**6.2.4. Selenium analogues of apatite**

**Compound Composition [wt.%] M**

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

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

Sr5(CrO4)3Cl 53.33 18.99 23.37 4.31 — 821.53 Sr5(CrO4)3Br 50.59 18.01 22.17 9.23 — 865.98

(pentavalent) oxidation state. The material shows the paramagnetic behavior.

ϕ = 25.1°

spheres mark F and O atoms, respectively. The unit cell is indicated by black lines [71].

chains and the coordination polyhedra of Cr and Sr atoms [76].

[CuO]<sup>−</sup>

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

**Fig. 11.** Structural representation of Sr10(CrO4)6F2 apatite with SrO6 octahedra and CrO4 tetrahedra: larger and smaller

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

**M Cr O Z H [g.mol−1]**

Selenium oxyanion-substituted hydroxyapatite (SeHAP) was synthesized as a promising material for the treatment of bone cancer to reduce the probability of recurrence, because selenium plays an important role in protein functions and it has significant effect on the induction of cancer cell apoptosis [77]. Another study indicated that selenite (SeO4 2−) or selenite (SeO3 2−) oxyanions exert their cancer chemopreventive effects by direct oxidation of critical thiol-containing cellular substrates and that they are more efficacious anticarcinogen‐ ic agents than selenomethionine or selenomethylselenocysteine with a lack of oxidation capability [3],[78].

Selenium was incorporated into the hydroxyapatite lattice by replacing some of the phos‐ phate groups with selenite groups. SeO4 2− (selenate) ion has tetrahedral structure like PO4 3− ion (**Fig. 5** and **Table 4** in **Section 1.2**), but it is slightly larger (2.49 Å) in diameter than phosphate ion, which is 2.38 Å in diameter. By contrast, SeO3 2− (selenite) ion has very similar diameter (2.39 Å), but it has a quite different flat trigonal pyramid geometry. The substitu‐ tion of bivalent selenium oxyanions forms positively charged vacancy compensated by simultaneous decalcification and dehydroxylation according to the reaction [3],[79]:

$$\begin{aligned} \text{Ca}\_{10}\text{(PO}\_{4}\text{)}\_{6}\text{(OH)}\_{2} + \text{x}\text{ (SeO}\_{\text{n}}\text{)}^{2-} &\rightarrow \text{Ca}\_{10-\text{x}}\text{(PO}\_{4}\text{)}\_{6-\text{x}}\text{(SeO}\_{\text{n}}\text{)}\_{\text{x}}\text{(OH)}\_{2-\text{x}}\\ + \text{x}\text{ (PO}\_{4}^{3-}+\text{Ca}^{2+}+\text{x}\text{ OH}^{-} &\end{aligned} \tag{23}$$

The SeHAP crystal lattice parameters increased slightly as the Se concentration increased when the Se/P ratios were less than 0.5 [80]. All samples prepared via the precipitation method from aqueous solution by KOLMAS et al [79] contained significant amount of carbonates, especially of B-type. Thus, for these samples, the formula Ca10−x(PO4)6−x(SeOn)x(OH)2−x (**Eq. 23**) should be written as Ca10−x−y(PO4)6−x−y(SeOn)x(CO3)y(OH)2−x−y. Hydroxyapatites containing selenate ions are non-toxic, whereas hydroxyapatite with the highest concentration of selenites is toxic.
