Bonding nature in tellurite glasses

We show that the variation of the structural unit in tellurite glasses is due to the charge transfer from modifier atoms to ${\mathrm{TeO}}_6$ octahedra with use of first-principles molecular-orbital calculations. Mulliken overlap populations indicate that the oxygen coordination number of a Te atom reduces as the charge transfer progresses. This is consistent with the trend found in metal tellurite structures. Orbital-overlap population analysis reveals that the transferred electrons to the Te-O antibonding orbital cause breaking in the Te-O bonds and lead to reducing the coordination number of the Te atom. The bond ionicity and charge states of a Te atom and an O atom in α-${\mathrm{TeO}}_2$ are also presented.