**6.2.1. Arsenate substitution in hydroxylapatite**

the absorption and desorption equilibria with natural minerals, including apatites. Cadmi‐ um has a slight preference for Ca(I) site in fluorapatite and for Ca(II) site in hydroxyapatite [7], [52]. The interactions between these two ions (Cd and Ca) during absorption and ionic change processes in apatites present therefore considerable practical and theoretical interest.

Calculated energy differences (*E*) between these sites are of 12 and 8 kJ·mol−1 for fluorapatite and hydroxylapatite, respectively. The preference is not strong, and however, a part of the sites of the other type is also occupied by cadmium ions. The relative site occupation can be

where *E* = *E*(Cd2+ or Zn2+ on Ca(1)) – *E*(Cd2+ or Zn2+ on Ca(2)). At *T* = 298 K, *P* = 85 and 17 Cd2+ in fluorapatite and hydroxylapatite, respectively. From the value of *P* and from the fact that the sum of the two probabilities is 1, one can calculate that the probability of the lower-energy

BADRAOUI et al [53] reported that the maximum amount of cadmium substitution for stronti‐ um in the system Sr10−xCdx(PO4)6Z2 (Z = OH and F) accounts for about 40 at% in HAP and for 60 at% in FAP. The increase of cadmium content induces stronger decrease of the c-axis with respect to the a-axis. The structure refinements evidence found a statistical distribution of Cd atoms in Sr10−xCdx(PO4)6(OH)2 and a light preference for M(1) site in Sr10−xCdx(PO4)6(F)2. The stability ofthe system M10−xM'x(PO4)6Z2 (M and M' = Ca, Pb, Cd, Sr andZ = OH and F) is strongly affected by the polarizability. As a matter of fact, complete miscibility is possible even when the cations exhibit great size differences, provided they are not both soft acids. Otherwise, the presence of two cations with quite different radii and relevant polarizabilities induces important distortions of the apatite unit cell and PO4 tetrahedra and consequently limits the

Pentavalent arsenic, vanadium and chromium substitution can completely replace phospho‐ rus in calcium, strontium and barium fluor- and chlorapatites. Calcium fluor-vanadate, arsenate and -chromate structures were distorted compared to normal hexagonal apatite. Manganese completely replaced phosphorus only in barium apatites, while chromium and manganese could not be incorporated into lead apatites. Excluding these exceptions, contin‐ uous solid solutions were formed between the phosphate and/or vanadate and the chromate or manganese analogues for given divalent and halide ions [54]. The substitution of CO3

at X- (carbonate-apatite of A-type) and Z-site (carbonate-apatite of B-type) was already

probability of substitution at Ca 1 site 4 E <sup>P</sup> exp probability of substitution at Ca 2 site 6 kT æ ö <sup>=</sup> = -ç ÷

( ) ( )

è ø (21)

2− ions

Cadmium is also a frequent heavy toxic pollutant element in water [7].

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

expressed by the equation [7]:

site occupancy is of 99% and 94%, respectively.

possibility of mutual substitution.

described in **Section 4.6.**

**6.2. Anionic substitution at X-site**

The arsenate (As5+) substitution in the hydroxylapatite structure was examined by LEE et al [55]. The investigation with samples of hydroxylapatite, the As5+-substituted analogue (synthetic analogue of mineral johnbaumite, **Section 1.6.3**) and of intermediate compositions does not provide any evidence of lowering the symmetry below P63/M. A series of arsenate-substitut‐ ed hydroxyapatite was also prepared through aqueous precipitation method by ZHU et al [56]. Prepared solid solutions (Ca5(PxAs1−xO4)3(OH)) showed apatite structure for the whole arsenate/phosphate series. With decreasing arsenate content, the particles changed from smaller needle-like to large tabular crystals and the unit cell dimensions *a* and *c* increased but not in fair agreement with Vegard's law4 [57]. In FT-IR spectra, the area of phosphate peak was gradually suppressed and the area of arsenate peak increased as the proportion of arsenate increased.

Complete PO4 3− ↔ AsO<sup>4</sup> 3− substitution was also recognized in experimental studies of apatite analogues, such as in the system Sr5(PO4)3OH-Sr5(AsO4)3OH [58]. The Rietveld refinement of Sr5(AsO4)3Cl (pentastrontium tris[arsenate(V)] chloride, 890.31 g·mol−1) from high-resolution synchrotron data was performed by BELL et al [59]. The hexagonal compound crystallizes in the same structure (**Fig. 8**) as other halogenoapatites in the space group P63/M with the cell parameters: *a* = 10.1969 Å, *c* = 7.28108 Å, *V* = 655.63 Å3 , *c*:*a* = 0.7140 and *Z* = 2. The structure consists of isolated tetrahedral AsO4 3− anions (As atom and two O atoms have m-symmetry), separated by two crystallographically independent Sr2+ cations, which are located on mirror planes and threefold rotation axes, respectively. One Sr atom is coordinated by nine O atoms and the other one by six. Chloride anions (site symmetry 3 ¯) are at 2*a* sites and are located in the channels of the structure.

**Fig. 8.** The structure of Sr5(AsO4)3Cl apatite (perspective view along the c-axis).

<sup>4</sup> Vegard's law, first pronounced in 1921, states that the lattice parameter of a solid solution of two phases with similar structures is a linear function of lattice parameters of the two end-members [57].
