title: Geometrical parameterization of the crystal chemistry of P6(3)/m apatites: comparison with experimental data and ab initio results.
abstract: Experimental structure refinements and ab initio simulation results for 18 published, fully ordered P6(3)/m (A;{\rm I}_4)(A;{\rm II}_6)(BO4)6X2 apatite end-member compositions have been analyzed in terms of a geometric crystal-chemical model that allows the prediction of unit-cell parameters (a and c) and all atom coordinates. To an accuracy of +/- 0.025 A, the magnitude of c was reproduced from crystal-chemical parameters characterizing chains of ...-A(II)-O3-B-O3-A(II)-... atoms, whereas that of a was determined from those describing (A(I)O6)-(BO4) polyhedral arrangements. The c/a ratio could be predicted to +/-0.2% using multi-variable functions based on geometric crystal-chemical model predictions, but could not be ascribed to the adjustment of a single crystal-chemical parameter. The correlations observed between algebraically independent crystal-chemical parameters representing the main observed polyhedral distortions reveal them as the minimum-energy solution to accommodate misfit components within this flexible structure type. For materials with given composition, good agreement (within +/- 0.5-2.0%) of ab initio crystal-chemical parameters was observed with only those from single-crystal refinements with R </= 4.0%. Agreement with single-crystal work with R > 4.0% was not as good, while the scatter with those from Rietveld refinements was considerable. Accordingly, ab initio cell data, atomic coordinates and crystal-chemical parameters were reported here for the following compositions awaiting experimental work: (Zn,Hg)10(PO4)6(Cl,F)2, (Ca,Cd)10(VO4)6Cl2 and (Ca,Pb,Cd)10(CrO4)6Cl2.
authors.affiliation: Institute for Chemical Process and Environmental Technology (ICPET), National Research Council of Canada, Ottawa, Ontario, Canada K1A 0R6.
authors.forename: Patrick H J
authors.initials: PH
authors.lastname: Mercier
authors.forename: Yvon
authors.initials: Y
authors.lastname: Le Page
authors.forename: Pamela S
authors.initials: PS
authors.lastname: Whitfield
authors.forename: Lyndon D
authors.initials: LD
authors.lastname: Mitchell
authors.forename: Isobel J
authors.initials: IJ
authors.lastname: Davidson
authors.forename: T J
authors.initials: TJ
authors.lastname: White
mesh_terms: D001031:Apatites / Q000737:chemistry*; D003460:Crystallization; D008958:Models, Molecular; D015394:Molecular Structure
chemical_list: D001031:Apatites