The importance of π-bonding in glass chemistry: borate glasses

Relationships between chemical bonding, electronic polarisability, optical basicity and coordination number are discussed for oxidic glasses. Particular attention is focused on the lithium borate glass system because of the startling observation that the basicity decreases as the initial lithium oxide content is increased. This arises Ming to a very large increase in the polarising power of the boron, gamma(B). Only part of the increase is due directly to the coordination number change, which is shown to be responsible for gamma(B) rising from 2.47 (the value for threefold coordinated boron) to 3.3. However, the coordination number change also results in the complete loss of pi-bonding in the BO(4) unit, and it is this factor that further increases gamma(B) to the experimentally observed value of 4.2. These changes in the chemical bonding result in the electronic charge borne by the oxygen atoms in the BO(4) unit being less than in the BO(3) network, and writing it in the form [BO(4)](-) could be misleading. Generally for the alkali borate glasses, the overall basicity arises from the combination of the two systems: the weakly basic BO(4) units, and the more basic BO(3) network. The separate trends for these systems in Na(2)O-B(2)O(3) glasses are calculated for increasing Na(2)O content, and discussed with reference to the 'two site' model (of Kamitsos et al) for borate glasses. The overall study highlights the importance of pi-bonding in oxidic glasses generally, and the very pronounced effect when it is absent.