**6.1.2. Magnesium-substituted apatite**

**Cluster Site occupancy**

*k* = 4 Site M Sr, Hg

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

*k* = 5 Site M Ca, Cd, Pb

*k* = 6 Site M Ca, Pb

*k* = 7 Site M Zn

Z Z Z F, Cl

**6.1. Cationic substitution at M(1) and M(2) sites**

ed in the bond distortion angle *ΨMI* <sup>−</sup>*O*<sup>1</sup>

**6.1.1. Strontium-substituted apatites**

X P Z Cl, Br

X P Z Cl, Br

X P

**Table 4.** The relationship linking various clusters shown in **Fig. 3** with the site occupancy in the apatite unit [23].

*MIz*=0 [23].

Even though Ca2+ and Hg2+ cations have roughly the same ionic size (1.18 and 1.23 Å at M(I) site), their electronegativity data indicates that Hg atoms (electronegativity value of 2 in Pauling scale) are relatively highly covalent compared to Ca atoms (electronegativity value of 1 in Pauling scale). In the structure map, this covalent character is predicted to be manifest‐

Sr2+ ion, which is larger than Ca2+, is ordered almost completely into the smaller Ca(2) site in the apatite structure (**Fig. 4**). The bond valence sums of Sr ions at two sites demonstrate that Sr is severely overbonded at apatite Ca site but less at Ca(2) site. Complete ordering of Sr into Ca(2) sites has important implications for the diffusion of that element in the apatite struc‐ ture. It is the subject of several recent studies. The diffusion of Sr in (001) was shown to be as rapid or even more rapid than the diffusion parallel to [001]. As there are neither sites available for Sr, which are linked in (001), nor any interstitial sites, which can contain Sr2+ ion, the

A series of Sr-substituted hydroxyapatites, (SrxCa1−x)5(PO4)3OH, where *x* = 0.00, 0.25, 0.50, 0.75 and 1.00, was investigated by O'DONNELL et al [25]. The lattice parameters (a and c), the unit cell volume and the density were shown to increase linearly with strontium addition and were consistent with the addition of slightly larger and heavier ion (Sr) instead of Ca. There was a

diffusion mechanism involving the vacancies or defects or both is indicated [24].

slight preference for strontium to enter Ca(2) site in mixed apatites.

X V, Cr, As Z Cl

> Magnesium-substituted hydroxyapatite (MgHAP) powders with different crystallinity levels, prepared at room temperature via a heterogeneous reaction between Mg(OH)2/Ca(OH)2 powders and (NH4)2HPO4 solution using the mechanochemical- hydrothermal route, were reported by SUCHANEK et al [27]. The as-prepared products contained unreacted Mg(OH)2 and therefore had to undergo the purification in ammonium citrate aqueous solutions at room temperature. MgHAP contained 0.24 - 28.4 wt.% of Mg and the concentration of Mg was slightly lower near the surface than that in the bulk.

> Two effects of different magnesium sources (magnesium nitrate and magnesium stearate) on the synthesis of Mg-substituted hydroxyapatite (Mg-n-HAP) nanoparticles by the co-precipi‐ tation method were investigated by LIJUAN et al [28]. There was no obvious difference of morphology, nanoparticle size and thermal stability between those two Mg-n-HAPs. However, Mg-n-HAP synthesized by magnesium stearate had lower crystallinity and better dispersibility, suggesting that magnesium stearate was a novel magnesium source to synthe‐ size Mg-n-HAP, which can effectively reduce the powder crystallinity and prevent the aggregation of Mg-n-HAP nanoparticles, owing to the introduction of organic magnesium source, so as to obtain a promising candidate material to prepare Mg-n-HAP/polymer composite used in a variety of bone applications.
