Joint structure refinement of x-ray and neutron diffraction data on disordered materials:application to liquid water

Abstract

X-ray diffraction data on liquids and disordered solids often provide useful complementarystructural information to neutron diffraction data. Interpretation of the x-ray diffractionpattern, which is produced by scattering from the atomic electrons rather than from theatomic nuclei as in the case of neutron diffraction, is, however, complicated by theQ-dependent electronic form factors, which cause the x-ray diffraction signal to decline rapidly with increasingQ,where Q is the wave vector change in the diffraction experiment. The problem is particularlyimportant in cases such as water where there is a significant molecular polarization causedby charge transfer within the molecule. This means that the electron form factorsapplicable to the molecule in the condensed environment often deviate from their free atomvalues. The technique of empirical potential structure refinement (EPSR) is used hereto focus on the problem of forming a single atomistic structural model whichis simultaneously consistent with both x-ray and neutron diffraction data. Thecase of liquid water is treated explicitly. It is found that x-ray data for water doindeed provide a powerful constraint on possible structural models, but that theQ-range of the different x-ray data sets (maximumQ rangesfrom 10.8 to ∼17.0 Å−1 for different x-ray experiments), combined with variations between different datasets, means that it is not possible to rigorously define the precise position andheight of the first peak in the OO radial distribution function. Equally, it is foundthat two different neutron datasets on water, although measured to a maximumQ of atleast 30 Å−1, give rise to further small uncertainties in the position of the hydrogenbond peaks. One general conclusion from the combined use of neutron andx-ray data is that many of the classical water potentials may have a corewhich is too repulsive at short distances. This produces too sharp a peak inr-space at too short a distance. A softer core potential is proposed here.