Structural groups and their mixing in borosilicate glasses of various compositions – an NMR study

Five binary borosilicate glasses and 54 ternary sodium borosilicate glasses covering the glass-forming regions have been examined by 11B, 23Na and 29Si solid state MAS-NMR. Linear dependencies of the chemical shifts on compositional parameters were observed. In contrast to most previous structural models, the results point to a much better mixing of copolymerized SiO4/2, BO3/2 and BO4/2 groups and therefore between borate and silica networks and a more uniform distribution of sodium cations in the whole composition range than hitherto assumed. Only the observed fractions of three-and four-coordinated borons and the quantitative amount of alkali to form either four-coordinated boron or non-bridging oxygen atoms cannot be described by a random model. The differences between molten and sol–gel prepared borosilicate glasses appear in a new light.