**6.1.1. Strontium-substituted apatites**

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

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

**Fig. 4.** The structure of natural Sr-bearing apatite refined by HUGHES et al [24] and viewed along the c-axis.

Strontium is often substituted for calcium in order to confer the radio-opacity in glasses used for dental cements, biocomposites and bioglass-ceramics. It can be concluded that strontium substitutes for calcium with little change in the glass structure as a result of their similar charge to size ratio. Glasses with low content of strontium nucleate in the bulk to form calcium apatite phase. Glasses with medium strontium content nucleate to mixed calcium-strontium apatite at the surface and glass fully substituted by strontium to strontium fluorapatite [26].
