Electronic properties of compounds of the Li2O-B2O3 system

The electronic properties of (Li2O)x(B2O3)1-x (x=0.0, 0.25, 0.33, 0.5, 0.75, and 1.0) compounds were studied by periodic quantum-chemical calculations at density functional theory level using localized atomic basis functions. A good agreement between calculated and measured values of the band gap was obtained for Li2O. The band gaps were predicted for other compounds of the Li2O-B2O3 system where less reliable experimental data are available. The calculated band gap of (Li2O)x(B2O3)1-x decreases with increasing mole fraction x of Li2O. This was interpreted by the decreasing bonding interaction between BOn groups. By an analysis of the electron density distribution, it was shown that in all systems under consideration the B-O bonds are mainly covalent, and that B-O bonds in BO3 units are stronger than in BO4. The Li-O interaction is predominantly ionic.