title: Rietveld structure refinement of precipitated carbonate apatite using neutron diffraction data.
abstract: X-ray and time of flight neutron diffraction data, FTIR and MAS-NMR spectra, and ICP-AES and carbonate analyses have been collected from a sodium-containing carbonate apatite (CO(3) content 12.5(7)wt%). A structural model based on Holly Springs hydroxyapatite without CO(3)(2-) ions showed an apparent reduction in the PO(4) tetrahedral volume which is ascribed to CO(3)(2-) replacing PO(4)(3-) ions in the lattice. Four structural models from the literature with the CO(3)(2-) ion explicitly modelled were fitted to the neutron diffraction data by the Rietveld method. The best fit was obtained with the CO(3)(2-) ion in disorder between the mirror symmetry related faces of a vacant PO(4)(3-) site and with the normal to the plane of the CO(3)(2-) ion at approximately 30 degrees to the c-axis. This angle is consistent with results from previous polarised IR measurements on single crystals of francolite (a fluorocarbonate apatite) and human dental enamel. Further refinement of the model revealed a hitherto unknown atom site close to the unit cell origin, (possibly a water molecule). The refined hexagonal unit cell parameters from the neutron diffraction data were a=9.3446(3)A and c=6.9199(4)A.
authors.affiliation: Dental Biophysics, Medical Sciences Building, Queen Mary, University of London, Mile End Road, London, UK E1 4NS. r.m.wilson@qmul.ac.uk
authors.forename: Rory M
authors.initials: RM
authors.lastname: Wilson
authors.forename: James C
authors.initials: JC
authors.lastname: Elliott
authors.forename: Stephanie E P
authors.initials: SE
authors.lastname: Dowker
authors.forename: Ron I
authors.initials: RI
authors.lastname: Smith
mesh_terms: D001031:Apatites / Q000032:analysis / Q000737:chemistry*; D001672:Biocompatible Materials / Q000737:chemistry*; D002254:Carbonates / Q000737:chemistry; D011232:Chemical Precipitation; D003198:Computer Simulation; D003461:Crystallography; D008422:Materials Testing / Q000379:methods*; D008956:Models, Chemical; D008958:Models, Molecular; D008968:Molecular Conformation; D033363:Neutron Diffraction / Q000379:methods*
chemical_list: D001031:Apatites; D001672:Biocompatible Materials; D002254:Carbonates; C030782:carboapatite