## 3 Results  
### Structural and Morphological Analysis  
The nHAp and nFAp crystalize in the hexagonal crystal lattice in the space group \(P6_{3}/m\). The existence of two types of tunnels in an apatite-type structure enables the presence of two non-equivalent cationic sites labelled Ca(1) and Ca(2). The crystallographic site Ca(1) with C\({}_{3}\) symmetry is coordinated by nine oxygen atoms that come from the PO\({}_{4}\)\({}^{3-}\) groups. The second Ca(2) site with C\({}_{s}\) symmetry is seven coordinated by six oxygen atoms from the PO\({}_{4}\)\({}^{3-}\) groups and one hydroxide ion in the case of nHAp or one fluoride ion in the case of nFAp. The X-ray diffraction measurements were performed to check the phase purity of the obtained materials (see Figure 2). The XRD patterns of nHAp and nFAp nanosized materials exhibit characteristic patterns of hexagonal structure (\(P6_{3}/m\) space group) that are compatible with the apatites (ICSD--26204 and ICSD--9444, respectively). The obtained materials are characterised by crystal phase purity. The main diffraction peaks are located at 31.9 of \(2\theta\) values corresponding to the (211) and 33.1 and corresponding to the (300) Miller indexes. The morphology of the apatites, determined by the SEM technique, is shown in Figure 3. The grains possess an elongated shape with well-defined edges and grain boundaries. The morphology of the materials obtained by the hydrothermal method is rod-like, and the size is in the nanoscale range. The particle size distributions of the studied systems were measured based on SEM images and are presented as histograms (Figure 3e,f). The average grain size of the Ca\({}_{10}\)(PO\({}_{4}\))\({}_{6}\)(OH)\({}_{2}\) powder is 56 nm in width and 107 nm in length, while the Ca\({}_{10}\)(PO\({}_{4}\))\({}_{6}\)F\({}_{2}\) powder is 37 nm in width and 92 nm in length. The appropriate content of elements in the obtained nHAp and nFAp nanosized powders was checked by using the EDS method. The ratio of \(n_{Ca}\) to \(n_{P}\) and the molar content of fluorine were calculated. The results are shown in Table 1. The amount of fluorine ions built into apatite is dependent on the pH value [58]. Therefore, the authors concluded that if the pH value is lower than this used for us (pH = 9), a higher amount of fluorine ions will be built up into the nFAp structure. This is connected with competition between F\({}^{-}\) and OH\({}^{-}\) ions. The higher the OH ion concentration (higher pH value) and the lower the F ion concentration, the higher the probability of HAp creation. The FT-IR spectroscopy was employed to identify the existence of functional groups in the obtained materials (see Figure 4). We identify typical vibrational bands of the phosphate groups in FT-IR spectra of the nHAp nanosized powder: the doubly degenerated \(v_{2}\) bending at 472 cm\({}^{-1}\), the triply degenerated \(v_{4}\) vibration at 564 cm\({}^{-1}\) and 600 cm\({}^{-1}\); the non-degenerate \(v_{1}\) symmetric stretching at 962 cm\({}^{-1}\); and the triply degenerate \(v_{3}\) antisymmetric stretching at 1025 cm\({}^{-1}\) and 1088 cm\({}^{-1}\). Moreover, the stretching and librating modes of hydroxyl groups at 630 cm\({}^{-1}\) and 3572 cm\({}^{-1}\) are present in the IR spectra, indicating that HAp was formed. In the case of FAp, typical vibrational bands of phosphate groups were also identified: the doubly degenerated \(v_{2}\) bending at 472 cm\({}^{-1}\), the triply degenerated \(v_{4}\) vibration at 564 cm\({}^{-1}\) and 598 cm\({}^{-1}\); the non-degenerate \(v_{1}\) symmetric stretching at 963 cm\({}^{-1}\); and the triply degenerate \(v_{3}\) antisymmetric stretching at 1017 cm\({}^{-1}\), 1046 cm\({}^{-1}\) and 1092 cm\({}^{-1}\). No vibrations were found at about 3560 and 630 cm\({}^{-1}\) (see arrows in Figure 4), characteristic of the hydroxyl groups, confirming the formation of a pure nFAp structure.