Mechanisms of boron coordination change between borosilicate glasses and melts

The structure of two borosilicate glasses and melts has been investigated by neutron diffraction coupled with Reverse Monte Carlo (RMC) simulation. For both compositions (10B2O3 ⋅ 30Na2O ⋅ 60SiO2 and 10B2O3 ⋅ 15Na2O ⋅ 15CaO ⋅ 60SiO2), the fraction of tetrahedral boron ([4]B) drops from 0.7 to 0.3 from the glass to the melt. The change from a modifying to a charge compensating role of cations during quench is related to the boron coordination change. These molecular scale modifications explain the low viscosity of alkali/alkaline-earth borosilicate melts relative to alkali/alkaline-earth silicate melts, as the viscous flow is enhanced with the proportion of [3]B. RMC simulations of soda borosilicate glass and melt show a preferential interbonding between B and Si polyhedra. Such medium range order interbonding is unfavored in the soda-lime borosilicate glass showing a nearly random network. The effect of Ca enhancing the proportion of B-B pairs is here highlighted.