The early stages of nucleation and crystallisation of an apatite glass-ceramic: Evidence for nano-scale crystallisation

Crystallisation of a fluorapatite (FAp) (Ca5(PO4)3F) glass-ceramic has been studied by differential scanning calorimetry (DSC), X-ray diffraction (XRD), small angle neutron scattering (SANS), time-of-flight (TOF) neutron diffraction and multinuclear 19F, 31P, 27Al, 29Si magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. During crystallisation of FAp, fluorine is used in forming the FAp phase, since fluorine is known to reduce the glass transition temperature (Tg), the Tg of the residual glass increases on FAp crystallisation, which will restrict the growth of the FAp crystals. The XRD patterns of the heat-treated glass show the presence of FAp in addition there is marked line broadening. Debye–Scherrer analysis of the line broadening gives an average crystallite size of approximately 11–21 nm. SANS shows the glass to be single phase with no evidence of amorphous phase separation. SANS analysis of the heat-treated glass shows a single peak at q = 0.036 Å− 1 corresponding to FAp crystallites of about 28 nm. 19F MAS NMR shows a broad resonance identified as F-Ca(n) species in the glass, which narrows down on heat treatment and moves to position close to the F-Ca(3) site in calcium FAp. 31P MAS NMR of the glass showed orthophosphate in structure (Q0) with a small amount of pyrophosphate (P-O-P and/or Al-O-P). On heat treatment the resonance narrows and shifts to 2 ppm corresponding to crystalline FAp. MAS NMR data suggests presence of small amount of Mg substituted into the lattice. It is thought that the observation of nano-crystals is aided by the low fluorine content of this glass. We have proposed semi-empirical rules for designing nano-crystalline oxyfluoride phases in glass-ceramic materials.