A model has been presented to correlate both the density and molar volume of Bi2O3–B2O3 glasses with the
structure. The model makes use of the available data of the fraction of four coordinated boron to get the
concentration of all structural units. The concentration of BO4 units shows a maximum at ~31 mol% Bi2O3.
There is a nonlinear decrease in the concentration of symmetric BO3 units and an increase in the concentration
of BiO6 units. Asymmetric BO3 units start forming for Bi2O3 N 42 mol%. It is deduced that in BiO6 units, oxygen
atoms are four coordinated with Bi atoms. The model is able to fit the calculated and experimental density and
molar volume and explains the minimum in the molar volume at 22 mol% Bi2O3.