Borate structures: crystalline and vitreous-

摘要

A comprehensive survey is presented of the structures of the crystalline phases formed in M2O-B2O3 and MO-B2O3 glass forming systems, with particular reference to those aspects of relevance to the structures of the corresponding glasses. Tables are given of the known crystalline phases in each system, together with the current status ofknowledge concerning their structure. The crystalline phases for which the full structure has been determined are employed to investigate the compositional dependence of various structural parameters, viz. the content of the five different basic borate structural units, B~((n)), and of the various species of superstructural unit. The information accumulated for the crystalline phases is employed as a guide to the structures of the equivalent glasses, and to the variation of the above structural parameters with composition in the vitreous state, especially the fraction, x4, of the boron atoms in a given glass that are 4-fold co-ordinated. Topological models have been presented in the literature for the variation of x4 with composition, and their predictions are compared with experimental data for both crystalline and vitreous samples. The simplest topological model assumes that BO_4~- units cannot be immediate neighbours, due to electrostatic repulsion, but such an arrangement is found in several superstructural units, presumably as a result of the unfavourable electrostatic configuration being more than balanced by the superstructural unit stabilisation energy. Experimental evidence for the presence of superstructural units in borate glasses is therefore examined, together with their possible effect on the structural unit fractions, x_n. The paper concludes by discussing a composite structural model for borate glasses, and by posing 10 questions of importance to continuing research on borate systems.