**6.3.1. Nitrogen-containing apatites**

Nitrogen was incorporated into hydroxyapatite by dry ammonia treatments at temperatures between 900 and 1200°C in the presence of graphite. The process of synthesis of cyanamida‐ patite (Ca10(PO4)6CN2, Ca10(PO4)6NCN) can be described by the following chemical equations [13]:

$$\text{2 }\text{NH}\_3 + \text{C} \rightarrow \text{H}\_2\text{CN}\_2 + \text{2 }\text{H}\_2\tag{24}$$

$$\text{Ca}\_{10}\text{(PO}\_4\text{)}\_6\text{(OH)}\_2 + \text{H}\_2\text{CN}\_2 \rightarrow \text{Ca}\_{10}\text{(PO}\_4\text{)}\_6\text{CN}\_2 + 2\text{ H}\_2\text{O}\tag{25}$$

Ammonia reacts with graphite during the thermal treatment forming [CN2] 2− ions (**Eq. 24**). These cyanamide ions interchange with moveable OH<sup>−</sup> ions situated on the sixfold screw axis of apatite to form cyanamidapatite (**Eq. 25**). A similar reaction is known for the synthesis of calcium cyamide from calcium oxide:

$$\text{CaO} + 2\text{ NH}\_3 + 2\text{ C} \rightarrow \text{CaCN}\_2 + \text{CO} + \text{3}\text{ H}\_2\tag{26}$$

The treatments at temperatures above 1200°C or long-term treatments destroy the apatite lattice completely through the phosphate reduction. Cyanimide ions lose their sites in the apatite lattice and the nitrogen content decreases [13]. The synthesis of Ca10(PO4)6CN2 apatite provides the evidence that the hydroxylapatite structure is able to incorporate larger organic molecules [81].

Direct transformation of TCP (Ca3(PO4)2) into cyanamidapatite according to the reaction:

$$\text{Ca}\_3\text{Ca}\_3\text{(PO}\_4\text{)}\_2 + \text{H}\_2\text{CN}\_2 \rightarrow \text{Ca}\_9\text{(PO}\_4\text{)}\_\text{g}\text{(HPO}\_4\text{)}\text{HCN}\_2\tag{27}$$

was also proposed by HABELITZ et al [82].

#### **6.3.2. Peroxide-doped apatites**

Although "oxygenated" apatites were not much investigated compared to other substituted apatites, some past studies have, however, reported the possibility of apatitic channels to incorporate oxygenated species such as H2O2 or O2 or molecular ions including O2 2− (the peroxide ion) and superoxide O2−. They are single-phase nanocrystalline apatites, where part of apatitic OH<sup>−</sup> ions are replaced by oxygenated species. Typically by peroxide ions (quanti‐ fied) and at least the traces of superoxide ions can be prepared by the precipitation from aqueous calcium and phosphate solutions in the presence of H2O2 under medium room temperature [83],[84].

The local structure of hydroxyl-peroxy apatite was described by YU et al [85]. Hydroxyl-peroxy apatite contains a small amount of partially dehydroxylated hydroxyapatite phase and calcium hydroxide. The incorporation of peroxide ions into the lattice of HAP causes the perturba‐ tions of hydrogen environments and slight changes in its crystal morphology. The distance between H in some structural OH- and adjacent O along the c-axis becomes longer instead of forming the hydrogen bond after the incorporation of peroxide ions.

According to the concentration of peroxide ions in hydroxyl-peroxy apatite and the theoreti‐ cal value, the corresponding formula for the hydroxyl-peroxy apatite is proposed as follows [85]:

Ca10(PO4)6(OH)1.34−2x(O2)0.33(O)x□0.33+x.

3 22 2 2 NH C H CN 2 H +® + (24)

2− ions (**Eq. 24**).

2− (the

ions situated on the sixfold screw axis

10 4 ( ) ( ) 2 2 10 4 2 2 ( ) 6 2 <sup>6</sup> Ca PO OH H CN Ca PO CN 2 H O +® + (25)

CaO 2 NH 2 C CaCN CO 3 H + + ® ++ 3 22 (26)

3 4 22 9 4 4 2 ( ) ( ) ( ) 2 5 3 Ca PO H CN Ca PO HPO HCN + ® (27)

Ammonia reacts with graphite during the thermal treatment forming [CN2]

of apatite to form cyanamidapatite (**Eq. 25**). A similar reaction is known for the synthesis of

The treatments at temperatures above 1200°C or long-term treatments destroy the apatite lattice completely through the phosphate reduction. Cyanimide ions lose their sites in the apatite lattice and the nitrogen content decreases [13]. The synthesis of Ca10(PO4)6CN2 apatite provides the evidence that the hydroxylapatite structure is able to incorporate larger organic

Direct transformation of TCP (Ca3(PO4)2) into cyanamidapatite according to the reaction:

Although "oxygenated" apatites were not much investigated compared to other substituted apatites, some past studies have, however, reported the possibility of apatitic channels to incorporate oxygenated species such as H2O2 or O2 or molecular ions including O2

peroxide ion) and superoxide O2−. They are single-phase nanocrystalline apatites, where part

fied) and at least the traces of superoxide ions can be prepared by the precipitation from aqueous calcium and phosphate solutions in the presence of H2O2 under medium room

The local structure of hydroxyl-peroxy apatite was described by YU et al [85]. Hydroxyl-peroxy apatite contains a small amount of partially dehydroxylated hydroxyapatite phase and calcium hydroxide. The incorporation of peroxide ions into the lattice of HAP causes the perturba‐ tions of hydrogen environments and slight changes in its crystal morphology. The distance between H in some structural OH- and adjacent O along the c-axis becomes longer instead of

forming the hydrogen bond after the incorporation of peroxide ions.

ions are replaced by oxygenated species. Typically by peroxide ions (quanti‐

These cyanamide ions interchange with moveable OH<sup>−</sup>

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

calcium cyamide from calcium oxide:

was also proposed by HABELITZ et al [82].

**6.3.2. Peroxide-doped apatites**

molecules [81].

of apatitic OH<sup>−</sup>

temperature [83],[84].

**Fig. 13.** Possible configuration of hydroxyl ions, peroxide or oxide ions and vacancies in the channel along the crystal‐ lographic c-axis in hydroxyl-peroxy apatite. O, H atoms and vacancies are presented by large gray circles, small open circles and gray squares, respectively. Filled small circles represent H atoms perturbed by the incorporation of perox‐ ide ions [85].

A scheme of possible configurations of hydroxyl ions, peroxide or oxide ions and vacancies in the channel along the crystallographic c-axis in hydroxyl-peroxy apatite is illustrated in **Fig. 13**. Peroxide ions incorporated into HAP are located in the channel of apatite structure through the substitution of a portion of OH<sup>−</sup> radicals, and the material is a solid solution of hydroxyland peroxide apatite.

ZHAO et al [86] reported that a new hydrogen bond was formed between peroxide ions and adjacent OH<sup>−</sup> radicals in hydroxyl-peroxy apatite. According to the literature [430,446], the formation of hydrogen bond would induce a downfield shift of corresponding proton resonance. Some authors reported a linear correlation of the isotopic proton chemical shift with the O-H…O distance, which was a measure of the hydrogen bond strength. ZHAO et al [86] suggested the following mechanism for the incorporation of O<sup>2</sup> 2−:

$$\text{2 OH}^-\text{O}^{2-} + \left[\begin{array}{c} \end{array}\right] + \text{H}\_2\text{O}\uparrow\tag{28}$$

where [] was the vacancy. O2− ion was active and could react with O2 to produce O<sup>2</sup> 2−.

$$\text{O}^{2-} + \frac{1}{2}\text{O}\_{2} \rightarrow \text{O}\_{2}^{2-} \tag{29}$$

Peroxide ions associated with the vacancies were situated placed in the channel of HA lattice along the c-axis through the substitution of a portion of OH radicals. The molecular ions constituted a symmetric vibrator with a stretching vibration active in Raman spectrometry. This vibration was recorded at 750 cm−1 in the Raman spectra of O2 2−-containing HA samples. The final product was a solid solution of hydroxyl- and peroxide-apatite. However, the existence of peroxide ions in the HA lattice caused the contraction of the unit-cell dimen‐ sions of HA materials. In addition, a new hydrogen bond was formed between peroxide ions and adjacent OH radicals, which was determined by using molecular spectroscopy analysis. During annealing treatment in air, peroxide ions decomposed and the substituted OH radicals re-entered the HA lattice, resulting in the elimination of the structural aberrations caused by the incorporation of peroxide ions. The concentration of peroxide ions present in HA sam‐ ples was measured by chemical analysis [86].

#### **6.3.3. Chalcogenide phosphate apatites**

The synthesis and the structure of four new chalcogenide5 [87] phosphate apatitic phases of the composition given by the formula:


were reported by HENNING et al [88].

<sup>5</sup> The elements from the chalcogenide group (or oxygen group family) belonging to Group 16 (VI A) of the periodic table: O, S, Se, Te and Po. Elements sulfur, selenium and tellurium are also termed as the elements from the sulfur subgroup [87].

**Fig. 14.** The structure of Ca(PO4)6S (a), Sr(PO4)6S (b), Ba(PO4)6S (c) and Ca(PO4)6Se (d) viewed along the c-axis.

These four apatites are isostructural and crystallize in the trigonal space group P3 over bar with the chalcogenide ion positioned at (001/2). Sulfoapatites show no ability to absorb H2S in the way that oxyapatite absorbs H2O at elevated temperatures. This can be attributed to the position of sulfide ion and the way it influences the crystal structure around vacant chalcoge‐ nide position at (000) [88].
