**4.4 Fluorapatites**

**Shape\* Approximated size range Method(s) of**

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

Dumbbell 2–3 μm (organized nanoparticles of ~50 nm size)

high-temperature processes (ht), synthesis from biogenic sources (bs), combination procedures (cp).

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

\*\* 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),

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

**Fig. 20.** The formation and the morphology evolution mechanism of Ca5(PO4)3OH samples with various morphologies

0.5–7 μm (pores 20–150 nm) hth, cp

1.5–2.5 μm (organized nanorods of 100–150

nm diameter and 1–2 μm length)

Porous microsphere, mesoporous

Bowknot, self-assembled

sphere

nanorods

\* Consult with **Section 3.1.14**.

[32],[127],[128].

based upon different pH values [127],[128].

**synthesis\*\***

cp

cc

### **4.4.1 Synthetic analogues of the mineral fluorapatite**

In literature various routes for the preparation of synthetic analogues of fluorapatite are described which include solid-state reactions of the type [129]:

$$\text{Ca}\_3\text{Ca}\_3\text{(PO}\_4\text{)}\_2 + \text{CaF}\_2 \rightarrow \text{Ca}\_{10}\text{(PO}\_4\text{)}\_6\text{F}\_2\tag{25}$$

At the temperature of 900°C hydroxylapatite reacts with calcium fluoride to give fluorapa‐ tite [69]:

$$\text{Ca}\_{10}\text{(PO}\_4\text{)}\_6\text{(OH)}\_2 + \text{CaF}\_2 \rightarrow \text{Ca}\_{10}\text{(PO}\_4\text{)}\_6\text{F}\_2 + \text{CaO} + \text{H}\_2\text{O}\text{(g)}\tag{26}$$

Fluorapatite can be also prepared directly by firing a mix of 3Ca3(PO4)2 with CaF2 at 1600°C, or from calcium pyrophosphate and calcium fluoride:

$$118\text{ Ca}\_2\text{P}\_2\text{O}\_7 + 14\text{ CaF}\_2 \rightarrow 5\text{ Ca}\_{10}\left(\text{PO}\_4\right)\_6\text{F}\_2 + 6\text{ POF}\_3\tag{27}$$

Chlorapatite can be prepared by similar method using calcium chloride. It can also be produced in the reversible reaction according to **Eq. 21**.

Original phase diagram **Fig. 21**(a) for the section Ca3(PO4)2–CaF2 of the ternary system CaO– P2O5–CaF2 was published by NACKEN [130]. The range of compositions was extended by BERAK [131] (b), and further refined (**Fig. 22**) by BERAK and T.-HUDINA [132]. Important features are congruent melting of Ca10(PO4)3F2 at 1650°C, eutectics with Ca3(PO4)2 at 1620°C and second one with CaF2 at 1203°C. Sufficiently precise phase diagram enables to determine necessary information on the flux growth of fluorapatite, so the crystal with only slight deficiency in fluorine compared to the theoretical one can be prepared. The problem concerning possible stable existence of spodiosite (Ca2(PO4)F) analogous to naturally occurring mineral remains unsettled, but it appears unlikely to be stable at liquidus temperatures [133].

**Fig. 21.** Phase diagram of Ca3(PO4)2–CaF2 section by NACKEN [130] (a) and BERAK [131 ] (b).

**Fig. 22.** Phase equilibrium in the system Ca3(PO4)2–CaF2: Ca10(PO4)6F2 (ApA) and Ca7(PO4)4F2 (ApB) [132].

The implication of the crystal growth of apatite and calcite in the systems Ca3(PO4)2–CaCO3– Ca(OH)2–CaF2 (**Fig. 23**(**a**)) and Ca3(PO4)2–Ca(OH)2–CaF2–H2O (**b**) enables the quaternary phase diagram provided by WILLIE [134].

**Fig. 23.** System Ca3(PO4)2–CaCO3–Ca(OH)2–CaF2 (a) and Ca3(PO4)2–Ca(OH)2–CaF2–H2O (b) at the pressure of 1 kbar [134].

Long and uniform HAP whiskers with high crystallinity, controlled morphology and high aspect ratio were synthesized by ZHANG and DARWELL [83] via the hydrothermal method using acetamide. Compared to urea as an additive, which is commonly used to raise the pH in order to drive the nucleation and growth of HA crystals [106], acetamide has low hydrolysis rate under required hydrothermal conditions. This allows better and easier control, giving rise to rapid growth of whiskers at low supersaturation. The whisker length and width were in turn given by the solution conditions, including the concentration of Ca and PO4 [83].

#### **4.4.2 Barium fluorapatite**

**Fig. 21.** Phase diagram of Ca3(PO4)2–CaF2 section by NACKEN [130] (a) and BERAK [131 ] (b).

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

**Fig. 22.** Phase equilibrium in the system Ca3(PO4)2–CaF2: Ca10(PO4)6F2 (ApA) and Ca7(PO4)4F2 (ApB) [132].

diagram provided by WILLIE [134].

The implication of the crystal growth of apatite and calcite in the systems Ca3(PO4)2–CaCO3– Ca(OH)2–CaF2 (**Fig. 23**(**a**)) and Ca3(PO4)2–Ca(OH)2–CaF2–H2O (**b**) enables the quaternary phase Barium apatite can be prepared by solid-state reaction [135]:

$$\text{66 BaHPO}\_4 + \text{3BaCO}\_3 + \text{BaF}\_2 \xrightarrow{\text{heat}} \text{Ba}\_{10}\left(\text{PO}\_4\right)\_6\text{F}\_2 + \text{3CO}\_2 + \text{3H}\_2\text{O} \tag{28}$$

It possesses typical hexagonal structure with the space group P63/M and *a* = 10.153 Å, *c* = 7.733 Å, c:a = 1:0.722, *V* = 10.153 Å3 and *Z* = 2 [135].

Ba(1) atoms are located in columns on three threefold axes and are coordinated by nine oxygen atoms. The Ba(2) sites form triangles around the F site and are coordinated by six oxygen atoms and one fluoride ion. Fluoride ions are statistically displaced by ∼0.25 Å from the Ba(2) triangles. This displacement of F ions is analogous to the displacement of OH ion in Ca10(PO4)6(OH)2 [138].
