On the uniqueness of structure extracted from diffraction experiments on liquids andglasses

Abstract

There is continued interest in the problem of extracting structures from x-ray and neutrondiffraction data on liquids and glasses. Traditional Fourier transform techniques, with theirinherent weakness of possible systematic and truncation artefacts being introduced into theestimated distribution functions, are increasingly being complemented by computersimulation methods. These allow three-dimensional models of the scattering system to bebuilt, at the correct atomic number density, which are consistent with both the diffractiondata themselves and with other known or estimated constraints such minimum particleseparations. Here the empirical potential structure refinement (EPSR) method isused to explore structure in supercooled liquid Ni, amorphous Ge and amorphousGeSe2, and to evaluate alternative versions of the radial distribution functions which areconsistent with the diffraction data. In the case of liquid Ni, it is found that thereis, based on the diffraction data, some uncertainty on the hardness and shapeof the repulsive core of the interatomic pair potential, and this may influencethe current debate about the existence of icosahedral order in this liquid. Foramorphous Ge two distinct radial distribution functions are generated, both consistentwith the diffraction data, one of which has strong tetrahedral local order withthe other having a predominantly triangular local coordination. For amorphousGeSe2 it is found the SeSe and GeSe radial distribution functions can be determined well from thedata, but the GeGe distribution is more uncertain, with the best fits implying both GeGeand SeSe homopolar bonds as originally proposed. The results are used to discuss theambiguities inherent in the structural interpretation of diffraction data, even for one- andtwo-component systems.