Acta Cryst. (2007). B63, 251-256 [ doi:10.1107/S0108768106053316 ]
Abstract: Apatites commonly adopt P63/m hexagonal symmetry. More rarely, monoclinic chemical analogues have been recognized, including the biologically significant hydroxyapatite, Ca10(PO4)6(OH)2, but the driving force towards lower symmetry has not been systematically examined. A combination of diffraction observations and ab initio calculations for Ca10(AsO4)6F2 and Ca10(VO4)6F2 show these materials are triclinic 
apatites in which the AsO4 and VO4 tetrahedra tilt to relieve stress at the metal and metalloid sites to yield reasonable bond-valence sums. An analysis of the triclinic non-stoichiometric apatites La10 - x(GeO4)6O3 - 1.5x and Ca10(PO4)6(OH)2 - xOx/2 confirms this scheme of tetrahedral rotations, while Cd10(PO4)6F2 and Ca10(CrO4)6F2 are predicted to be isostructural. These distortions are in contrast to the better known P1121/b monoclinic dimorphs of chloroapatite and hydroxyapatite, where the impetus for symmetry reduction is ordered anion (OH- and Cl-) displacements which are necessary to obtain acceptable bond lengths. These results are important for designing apatites with specific structural and crystal-chemical characteristics.
Keywords: apatites; symmetry; neutron powder diffraction; bond-valence sum.
 |   Structure factor file (CIF format) (61.6 kbytes) |
| Structure factor file (CIF format) (62.2 kbytes) |
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