Medium-range order and random networks

Micro-crystallite and continuous random network (CRN) models started life in the same decade. Their fates have been quite different. With the discovery of each new family of glasses, micro-crystallite models have been tested in extreme versions and, rightly, found to be inadequate. CRN models have been treated much more elastically, with modifications inserted to accommodate new experimental facts. It is now hard to find a clear statement of what the term CRN means, and most definitions are system-specific. An attempt is made here to present a view of the structure of network glasses, which recognises the strengths and weaknesses of ordered and random models, and the impact of recent experimental investigations of medium-range structure.