Triclinic apatites

Apatites commonly adopt P6(3)/m hexagonal symmetry. More rarely, monoclinic chemical analogues have been recognized, including the biologically significant hydroxyapatite, Ca-10(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 Ca-10(AsO4)(6)F-2 and Ca-10(VO4)(6)F-2 show these materials are triclinic P $(1) over bar $ 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 La-10 - x(GeO4)(6)O3 - 1.5x and Ca-10(PO4)(6)(OH)(2 - x)O-x/2 confirms this scheme of tetrahedral rotations, while Cd-10(PO4)(6)F-2 and Ca-10(CrO4)(6)F-2 are predicted to be isostructural. These distortions are in contrast to the better known P112(1)/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.