Triclinic apatites

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 P\bar 1 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 − x O x/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.