Density–structure correlations in fluoride-containing bioactive glasses

Fluoride-containing bioactive glasses are of interest for dental and orthopaedic applications. On the basis of MAS NMR results, a model has been developed for the density and molar volume of SiO2–P2O5–CaO–Na2O–CaF2 bioactive glasses. The model correlates the density with the concentration and volume of the structural units forming the glass. CaF2 is assumed to be present in glass in the form of CaF4 units; the volume of CaF4 in the studied glasses is about 1.02% greater than in the CaF2 crystal. Calculated density and molar volume are consistent with the experimental data and the good agreement between calculated and measured density provides validation of the presence of F–Ca(n) units from 19F MAS NMR studies. Because of constant network connectivity of the silicate network, the increase in both the density and molar volume is related only to the increase in the concentration of CaF4 units.