**4.3 Synthetic analogues of the mineral hydroxylapatite**

The synthetic analogue of the mineral hydroxylapatite can by prepared by the reaction [126]:

$$\begin{aligned} &10\text{ }\text{Ca}(\text{NO}\_3)\_2 + 6\text{ }(\text{NH}\_4)\_2\text{HPO}\_4 + 8\text{ }\text{NH}\_4\text{OH} \rightarrow \text{Ca}\_{10}\text{(PO}\_4\text{)}\_6\text{(OH)}\_2\\ &+20\text{ }\text{NH}\_4\text{NO}\_3 + 6\text{ H}\_2\text{O} \end{aligned} \tag{23}$$

Aqueous solutions of 0.167 mol·cm−3 of Ca(NO3)2 and 0.100 mol·cm−3 of (NH4)2HPO4 were prepared, and their pH values were adjusted to above 8 by the addition of ammonium hydroxide. (NH4)2HPO4 solution was heated to about 85°C and then slowly dropped into equal volume of vigorously stirred solution of Ca(NO3)2. The temperature of the reaction mixture

was kept at 85°C and stirring was maintained for further 3 days. In order to remove CO2, the flow of N2 was introduced to the suspension in reaction vessel. The suspension was then filtered and washed.

The survey of known chemical reactions successfully used for the synthesis of hydroxylapa‐ tite was provided by SHOJAI et al [32]. Depending on applied method (**Table 2**) and condi‐ tions (**Fig. 20**), different shapes of apatite particles can be prepared.

1900

1800

Temperature/°C

[118].

1700

1600

1:1 + Liq.

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

30 40

Nd2O3 + 1:1

along the [0001] direction. The largest crystals were approximately 6×1×1 mm.

**4.3 Synthetic analogues of the mineral hydroxylapatite**

43 2

20 NH NO 6 H O

7:9(Ap) + Liq.

1:1

Nd9.33(SiO4)6O2

+ + Ap

50

**Fig. 19.** Reconstructed pseudobinary phase diagram around the apatite phase Nd9.33(SiO4)6O2 in the Nd2O3-SiO system

The growth of single crystal of synthetic analogue of vanadinite (lead vanado-chlorapatite, Pb5(VO4)3Cl) using the CsCl flux method was performed by MASAOKA and KYONO [124]. No impurity phases were formed from this crystal growth method. Crystals obtained via this method exhibit well-developed hexagonal prismatic form of the size of several millimeters

The first hydrothermal growth of single crystals of chlorapatite was reported by ROUFOSSE et al [125]. Crystals grown from the system chlorapatite-HCl-H2O at 50 000 psi and pH = 1 with the growth zone at 465°C and dissolution zone at 360°C were found to be of high stoichiometry.

The synthetic analogue of the mineral hydroxylapatite can by prepared by the reaction [126]:

() () <sup>3</sup> <sup>4</sup> <sup>4</sup> <sup>4</sup> 10 4 ( )( ) 2 2 6 2

Aqueous solutions of 0.167 mol·cm−3 of Ca(NO3)2 and 0.100 mol·cm−3 of (NH4)2HPO4 were prepared, and their pH values were adjusted to above 8 by the addition of ammonium hydroxide. (NH4)2HPO4 solution was heated to about 85°C and then slowly dropped into equal volume of vigorously stirred solution of Ca(NO3)2. The temperature of the reaction mixture

+ + (23)

10 Ca NO 6 NH HPO 8 NH OH Ca PO OH

+ +®

SiO2 mol%

1:2

60 70

Ap

1:2 + Liq.

1:2 + SiO2



\* Consult with **Section 3.1.14**.

\*\* Solid-state synthesis (ss), mechanochemical method (mch), conventional chemical precipitation (cc), hydrolysis method (hl), sol-gel method (sg), hydrothermal method (hth), emulsion method (em), sonochemical method (sch), high-temperature processes (ht), synthesis from biogenic sources (bs), combination procedures (cp).

**Table 2.** Shape of hydroxylapatite particles prepared by given synthesis methods [32].

The influence of conditions on the morphology of hydroxylapatite particles is shown in **Fig. 20**. During hydrothermal synthesis, the particle size of HAP decreases with increasing pH value [32],[127],[128].

**Fig. 20.** The formation and the morphology evolution mechanism of Ca5(PO4)3OH samples with various morphologies based upon different pH values [127],[128].

Complete replacement of halogen occurs when either fluorapatite or chlorapatite is heated in the steam of H2 or H2O at high temperatures [69]:

$$\begin{array}{rcl} \mathbf{Ca}\_{10} \left( \mathbf{PO}\_{4} \right)\_{6} \mathbf{F}\_{2} & \xrightarrow{\mathbf{H}\_{2}, \ 16 \& \mathbf{PO}^{\circ}} \mathbf{Ca}\_{10} \left( \mathbf{PO}\_{4} \right)\_{6} \left( \mathbf{OH} \right)\_{2} \\ \mathbf{H}\_{12} \mathbf{O}\_{1} \left( \mathbf{PO}\_{4} \right)\_{6} & \\ \mathbf{C} & \mathbf{Ca}\_{10} \left( \mathbf{PO}\_{4} \right)\_{6} \mathbf{Cl}\_{2} \end{array} \tag{24}$$
