Role of fluorine in the devitrification of SiO₂·Al₂O₃·P₂O₅·CaO·CaF₂ glasses

A series of eight glasses based on a glass system with the generic composition 1.5(5-Z)SiO₂·(5-Z) Al₂O₃·1.5P₂O₅·(5-Z)CaO·ZCaF₂ were studied where Z = 2, 1.75, 1.5, 1.25, 1, 0.5, 0.25, 0. These glasses were characterized using differential scanning calorimetry (DSC) and x-ray powder diffraction (XRD). Glasses with high fluorine contents were found to crystallize readily to fluorapatite via a homogeneous nucleation route probably involving prior amorphous phase separation. These results are explained in terms of an approach which views glasses as being inorganic polymers where the presence of fluorine disrupts the glass network and thereby reduces the energy barrier to homogeneous nucleation and crystallization of fluorapatite.