They concluded that the process of FAp formation, constitutes three
steps, which is also in good agreement with the modified evolutionary
theory of apatite crystals that confirm the presence of
CaF2 as a precursor for FAp formation. Both a- and
c-lattice parameters of HAp phase were Rietveld refined and calculated
as a function of temperature along with a-lattice parameter of
CaF2 phase. Contraction of a-axes of samples, were seen
to be about -0.05 Å compared to the HAp control sample. This change
might be explained by the fluoride-hydroxyl substitution within the
apatite structure, due to the smaller size of the F-compared to OH-, in agreement
with.38,39 Slight expansion of the c-lattice parameter
in 90ᵒC was evident compared to the control HAp
sample, as the CaF2 wt% follows a decreased trend with
increasing temperature (Figure 2b-d). In order to quantify the amount of
HAp and compare it with S-XRD data, 1H MAS-NMR would
be fundamental in future work along with those already gathered data in
order to quantify the amount of each phase, HAp, FAp and
CaF2 relatively.
19F MAS-NMR Characterization
19F chemical shifts peak positions of apatite grown at
different temperatures were investigated to identify their chemistry and
F- interactions. Figure 3 shows the peak positions for
the mineralized powders at the different temperatures;
37ᵒC,
70ᵒC, and 90ᵒC. Using DMFIT, NMR
peaks deconvolution detected 5 peaks, two major crystalline phases could
be found, the first being FAp at approximately −103.5 ppm and the second
being CaF2 having a peak at approximately −109 ppm.
Furthermore, a -95 ppm peak was observed, which is identified as a
CO3 peak. It was observed that at
37ᵒC, the reaction is driven kinetically between
Ca2+ ions and F- ions forming
CaF2 as it is more kinetically stable than FAp. On the
other hand, with increasing the temperature the reaction will be driven
thermodynamically as the FAp formation is favoured and is more
thermodynamically stable than CaF2. CaF2crystals in the latter case may act as a reservoir of
Ca2+ and F- ions that would react with free phosphate
groups in solution to form FAp crystals. Moreover, the NMR peak -95 ppm
that corresponds to the CO32-, was more evident at 90ᵒC compared to the
other temperatures. In comparing to the FTIR results, at this
temperature, CO32- stretch vibrations were also seen unlike the other
two temperatures, this might give an indication that this weak peak
found in the NMR spectra could be related to different interaction of F-
ions inside apatite lattice when CO32- is included. Exploration of this
hypothesis might be useful in future studies. Scholz et
al.43 also identify an isotropic 19F chemical shift of
-108.6 ppm of CaF2 that was synthesized mechanochemically using
high-energy ball milling. Additionally, it was also found that by
increasing the time of milling the 19F line width resonance increases
and subsequently its amount decreases. However, Chen et
al.34 have used a similar wet chemical method at 70ᵒC,
where they found only one peak of FAp at -103 ppm while the CaF2 was
totally absent which is considered as a contradictory finding to our
MAS-NMR result. This could be explained by the time of incubation, as
they incubated the samples for 5 days compared to 8 days.