Atomic positional disorder of a single-phase natural carbonate fluorapatite (francolite) is revealed from analysis of the atomic displacement parameters (ADPs) refined from neutron powder diffraction data as a function of temperature and carbonate content. The ADPs of the francolite show a strong disturbance at the P, O3, and F sites. When it is heat treated to partially or completely remove the carbonate, the ADPs as well as the other structural parameters resemble those of a fluorapatite (Harding pegmatite) that was measured under the same conditions. The various structural changes are consistent with a substitution mechanism whereby the planar carbonate group replaces a phosphate group and lies on the mirror plane of the apatite structure.
We present a comparison between detailed structural parameters of a low temperature synthetic carbonate hydroxyapatite, a hydroxyapatite, and a natural carbonate fluorapatite from Rietveld refinements of neutron powder diffraction data. The observed modifications in the crystallographic parameters of the synthetic carbonate apatite are consistent with the carbonate substitution on the mirror plane of the tetrahedral site as in the case of the natural carbonate apatite.
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