Comparison of crystal structure parameters of natural and synthetic apatites from neutron powder diffraction

A systematic behavior in the crystal structure parameters of natural, synthetic, carbonate, and non-carbonate apatite is revealed from Rietveld refinements of neutron powder diffraction experiments. The results of this work on synthetic carbonate hydroxyapatites (CHAps) are consistent with the mechanism of carbonate substitution on the mirror plane of the phosphate tetrahedron, as it was introduced for the natural carbonate fluorapatite (CFAp). The present comparison shows that the tetrahedral bond lengths P-O1 and P-O2 decrease by 3-4% in all carbonate apatites. The atomic displacement parameters (ADPs) of the tetrahedral (T) and the O3 sites are greater in the carbonate than in the non-carbonate apatites. The atomic positional disorder of the T site (P/C site) is greater in the CFAp than in the CHAps, while the opposite happens al the O3 sites. Finally, the room-temperature ADPs of all of the atoms in the CFAp and CHAps show the same behavior as in the corresponding non-carbonate materials.