**6.2.2. Vanadate substitution in hydroxylapatite**

The synthesis of synthetic alkaline-earth vanadate hydroxylapatites from hydroxide fluxes was performed by MUGAVERO et al [60]. The hexagonal Sr5(VO4)3OH apatite (pentastrontium tris[vanadate(V)] hydroxide, **Fig. 9**(**a**)) possesses the P63/M space group with the cell parame‐ ters: *a* = 10.0570 Å, *c* = 7.4349 Å, *c*:*a* = 0.7393 and *V* = 651.24 Å3 . The structure of Ba5(VO4)3OH (pentabarium tris[vanadate(V)] hydroxide) apatite is shown in **Fig. 9**(**b**). It crystallizes in hexagonal system with the space group P63/M and the cell parameters: *a* = 10.4589 Å, *c* = 7.8476 Å, *c*:*a* = 0.7503 and *V* = 743.43 Å3 .

**Fig. 9.** The structure of Sr5(VO4)3OH (a) and Ba5(VO4)3OH (b) apatite viewed along the c-axis.

The compounds (solid solution) of the composition of Pb5(PxV1−xO4)3Cl (0 ≤ *x* ≤ 1), which are synthetic analogues of minerals pyromorphite, vanadinite and endlichite, were synthesized for the first time by CHERNORUKOV et al [61] via high-temperature solid-phase reactions:

$$\begin{aligned} \text{4.5 Pb(NO}\_3\text{)}\_2 + \text{0.5 PbCl}\_2 + \text{3x (NH}\_4\text{)}\_2\text{HPO}\_4 + \text{1.5(l-x)}\text{ V}\_2\text{O}\_5 &\rightarrow \\ \text{Pb}\_3\text{(P}\_\text{x}\text{V}\_{1-x}\text{O}\_4\text{)}\_3\text{Cl} + \text{9 NO}\_2 + \text{2.25 O}\_2 + \text{6x }\text{NH}\_3 + \text{4.5x H}\_2\text{O} \end{aligned} \tag{22}$$

**Fig. 10.** Fragment of Pb5(VO4)3Cl crystal structure [61].

The variations of unit cell parameters as a function of composition respect Vegard's law. These compounds are structurally built of discrete phosphate or vanadate tetrahedra linked to one

another by lead polyhedra, which form joint layers (**Fig. 10**). Apatite-type structures offer typically two crystallographic positions for cations differing in the coordination number and local symmetry. Lead atoms occupying the first positions form polyhedra shaped as threecapped trigonal prisms of PbO9 having the symmetry C3, the columns of which run along the threefold axis. Distorted two-capped trigonal prisms of PbO6Cl2 residing in the second positions have local symmetry С1.
