**4.7 Bromoapatites**

#### **4.7.1 Calcium bromapatite**

Calcium bromapatite has typical hexagonal apatite structure with the space group P63/M, *a* = 9.761 Å, *c* = 6.739 Å, c:a = 1: 0.6904, *V* = 556.06 Å3 and *Z* = 2 (**Fig. 29**) [186]. The synthesis of **calcium bromapatite** (Ca10(PO4)6Br2) in the tubular quartz reactor (sealed-tube method) can be described by the reaction [135],[187],[188],[189]:

$$\text{Ca}\_{10}\text{(PO}\_4\text{)}\_6\text{OH}\_2 + 2\text{ HBr} \overset{1193\text{ K}, 15\text{ h}}{\rightarrow} \text{Ca}\_{10}\text{(PO}\_4\text{)}\_6\text{Br}\_2 + 2\text{ H}\_2\text{O} \tag{47}$$

**Fig. 29.** The structure of Ca5(PO4)3Br (perspective view along the c-axis).

The phase can also be prepared via solid-state synthesis reaction [135]:

#### Synthetic Phase with the Structure of Apatite http://dx.doi.org/10.5772/62212 219

$$\begin{aligned} \text{6. } \text{CaHPO}\_4 + \text{3. } \text{CaCO}\_3 + \text{CaBr}\_2 &\xrightarrow{\text{heat}} \text{Ca}\_{10} \left( \text{PO}\_4 \right)\_6 \text{Br}\_2\\ + \text{3. } \text{CO}\_2 + \text{3. } \text{H}\_2\text{O} \end{aligned} \tag{48}$$

#### **4.7.2 Lead bromapatite**

Carbonate can be introduced into the structure of carbonated barium-chlorapatite by stirring

The attempts to prepare carbonated barium-chlorapatite in a one-step synthesis results in a mixture of BaCO3 and Ba3(PO4)3. The variations in the manner in which carbonate was added to the reaction mixture, such as co-titrating a carbonate solution along with BaCl2 and NH4H2PO4, pre-mixing it with NH4H2PO4, or adding it first or last did not eliminate the precipitation of simple salts. The inability at 60°C and at the pH of 10 to produce carbonated barium-chlorapatite at any ratio of carbonate to phosphate in the aqueous solution is proba‐

Calcium bromapatite has typical hexagonal apatite structure with the space group P63/M, *a* = 9.761 Å, *c* = 6.739 Å, c:a = 1: 0.6904, *V* = 556.06 Å3 and *Z* = 2 (**Fig. 29**) [186]. The synthesis of **calcium bromapatite** (Ca10(PO4)6Br2) in the tubular quartz reactor (sealed-tube method) can be

( ) ( )

1193 K, 15 h

Ca P O Br

Ca PO OH 2 HBr Ca PO Br 2 H O 10 4 2 6 6 +® + 10 4 2 2 (47)


( ) ( ) <sup>2</sup> 3 2 Ba PO Cl CO Ba PO Cl PO Cl Ba 10 4 2 3 6 5 9 4 <sup>4</sup>

apatite in an (NH4)2CO3 solution for 1 week [185]:

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

bly due to close molar solubility of simple salts [185].

described by the reaction [135],[187],[188],[189]:

**Fig. 29.** The structure of Ca5(PO4)3Br (perspective view along the c-axis).

The phase can also be prepared via solid-state synthesis reaction [135]:

**4.7 Bromoapatites**

b

a

**4.7.1 Calcium bromapatite**

The synthesis of **lead bromapatite** (Pb10(PO4)6Br2) by solid-state synthesis via sintering of equal amount of Pb9(PO4)6 and PbBr2 at the temperature of 250°C in a platinum tube was descri‐ bed by BHATNAGAR [191]. Br<sup>−</sup> (1.95 Å) anions at the *Z*-site in general formula of apatite (Pb10(PO4)6Z2) can be readily substituted by other usual monovalent anions (Cl<sup>−</sup> , F<sup>−</sup> and OH<sup>−</sup> ).

**Fig. 30.** The structure of lead bromapatite identifying the channel polyhedron (broken lines) formed by Pb(2) cations in apatite channel wall: open Pb(2) circles are at the height z = 1/4 and closed circles are at z = 3/4; triangles are PO4 tetra‐ hedra centered at z = 1/4 (open) and z = ¾ (shaded); numbers (1, 2, 3) identify oxygen atoms forming the corners of tetrahedra [190]

LIU et al [190] prepared synthetic lead bromapatite via solid-state reaction at pressure up to 1 GPa. In the structure of this phase (**Fig. 30**), isolated PO4 tetrahedra centered at z = 1/4, 3/4 are linked by Pb(1) in nine-fold (6 + 3) coordination and Pb(2) in an irregular sevenfold (6 + 1) coordination. A prominent feature is large c-axis channel which is defined by tri-clusters of M(2) cations at z = 1/4, ¾ and accommodates a variety of anion components.

#### **4.7.3 Strontium bromapatite**

Strontium bromapatite (strontium bromoapatite) can be prepared via solid-state reaction (**Eq. 50**) and wet (solution) method (**Eq. 51**) according to the following reactions [135]:

$$\begin{aligned} &6\text{ SrHPO}\_4 + 3\text{ SrCO}\_3 + \text{SrBr}\_2 \xrightarrow{\text{heat}} \text{Sr}\_{10} \left(\text{PO}\_4\right)\_6\text{Br}\_2\\ &+ 3\text{ CO}\_2 + 3\text{ H}\_2\text{O} \end{aligned} \tag{49}$$

$$\begin{aligned} 110 \text{ SrBr}\_2\text{(aq)} + 6 \text{ Na}\_3\text{PO}\_4\text{(aq)} &\to \text{Sr}\_{10}\text{(PO}\_4\text{)}\_6\text{Br}\_2\text{(s)}\\ 1 + 18 \text{ NaBr}(\text{aq}) \end{aligned} \tag{50}$$

Since the precipitate contains Na+ ions, it must be washed thoroughly to obtain pure product. A small amount of hydroxylapatite may also be present.

Strontium bromapatite forms softer crystal than fluorapatite or strontium chlorapatite. Since it is not stable under the mercury-vapor discharge in fluorescent lamp (**Section 10.6**), stronti‐ um bromapatite cannot be used for the production of lighting phosphor [135].
