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

A systematic behavior in the crystal structure parameters of natural, synth etic, carbonate, and non-carbonate apatite is revealed from Rielveld refine ments of neutron powder diffraction experiments. The results of this work o n synthetic carbonate hydroxyapatites (CHAps) are consistent with the mecha nism of carbonate substitution on the mirror plane of the phosphate tetrahe dron, as it was introduced for the natural carbonate fluorapatite (CFAp). T he present comparison shows that the tetrahedral bond lengths P-O1 and P-O2 decrease by 3-4% in all carbonate apatites. The atomic displacement parame ters (ADPs) of the tetrahedral (T) and the O3 sites are greater in the carb onate 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 at 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 corres ponding non-carbonate materials.