Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 290

In B₂O₃ glass the presence of rigid, planar boroxol B₃O₆ units made up of three trigonal BO₃ units is well established [20]. In contrast to basic polyhedral units, AB_V, where a single A atom is coordinated by B atoms, super‐structural units such as boroxol units contain more than one A atom. Do super‐structural units then exist in other borate glasses? It is a question that has long persisted in the oxide‐glass science.

Super‐structural units may be energetically more favorable in systems with long‐range interactions. However, their larger size raises difficulties to match the density of networks with the observed density of glass. For example, the boroxol units are topologically equivalent to the basic BO₃ trigonal unit (both have δ = 2 and V = 3). If all BO₃ trigonal units in B₂O₃ glass are replaced by B₃O₆ boroxol units, the length scale of the structural unit is doubled (the volume thus increasing by a factor of 8) while the mass of the unit is only tripled so that the density of a network of boroxol units is only 3/8 of that of a network of trigonal units. Further, based on topological considerations mentioned before, an extended 3‐D network can incorporate only a small fraction of super‐structural units with V > 4. For this reason, the di‐pentaborate and the di‐triborate groups, two of the six super‐structural units listed by Wright [20], probably do not exist in significant concentrations.