Rietveld structure refinement of precipitated carbonate apatite using neutron diffraction data

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 (CO3 content 12.5(7) wt%). A structural model based on Holly Springs hydroxyapatite without CO32− ions showed an apparent reduction in the PO4 tetrahedral volume which is ascribed to CO32− replacing PO43− ions in the lattice. Four structural models from the literature with the CO32− ion explicitly modelled were fitted to the neutron diffraction data by the Rietveld method. The best fit was obtained with the CO32− ion in disorder between the mirror symmetry related faces of a vacant PO43− site and with the normal to the plane of the CO32− ion at 30° 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) Å and c=6.9199(4) Å.