Beyond the Gaussian Approximation in Exafs

Abstract

The extended x-ray absorption fine structure (EXAFS) technique was first introduced as an atomic structure determining technique [1] in 1971. Since then its applications have grown dramatically, expanding into many diverse fields [2–9]. With the decade or so of experience in using EXAFS, its limitations and strengths have been defined. Yet there still appears to be a misconception that EXAFS cannot be used to determine the atomic arrangement in highly disordered materials such as amorphous and anharmonic solids. This misconception arose because, although the original formulation of the EXAFS formula was for a general distribution of atoms [10], the formula most commonly employed [11] assumed a Gaussian distribution of the atoms about their average value, introducing a Debye-Waller-type factor. The Gaussian distribution is not generally valid when the disorder is large or anharmonic. In addition, it was thought [10] that multiple scattering effects in the x-ray absorption near-edge structure (XANES) regions would limit the low k information available from EXAFS to above k ≃ 3 Å^-1. Loss of this low k information would mean that EXAFS cannot determine rms disoorder greater than about 0.1 Å.