The Structure of Borosilicate Glasses

Abstract

In the author's previous paper, it was stated that due to the disposition that the cations of the modifying oxide do not approach to each other, atomic groups denominated as xy4 are formed in glasses containing B2O3 at the viscosity of about 1.5×1012 poises, i.e., near the contraction temperature of the glass. In this paper, further considerations concerning the xy4-group formation in borosilicate glasses are given from the point of view of the ionic refraction of various oxygen in sodium borate glasses.It is deduced that the B-O bonds in the BO4 tetrahedron which is located in the central part of the xy4-group is very strong, and this tetrahedron is formed at the expense of the loosening of the B-O bonds in the BO3 triangles around it, i.e., these BO3 triangles are unstable in comparison with those in the normal state. Accordingly, the predominance in number of the atomic groups xy4 in comparison with other groups such as xy3z, xy2z2, xyz3, or xz4, where x, y, and z represent BO4 tetrahedron, BO3 triangle and SiO4 tetrahedron, respectively, is explained.When the Na2O content in the sodium borate glass is increased above 0.167, Na ions begin to cut the B-O bonds around xy4-groups, and as the result, the unstable BO3 triangles around BO4 tetrahedron must be stabilized to some degree. It may be supposed that these Na ions should then be trapped in deep potential holes on cutting off the B-O bonds. These Na ions should not be so free as those added later, and they may be regarded as the so-called “enclosed” Na ions.It may also be supposed that for each one xy4-group, one Na ion could be the “enclosed” ion, i.e., free Na ions should begin to be produced at the composition 2Na2O⋅5B2O3 or 28.6 mol% Na2O. Thus the location of the maximum transition temperature curve in the ternary diagram Na2O-B2O3-SiO2, and the stability of the network of borosilicate glasses are discussed.