XANES and EXAFS modelling of configurational disorder in silver borate glasses

The sensitivity of x-ray absorption spectroscopy (XAS) to the local structure around a selected atomic species symmetry, distances and angles is commonly exploited to quantitatively describe systems where all the configurations around the absorbing atoms may be approximated by the mean one. However, in many cases of disordered systems a simple description of the local structure with single-configuration models cannot be reconciled with experimental spectra. In this paper the necessity to analyse the XAS spectra by considering a multiplicity of atomic structural configurations is demonstrated in the case of silver borate glasses. The local coordination around Ag cations in (Ag2O.nB2O3) glasses has been studied by comparing the XAS experimental spectra at the Ag K edge with theoretical XANES and EXAFS spectra obtained by a configurational average of model structures. In order to provide a reasonable agreement with experiment, these models have to take into account that there are many geometrically distinct Ag neighbourhoods present in the system, and that the measured XANES and EXAFS signals are in fact averages over spectra originated at all the non-equivalent photoabsorbing sites (multi-configuration approach)