The influence of strontium substitution in fluorapatite glasses and glass-ceramics

Strontium is often substituted for calcium in order to confer radio-opacity in glasses used for dental cements, biocomposites and bioglass-ceramics. The present paper investigates the influence of substituting strontium for calcium in a glass of the following composition: 4.5SiO23Al2O31.5P2O53CaO2CaF2, having a Ca:P ratio of 1.67 corresponding to calcium fluorapatite (Ca5(PO4)3F). The glasses were characterized by magic angle spinning nuclear magnetic resonance (MAS-NMR), by differential scanning calorimetry (DSC) and X-ray powder diffraction (XRD). The 29Si, 27Al and 31P NMR spectra for the glasses with different strontium contents were identical. The 19F spectra indicated the presence of F–Ca(n) and Al–F–Ca(n) species in the calcium glasses and in the strontium glasses F–Sr(n) and Al–F–Sr(n). It can be concluded that strontium substitutes for calcium with little change in the glass structure as a result of their similar charge to size ratio. The low strontium glasses bulk nucleated to a calcium apatite phase. Intermediate strontium content glasses surface nucleated to a mixed calcium–strontium apatite and the fully strontium substituted glass to strontium fluorapatite.