**6.2.3. Chromium analogues of apatite**

The syntheses of chromium (Cr(V) [62]) analogues of apatite were described in literature including the following compounds [63],[64],[65],[66],[67],[67],[69],[70],[71],[72]:


**6.2.2. Vanadate substitution in hydroxylapatite**

Å, *c*:*a* = 0.7503 and *V* = 743.43 Å3

( )

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

*a*1 *a*2 *c*

ters: *a* = 10.0570 Å, *c* = 7.4349 Å, *c*:*a* = 0.7393 and *V* = 651.24 Å3

308 Apatites and their Synthetic Analogues - Synthesis, Structure, Properties and Applications

.

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

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‐

(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

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:

3 2 2 2 4 4 2 5

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

+ + +- ®

++ + + (22)

( ) ( ) ( )

5 x 1x 4 3 2 2 3 2 4.5 Pb NO 0.5 PbCl 3x NH HPO 1.5 1 x V O Pb P V O Cl 9 NO 2.25 O 6x NH 4.5x H O -

. The structure of Ba5(VO4)3OH


These compounds are in general prepared by the ignition of mixture of alkaline-earth carbonates, hydroxides or oxides with Cr2O3 in the presence of water vapor. Ca3(CrO4)2 compound (orthochromate), which is isomorphous with Ca3(PO4)2, is formed as an intermedi‐ ate by carrying out the synthesis in dry atmosphere; this compound is often identified as 9CaO·4CrO3·Cr2O3 [63],[73],[74],[75].

Theoretical compositions and formula weights of chromium apatite analogues are given in **Table 5**.



**Table 5.** Theoretical compositions of chromium apatite analogues (M(CrO3)4Z).

Sr10(CrO4)6F2 possesses typical hexagonal structure of apatite with the space group P63/M, which was refined using the powder neutron diffraction (**Fig. 11**) for the first time by BAIKIE et al [71]. As other chromium analogues of apatite, the material contains chromium in +5 (pentavalent) oxidation state. The material shows the paramagnetic behavior.

**Fig. 11.** Structural representation of Sr10(CrO4)6F2 apatite with SrO6 octahedra and CrO4 tetrahedra: larger and smaller spheres mark F and O atoms, respectively. The unit cell is indicated by black lines [71].

The crystal structure (**Fig. 12**) and the magnetic properties of strontium chromate phase (Sr5(CrO4)3(Cu0.586O)) with apatite-like structure were determined by TYUTYUNNIK and BAZUEV [76]. The sample was prepared by solid-state synthesis via the thermal treatment of the mixture of stoichiometric amount of SrCO3, Cr2O3 and CuO at the temperature of 1200°C in air for 36 h.

**Fig. 12.** Crystal structure of Sr5(CrO4)3(Cu0.586O): (a) projection along the c-axis and (b) side view showing the infinite [CuO]<sup>−</sup> chains and the coordination polyhedra of Cr and Sr atoms [76].

This phase crystallizes in the space group P63/M with hexagonal unit cell parameters: *a* = 10.0292 Å and *c* = 7.4623 Å, *V* = 650.033 Å and *Z* = 2. The compound is stable up to 1200°C in air. It was found that copper in the form of Cu+ cations is located in infinite linear (CuO)<sup>−</sup> chains inserted into the tunnels parallel to hexagonal c-axis. The chains contain about 40% vacancies in copper positions. The valence states of Cr and Cu may be mainly +5 and +1, respectively [76].
