Investigation of the anharmonic EXAFS oscillation of distorted HCP crystals based on extending quantum anharmonic correlated Einstein model

In this work, we have expanded and developed an efficient model to calculate and analyze the extended X-ray absorption fine structure (EXAFS) oscillation of distorted hexagonal close-packed (HCP) crystals with non-ideal axial ratio c/a in the wavenumber and temperature dependencies. The analysis procedure is performed by evaluating the influence of EXAFS cumulants on the amplitude reduction and phase shift of the anharmonic EXAFS oscillation that is described in terms of the Debye-Waller factor using the cumulant expansion approach. The calculation model of the anharmonic EXAFS cumulants is expanded and developed from the correlated Einstein model using the anharmonic effective potential and the first-order perturbation. The numerical results for crystalline zinc agree well with those obtained from other theoretical methods and experiments at various temperatures. The obtained results show that the present theoretical model is necessary to analyze experimental EXAFS signals of distorted HCP crystals under the effect of non-ideal axial ratio c/a. (author)