**4.9.1 Anion deficient lacunar lead apatites**

The apatitic structure can accommodate a great variety of other substituent's and vacancies in anionic sites (**Chapter 6**). Previous studies on apatites showed that the only system, where the compounds with the apatite structure could be prepared without *Z* anion, is the lead system. These apatites have the vacancies in Z anion sites (**Fig. 31**) with general formula APb4(XO4)3, where A is monovalent cation Na, Ag, K, etc. Pb2+on plays a crucial role allowing to preserve the ideal apatitic network. This role is related to the presence of lone 6*s*<sup>2</sup> pairs (**Section 2.1.2**) which can compensate for the Coulomb imbalance due to the existence of anion gaps in the tunnels of apatites [198],[199].

**Fig. 31.** Polyhedral view in the ab-plane of the crystal structure of NaCaPb3(PO4)3 showing the tunnels [199].

Lead in apatite is of great interest from two points of view. First, lead is known as a "bone seeker" as it accumulates in bones and teeth, second, it may contribute to the deviation from the general formula of apatites. Moreover a new voltammetric sensor for the quantification of mercury based on NaCaPb3(PO4)3 modified carbon paste electrode can be prepared. Because of the importance of these types of lacunar apatites and the problems which they may cause in biomaterial applications, particular attention has been paid during past few years to synthesize new lacunar anionic apatites [198],[199].

Silver lead apatite (Ag2Pb8(PO4)6, P63/M, *a* = 9.765 and *c* = 7.198 Å) and sodium lead apatite (Na2Pb8(PO4)6, P63/M, *a* = 9.731, *c* = 7.200 Å and Z = 2) were prepared by solid-state synthesis by TERNANE et al [200] from the stoichiometric mixture of Pb3(PO4)2 with Ag3PO4 (at 215°C and 100 atm.) and PbO, Na2CO3 and (NH4)2HPO4, respectively. The synthesis of sodium lead apatite can be described by chemical equation:

$$\begin{array}{rcl} \text{R PbO} + \text{Na}\_2\text{CO}\_3 + 6 \left(\text{NH}\_4\right)\_2\text{HPO}\_4 & \rightarrow & \text{Pb}\_3\text{Na}\_2\left(\text{PO}\_4\right)\_6 \\ + 12 \text{ NH}\_3 + 9 \text{H}\_2\text{O} + \text{CO}\_2 & & \end{array} \tag{52}$$

The unit cell contains eight divalent Pb2+cations, two monovalent cations (Na+ or Ag+ ) and six [PO4] 3− ions. The triangle sites, 6*h*, are occupied by Pb2+ions only while the column positions, 4*f*, are occupied by nearly equal amounts of Pb2+ and monovalent ions (Na+ or Ag+ ).

chalcogenide ion positioned at (0 0 ½). The sulfoapatites show no ability to absorb H2S in the way 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 the vacant chalcogenide position at (000). Calcium sulfoapatites, Ca10(PO4)6S, can be successfully synthesized only using oxide starting materials with sulfur vapor under H2 atmosphere instead of toxic H2S gas

The apatitic structure can accommodate a great variety of other substituent's and vacancies in anionic sites (**Chapter 6**). Previous studies on apatites showed that the only system, where the compounds with the apatite structure could be prepared without *Z* anion, is the lead system. These apatites have the vacancies in Z anion sites (**Fig. 31**) with general formula APb4(XO4)3, where A is monovalent cation Na, Ag, K, etc. Pb2+on plays a crucial role allowing to preserve

which can compensate for the Coulomb imbalance due to the existence of anion gaps in the

**Fig. 31.** Polyhedral view in the ab-plane of the crystal structure of NaCaPb3(PO4)3 showing the tunnels [199].

synthesize new lacunar anionic apatites [198],[199].

apatite can be described by chemical equation:

Lead in apatite is of great interest from two points of view. First, lead is known as a "bone seeker" as it accumulates in bones and teeth, second, it may contribute to the deviation from the general formula of apatites. Moreover a new voltammetric sensor for the quantification of mercury based on NaCaPb3(PO4)3 modified carbon paste electrode can be prepared. Because of the importance of these types of lacunar apatites and the problems which they may cause in biomaterial applications, particular attention has been paid during past few years to

Silver lead apatite (Ag2Pb8(PO4)6, P63/M, *a* = 9.765 and *c* = 7.198 Å) and sodium lead apatite (Na2Pb8(PO4)6, P63/M, *a* = 9.731, *c* = 7.200 Å and Z = 2) were prepared by solid-state synthesis by TERNANE et al [200] from the stoichiometric mixture of Pb3(PO4)2 with Ag3PO4 (at 215°C and 100 atm.) and PbO, Na2CO3 and (NH4)2HPO4, respectively. The synthesis of sodium lead

pairs (**Section 2.1.2**)

the ideal apatitic network. This role is related to the presence of lone 6*s*<sup>2</sup>

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

[197].

**4.9.1 Anion deficient lacunar lead apatites**

tunnels of apatites [198],[199].

The structure of this phase was also investigated by KOUMIRI et al [201]: *a* = 9.7249 Å, *c* = 7.190 Å and Z = 2. From the interatomic distances it appears that lead cations in the lacunar apatite NaPb4(PO4)3 behave in two different ways:


**Fig. 32.** The correlation chart for PO4 3− fundamental modes under free-ion, site-group and factor group analyses in Pb8M2(PO4)6 where M = Ag and Na [200].

All [PO4] 3− groups are crystallographically equivalent in the cell and have *C*s as the site group. P, O(1) and O(2) atoms are in 6*h* positions; remaining O(3) oxygen atoms occupy the (121) positions. On this basis, the optical modes at k = 0 are distributed as follows [200]:

$$\Gamma\_{\rm opt} = 1\,\text{2}\,\text{A}\_{\rm g} + 8\,\text{E}\_{\rm lg} + 1\,\text{3}\,\text{E}\_{2g} + 8\,\text{A}\_{\rm u} + 1\,\text{2}\,\text{E}\_{\rm tu} \tag{53}$$

where *A*g, *E*1g and *E*2g are Raman-active normal modes, *A*u and *E*1u normal modes are infra‐ red active. The internal modes from tetrahedral phosphate ions, six per unit cell, can be deduced by the group factor analysis given in **Fig. 32**.

NADDARI et al [202] performed the solid-state synthesis of colorless calcium-lithium lead apatite (Pb6Li2Ca2(PO4)6, LCPbAp, P63/M, *a* = 9.679 and *c* = 7.113 Å, *V* = 577.09 Å3 , *Z* = 1 and *ρ*calc. = 5.48 g·cm−3) via the thermal treatment of mixture of Li2CO3, (NH4)2HPO4, CaCO3 and PbO at 800°C in air for 12 h and subsequently at 900°C for 12 hours. The structure of Pb6Li2Ca2(PO4)6 is shown in the perspective view in **Fig. 33**(**a**). Site (I) is occupied by 0.88 Ca2+, 1.96 Li+ and 1.148 Pb2+. These cations are coordinated to nine oxide anions forming a tricapped trigonal prism. In the tunnel set around the *c* axis, site (II) is occupied by 4.98 Pb2+ and 1.02 Ca2+. These cations constitute the walls of the tunnel and are arranged in equilateral triangles (**Fig. 33**(**b**)).

**Fig. 33.** Perspective view of Pb6Ca2Li2(PO4)6 structure (a) and Pb(II)-Pb(II) stacking in Pb6Ca2Li2(PO4)6 showing possible arrangement of electron lone pairs [202].

Lithium ions occupy preferentially site (I) and this structure is anionic lacunary apatite stabilized by the interaction of Pb(II) electron lone pair. The electrical conductivity as a function of temperature can be interpreted assuming a hopping mechanism of Li ions in the tunnels [202].

Tricationic lacunar apatites Na1−xKxPb4(AsO4)3 (0 ≤ *x* ≤ 1) were synthesized as single phases by solid-state method at 700°C (48 h) by MANOUN et al [198]:

$$\begin{aligned} \text{4 PbO} + 0.5 \text{x } \text{K}\_2\text{CO}\_3 + 0.5 \text{(l-x) } \text{Na}\_2\text{CO}\_3 + 3 \text{ (NH}\_4\text{)}\_2 \text{HAsO}\_4 &\rightarrow\\ \text{Na}\_{1-x} \text{K}\_x \text{Pb}\_4 \text{(AsO}\_4\text{)}\_3 + 0.5 \text{ CO}\_2 + 1.5 \text{ H}\_2\text{O} + 3 \text{ NH}\_3 \end{aligned} \tag{54}$$

It was found that Pb(II) ions in the solid solutions preferentially occupied the M(1) and M(2) sites in the lacunar anionic apatite structure. The structure contains the channels running along the *c*-axis and centered at (00z). The channels are most probably occupied by lone electron pairs of Pb2+ cations.

Γ 12 A 8 E 13 E 8 A 12 E opt = ++ ++ g 1g 2g u 1u (53)

, *Z* = 1 and *ρ*calc.

and 1.148

where *A*g, *E*1g and *E*2g are Raman-active normal modes, *A*u and *E*1u normal modes are infra‐ red active. The internal modes from tetrahedral phosphate ions, six per unit cell, can be

NADDARI et al [202] performed the solid-state synthesis of colorless calcium-lithium lead

= 5.48 g·cm−3) via the thermal treatment of mixture of Li2CO3, (NH4)2HPO4, CaCO3 and PbO at 800°C in air for 12 h and subsequently at 900°C for 12 hours. The structure of Pb6Li2Ca2(PO4)6

Pb2+. These cations are coordinated to nine oxide anions forming a tricapped trigonal prism. In the tunnel set around the *c* axis, site (II) is occupied by 4.98 Pb2+ and 1.02 Ca2+. These cations

**Fig. 33.** Perspective view of Pb6Ca2Li2(PO4)6 structure (a) and Pb(II)-Pb(II) stacking in Pb6Ca2Li2(PO4)6 showing possible

Lithium ions occupy preferentially site (I) and this structure is anionic lacunary apatite stabilized by the interaction of Pb(II) electron lone pair. The electrical conductivity as a function of temperature can be interpreted assuming a hopping mechanism of Li ions in the tunnels

Tricationic lacunar apatites Na1−xKxPb4(AsO4)3 (0 ≤ *x* ≤ 1) were synthesized as single phases by

+ +- + ®

( ) ( )

+++ (54)

2 3 23 4 4 2

apatite (Pb6Li2Ca2(PO4)6, LCPbAp, P63/M, *a* = 9.679 and *c* = 7.113 Å, *V* = 577.09 Å3

is shown in the perspective view in **Fig. 33**(**a**). Site (I) is occupied by 0.88 Ca2+, 1.96 Li+

constitute the walls of the tunnel and are arranged in equilateral triangles (**Fig. 33**(**b**)).

deduced by the group factor analysis given in **Fig. 32**.

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

arrangement of electron lone pairs [202].

solid-state method at 700°C (48 h) by MANOUN et al [198]:

( )

1x x 4 4 3 2 2 3

Na K Pb AsO 0.5 CO 1.5 H O 3 NH -

4 PbO 0.5x K CO 0.5 1 x Na CO 3 NH HAsO

[202].

The factor group analysis [198] of the hexagonal structure (P63/M) shows that the normal modes of vibration can be classified among the irreducible representations of C6h as follows:

$$\Gamma = \mathrm{l}2\mathrm{A}\_{\mathrm{g}} + 8\mathrm{E}\_{\mathrm{lg}} + \mathrm{l}\mathrm{BE}\_{2\mathrm{g}} + 9\mathrm{A}\_{\mathrm{u}} + \mathrm{l}\mathrm{2B}\_{\mathrm{u}} + 9\mathrm{B}\_{\mathrm{g}} + \mathrm{l}\mathrm{BE}\_{\mathrm{lu}} + 8\mathrm{E}\_{2\mathrm{u}} \tag{55}$$

where the internal mode contribution of (AsO4) groups to the IR- and Raman-active vibra‐ tions is:

$$\begin{aligned} \Gamma\_{AsO\_4} &= 6A\_{\rm g} \left(\nu\_1 + \nu\_2 + 2\nu\_3 + 2\nu\_4\right) + 3E\_{\rm lg} \left(\nu\_2 + \nu\_3 + \nu\_4\right) + \\ 6E\_{\rm 2g} \left(\nu\_1 + \nu\_2 + 2\nu\_3 + 2\nu\_4\right) + 3A\_{\rm u} \left(\nu\_2 + \nu\_3 + \nu\_4\right) + \\ 6E\_{\rm lu} \left(\nu\_1 + \nu\_2 + 2\nu\_3 + 2\nu\_4\right) \end{aligned} \tag{56}$$

where *g* and *u* modes are Raman-and IR-active, respectively [198],[203],[204].

The syntheses of apatites, Na1−xKxCaPb3(PO4)3 (0 ≤ x ≤ 1), with anion vacancy were carried out using the solid-state reactions at 700°C for 48 h [199]:

$$\begin{aligned} \frac{1-\text{x}}{2} & \text{Na}\_2\text{CO}\_3 + \frac{\text{x}}{2}\text{K}\_2\text{CO}\_3 + 3\text{ PbO} + \text{Ca}\left(\text{NO}\_3\right)\_2 \cdot 4\text{H}\_2\text{O} + \\ 3\text{ (NH}\_4\text{)}\text{H}\_2\text{PO}\_4 &\rightarrow \text{Na}\_{1-\text{X}}\text{K}\_x\text{CaPb}\_3\left(\text{PO}\_4\right)\_3 + 0.5\text{ CO}\_2 + \\ 3\text{ (NH}\_3 + \text{NO}\_2 + \text{NO}\_3 + 8.5\text{ H}\_2\text{O} \end{aligned} \tag{57}$$

The lattice constants of the solid solutions varied linearly with x. It was found that Pb ions in the solid solutions occupied M(1) and M(2) sites in the lacunar apatite structure. The struc‐ ture was described as built up from [PO4] 3− tetrahedra and Pb2+ of six-fold coordination cavities (*6h* positions), which delimit void hexagonal tunnels running along [001]. The tunnels are connected by cations of mixed sites (*4f*) half occupied by Pb2+ and half by Na+ /K+ mixed alkali cations.

The factor group analysis of the hexagonal structure (P63/M) shows that the normal modes of vibration can be classified among the irreducible representations of C6h by **Eq. 56** where the internal mode contribution of (PO4) groups to the IR and Raman-active vibrations is [199]:

$$\begin{aligned} \Gamma\_{PO\_4} &= 6A\_{\rm g} \left( \nu\_1 + \nu\_2 + 2\nu\_3 + 2\nu\_4 \right) + 3E\_{\rm lg} \left( \nu\_2 + \nu\_3 + \nu\_4 \right) \\ &+ 6E\_{\rm g} \left( \nu\_1 + \nu\_2 + 2\nu\_3 + 2\nu\_4 \right) + 3A\_{\rm h} \left( \nu\_2 + \nu\_3 + \nu\_4 \right) \\ &+ 6E\_{\rm h} \left( \nu\_1 + \nu\_2 + 2\nu\_3 + 2\nu\_4 \right) \end{aligned} \tag{58}$$
