Chemical bonding character of the network forming bonds in oxide glasses

Chemical bonding character of the glass network forming M-O bonds (M = Si, B, Ge, and Al) was evaluated based on the Mulliken population analyses, in which the bond overlap population Q_MO was obtained from the molecular orbital calculations. It was found in alkali silicates that the interaction of alkali ion with bridging oxygen reduced Q_sio in Si-O-Si bridging bonds, and Q_sio in Si-O-Na terminal bonds was much larger than that in Si-O-Si bridges. In M-O-M bridges, that is, Si-O-Si, B3-O-B3, B4-O-B4, Ge4-O-Ge4 (Bn, Gen: n-fold coordinated boron and germanium), Q_MO was distributed around 0.6 ± 0.I. In M-O-M′ bridges formed by the heterogeneous combinations of B3-O-B4, Ge4-O-Ge5, Ge4-O-Ge6, Si-O-B4, and Si-O-A14, Q_MO of M-O bond was larger than Q_M′O of M′-O bond, and the difference between Q_MO and Q_MO was about 0.2. It was suggested that the dispropoitional sharing of electrons reflected the π-bonding character, that is, more electrons were localized on the bonds with larger it-bonding character.