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 90C 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; 37C, 70C, and 90C. 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 37C, 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.