The electronic properties of ${\left({\mathrm{Li}}_2\mathrm{O}\right)}_x{\left({\mathrm{B}}_2{\mathrm{O}}_3\right)}_{1-x}$ ($x=0.0$, 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 ${\mathrm{Li}}_2\mathrm{O}$. The band gaps were predicted for other compounds of the ${\mathrm{Li}}_2\mathrm{O}\text{−}{\mathrm{B}}_2{\mathrm{O}}_3$ system where less reliable experimental data are available. The calculated band gap of ${\left({\mathrm{Li}}_2\mathrm{O}\right)}_x{\left({\mathrm{B}}_2{\mathrm{O}}_3\right)}_{1-x}$ decreases with increasing mole fraction $x$ of ${\mathrm{Li}}_2\mathrm{O}$. This was interpreted by the decreasing bonding interaction between $\mathrm{B}{\mathrm{O}}_n$ groups. By an analysis of the electron density distribution, it was shown that in all systems under consideration the $\mathrm{B}\text{−}\mathrm{O}$ bonds are mainly covalent, and that $\mathrm{B}\text{−}\mathrm{O}$ bonds in $\mathrm{B}{\mathrm{O}}_3$ units are stronger than in $\mathrm{B}{\mathrm{O}}_4$. The $\mathrm{Li}\text{−}\mathrm{O}$ interaction is predominantly ionic.

• Received 11 March 2005

DOI:https://doi.org/10.1103/PhysRevB.72.125101