**4.10.2 Bismuth calcium oxyapatites**

The synthesis, the characterization and ionic conductivity of Ca8−xSrxBi2(PO4)6O2 phase where *x* = 3, 4 and 5 was reported by TRABELSI et al [214]. Sr2+ ions were noted to occupy two sites (4*f*) and (6*h*), with a strong preference for (6*h*) sites. Heavy Bi3+ atoms preferentially occupied the (6*h*) site.

New bismuth calcium silicon oxide Ca4Bi4.3(SiO4)(HSiO4)5O0.95, with the apatite structure was synthesized by UVAROV et al [215]. The structure was refined from the powder X-ray diffrac‐ tion data. The refinement revealed that the phase had the P63/M space group with the unit cell parameters *a* = 9.6090 Å, *c* = 7.0521 Å, *V* = 563.9 Å3 and *c*:*a* = 0.734.

Also the structure of bismuth calcium vanadium oxide (BiCa4V3O13, BiCa4(VO4)3O) was reported by HUANG and SLEIGHT [216] as apatite without an inversion center. The phase crystallizes in hexagonal system with the space group P63, *a* = 9.819 Å, *b* = 7.033 Å, *V* = 587.2 Å3 and*Z* = 2. Calculated density is 4.129 g·cm−3. Lower symmetry ofthe structure may be related to the site selective distribution of Bi atoms at the Ca sites.
