Optical gaps of alkali borate and alkali fluoroborate glasses

Abstract

The optical gaps of B2O3 alkali borate, and alkali fluoroborate glasses were determined by using blown films. The gap of B2O3 glass was 8.0 eV and the value decreased monotonically with decreasing the B2O3 content in both fluoroborate and borate glasses. When compared at a constant B2O3 content, the gap is in the order Li>Na>K and fluoroborate>borate. Ab initio molecular orbital calculation on the model clusters of the glasses revealed that the lowest excitations in B2O3 and alkali (fluoro) borate glasses are the transitions from nonbonding 2p orbitals of oxygen atoms in boroxol rings to vacant nonbonding 2p orbitals of boron atoms on “loose” BO3 triangles and from nonbonding 2p orbitals of oxygen atoms bonded with borons in tetrahedral coordination to the vacant ns orbitals of neighboring alkali ions, respectively. The vacant ns orbitals of the alkali ions neighboring oxygen atoms attached to tetrahedral borons primarily contribute to the lowest unoccupied molecular orbital (LUMO) levels. On the other hand, fluorine indirectly contributes to lowering the highest occupied molecular orbital (HOMO) levels via inductive effect (polarization of B–F σ-bond). It was therefore concluded that the effect of alkali addition on lowering LUMO levels is larger than that of fluorine addition on lowering the HOMO level.