Powder diffraction studies of synthetic calcium and lead apatites

The crystal structures of M-10(PO4)(6)X-2, where M = Ca or Pb and X = OH , F , Cl or Br , have been determined by Rietveld refinement of powder synchrotron X-ray and neutron diffraction data. All the compounds are hexagonal with space group P63/m. For the calcium compounds, the lattice parameters are a = 9.4302(5), 9.3475(3), 9.5902(6), 9.6482(6) and c = 6.8911(2), 6.8646(1), 6.7666(2), 6.7788(2) Angstrom, for X = OH , F -, Cl -, Br -, respectively. For the lead compounds, the corresponding lattice parameters are a = 9.8612( 4), 9.7547(5), 9.9767(4), 10.0618(3) and c = 7.4242(2), 7.2832(2), 7.3255(1), 7.3592(1) Angstrom. In these compounds there are two cation sites, a channel of M(I) atoms and a triangle of M(II) atoms. The anion interacts most strongly with the M(II) atoms with the interaction dictating the position of the anion relative to the M(II) triangle. In Ca-10(PO4)(6)X-2, the F ion sits within the triangle planes, while the larger OH - and Cl - anions are disordered above and below the M(II) triangles. The even larger Br is midway between two triangles at (0, 0, ). Despite the larger size of the isostructural lead compounds, no anions are found in the triangles. The F -, Cl - and Br - ions are at (0, 0, ) and the OH - ion is disordered at (0, 0, z). This difference in behaviour is possibly related to the lead 6s electrons. In this paper, the experimental results are presented and possible reasons for the observed differences are discussed.