Similarly, a carbonate stretch vibration was also observed on HAp Captal^® R at 1420 cm^-1. On the other hand, hydroxyl (OH) peaks that correspond to OH liberation and OH stretch were only detected on the HAp Captal^® R at 631 and 3574 cm^-1 respectively (Supplementary S1), however were absent in all our test groups at different temperatures. This observation gives an indication of the OH group substitution in the lattice, which would be in that case substituted with F- ions, and the coupling to F-in our study was enough to mask the OH group agreed with the observation seen in previous studies.^38,39 In contrast, Chen et al.³⁴ used a wet chemical method, where they changed the pH, while keeping the temperature constant; they found that with increasing pH, more CO₃^2- and OH^- have been incorporated into the lattice, on the contrary, our study did not find similar trend as a function of temperature. On the other hand, at the high temperature group (90ᵒC), we observed the presence of CO₃^2-, this can be explained by the inclusion of CO₂ in the solutions at the high temperature. Wang et al,⁴⁰ reported that the relative intensity of CO₃^2- vibration compared to phosphate vibration increase with time, whilst, in our study, the time constant was kept constant, therefore it will be useful in the to vary the reaction time to test this hypothesis, as this part is beyond the scope of our experiment.

The Miller indices (hkl) of all samples were investigated as a function of temperature, suggesting the presence of HAp, as its main characteristic Bragg peaks were seen as 002, 211, 112 and 300 (Figure 2a). All synchrotron XRD patterns at different temperatures; 37^ᵒC, 70^ᵒC and 90^ᵒC demonstrate the presence of mixed phases within the samples. As the temperature increased, the diffraction patterns tended to show more similarities to the control group. For example, our test groups exhibit the presence of the typical high intense HAp peaks such as 002, 211, and 300. Those peaks indicate the presence of HAp (purple tick marks- Figure 2a-d), which were well-fitted to our calculated data. On the other hand, changes in the relative intensities between 102 and 210 were observed, in comparison to the control sample. This observed change in the relative intensities seemed to be higher at the 37^ᵒC, followed by 70^ᵒC, and 90^ᵒC respectively. This change may be attributed to the presence of calcium fluoride (CaF₂) (light blue tick marks – Figure 2b-d). Furthermore, the broadening of 222 reflection of HAp seemed to be inversely proportional with the increased temperature, as more evident at both 37^ᵒC and 70^ᵒC. Those previous observations indicate the presence of a different phase along the HAp, which is the calcium fluoride (CaF₂). This is due to the diffraction reflection from 111 plane of CaF₂, which overlaps with the reflection of 102 plane of apatite, causing the relative peak variation with 210 plane. As well as the reflection from 220 plane overlaps over the 222 apatite plane leading to the broadening of the associated peak. Therefore, the presence of CaF₂ at all temperatures was confirmed as calculated in Table 2, however, with different ratios. Shapkin et al.⁴¹ studied the formation of FAp from both the intermediate amorphous phase and CaF₂, as a function of time using synchrotron X-ray diffraction. They observed similar overlapping events of apatite and CaF₂, as reported in the current study.