The known structure of a crystalline phase is almost always useful in investigating the unknown structure of the compositionally equivalent glass. For the local environment around elements like Si, B and P, the correspondence between site geometry and symmetry can be impressively close. Beyond near neighbours, any relationship becomes less obvious – at least in real-space data. Progress in understanding the medium-range structures of glasses has been painfully slow as a result. One essential clue is given by reciprocal-space features at low Q (scattering vector) in X-ray or neutron scattering data, which are clearly related to the medium-range structure. Interpretation of these features as ‘quasi-Bragg’ scattering allows direct comparison between the structures of the glass and equivalent crystalline phases. Applications of this method will be illustrated in borates and silicates, together with some chalcogenide glasses. Correspondence between low-Q features for these glasses and compositionally-equivalent crystals is qualitatively good. In some cases there is semi-quantitative agreement too. Thus the essential flavour of the medium-range structure of several typical glasses appears to be interpretable, rather easily.

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