector is perpendicular to the film plane. For phyllosilicates, no significant deviation from single crystal dichroism occurs when the mosaic spread is less than ±20–25° half-width at half-maximum (HWHM) for parallel measurement, and ±15–20° HWHM for normal measurement. Graphs are given for correcting EXAFS-derived coordination numbers for texture effects.