THE APPLICATIONS OF ANOMALOUS DIFFRACTION IN POWDER DIFFRACTOMETRY

Abstract

An atom scatters X-ray with an amplitude and a phase that can be represented by the complex number ; ftof (K,E) = fo (K) + f' (K,E)+ if (K, E) . fo(K) represents the atomic scattering factor for the radiations with frequencies smaller than the absorption edge frequencies. The anomalous scattering terms f' and f, which are functions of the wavelength, describe the in-phase an out phase components of the change due to finite binding energies of the electrons .in the atom. Near an absorption edge of an atom, where the x-ray energy is close to the binding energy of an inner shell electron both f' and vary rapidly with wavelength , jumps rapidly up to a maximum while f' dips and then rises again as the x-ray energy goes from below the edge to above it. The term results in breaking of Friedel's law, this effect has long been used to find the solution of the crystallographic phase problem. Very few applications were performed to show the useful aspect of anomalous dispersion on the powder dif f ractometry . Even though the principle is simple and straight forword the implementation was not a trivia tasK. The main difficulty was to obtain reliable data. As the energy dependence of diffraction intensity is usually small then a very high accurancy of measurments were required. The recent availability of synchrotron radiation as a tunable source of x-ray, with very high intensity made it possible to use this effect to obtain structural information from a powder diffractometry. For this purpose a powder automated diffractometer has been designed to be adapted to such new sources (L.U.R.E. DCI) . This diffractometer consistes of three main parts :