TABLE OF CONTENTS The current model for the structure of vitreous boric oxide differs significantly from that for vitreous silica. Although boron occurs in both triangular and tetrahedral coordination in crystalline compounds, it is believed to occur only in the triangular state in vitreous boric oxide. All such triangles are connected by BO at all three corners to form a completely linked network. However, since the basic building block of this network is planar rather than 3-dimensional, the 3-dimensional linkage which occurs in a network of tetrahedra does not exist for vitreous boric oxide. A 3-dimensional structure is developed by "crumpling" of the network, in much the same way that a two dimensional drawing on a sheet of paper develops a third dimension when the paper is crumpled into a ball. Since the primary bonds exist only within the plane of the paper, bonds in a third dimension (van der Waals bonds in this case) are very weak and the structure is easily disrupted. One result of this structure, for example, can be found in the glass transformation temperature of vitreous boric oxide, which is only ? 260 C, as opposed to the T g of vitreous silica, which is ? 1100 C.
The structure of vitreous boric oxide is also believed to contain a large concentration of an intermediate unit, consisting of three boron-oxygen triangles joined to form a structure known as a boroxol ring or boroxol...
