Apatite

The problem of the carbonate substitution in bioapatites is studied via x-ray and neutron powder diffraction. A series of samples of low temperature carbonate hydroxyapatites with varying carbonate content was synthesized. The carbonate solubility in the hydroxyapatite system was studied with x-ray diffraction. The detailed crystal structure parameters of the synthetic carbonate hydroxyapatites were studied as a function of temperature and carbonate content from the analysis of Rietveld refinements of neutron powder diffraction data. The phosphate tetrahedron is distorted by the carbonate substitution. The tetrahedral bond lengths P-O1 and P-O2 decrease by 2--3%. The atomic displacement parameters (ADPs) of the tetrahedral P, O1, O2, and O3 sites reveal a static disorder in the carbonate hydroxyapatite specimens. The results of the present comparison are consistent with the mechanism of carbonate substitution on the mirror plane of the phosphate tetrahedron.

The mechanism of the carbonate substitution in the apatite structure is unraveled through analysis of Rietveld refinements of powder diffraction data from a single-phase natural carbonate fluorapatite (francolite). The refined values of the P-O bond lengths give a ~1.6% distortion of the phosphate tetrahedron caused by the carbonate substitution. The two bond lengths P-O1 and P-O2 and their corresponding angles on the mirror plane of the phosphate tetrahedron are mainly disturbed by this substitution. A static positional disorder, mainly of the atoms at the tetrahedral sites, is revealed from the temperature dependence of the atomic anisotropic displacement parameters. A model for the mechanism of the carbonate entrance in the apatite structure is proposed.