Letter to the Editor: The extraction of atomic co-ordination numbers from x-ray diffraction data for network glasses

It has been demonstrated that both the detailed electron distributions of the constituent atoms/ions and non-Gaussian Gaussian peaks in T(r) can seriously limit the accuracy of co-ordination numbers extracted from x-ray diffraction data, whether the fit is performed in real or reciprocal space. In general, however, it is preferable to perform the fit in real space, where differences in shape between the experimental and fitted peaks are much more obvious, together with the effect of any experimental uncertainties, as represented by the noise in the resulting correlation function in the region below the first true peak. Note, also, that in reciprocal space the effects of a non-Gaussian peak shape on the extrapolation of the sinusoidal envelope to zero Q, from a high-Q fit to the interference function, Qi(Q), apply equally to neutron diffraction data, similarly limiting the accuracy of the resulting co-ordination numbers.