On the connectivity of borate tetrahedra in borate and borosilicate glasses

摘要

In borosilicate glasses, not only the question of the fraction of four- and three-fold coordinated boron deserves special consideration, but also the connectivity of the tetrahedral and trigonal borate groups with each other and with the silicate network. Older structural studies and models of borate and borosilicate glasses often invoke an avoidance rule in the context that negatively charged borate tetrahedra will not be directly linked in glasses. This statement is analogous to the Loewenstein rule which was postulated for aluminate tetrahedra in zeolites. However, contrary to aluminium, directly linked [B empty set(4)](-) tetrahedra are known to exist in natural minerals such as danburite, and not surprisingly, they also occur in glasses (empty set denotes bridging oxygen atoms). Raman spectroscopy confirms, for example, the presence of borate rings consisting of one neutral [BO3](0) trigonal and two charged [B empty set(4)](0) tetrahedral units. Four membered danburite-like rings consisting of two linked silicate and two linked borate tetrahedra were also identified by their Raman band at 630 cm in glasses with higher Na2O fractions. Additionally, borate rings consisting of three tetrahedral orthoborate units, [B empty set(2)O(2)](3-), were identified by vibrational spectroscopy in borate glasses containing high levels of MnO or Eu2O3. The presence of such rings strengthens the glass network and results in a second boron oxide anomaly as evidenced by an increase in the glass transition temperature at very high metal oxide contents. Two dimensional NMR experiments in alkali poor borosilicate glasses reveal a preference of [B empty set(4)](-) tetrahedra to bond to borate rather than silicate entities of the glass network. For these low alkaline glasses, R<0.5 (R is the fraction Na2O:B2O3), B-4-O-Si links are formed in quenched glasses but tend to disappear after annealing, whereas the remaining connections between the borate and silicate sub-networks involve predominantly B-3-O-Si linkages.