Structural studies of a carbonate fluorapatite from powder neutron diffraction data

Neutron powder diffraction data from 295 K to 10 K of a natural carbonate fluorapatite (francolite) were refined to examine static versus dynamic disorder as revealed by the temperature dependence of the atomic displacement parameters (ADP). The refined P-O1 and P-O2 bond lengths are correspondingly approximately 4% and 6% shorter than the P-O3 bonds. The deformation of the P-O tetrahedron can be explained by replacement of the larger P⁺⁵ ion by the smaller C⁺⁴ ion. As compared to fluorapatite and hydroxyapatite without carbonate the ADP's in francolite for the tetrahedral and F sites are significantly larger and the Ca2, O1 and O3 sites are slightly larger. The temperature dependence of the ADP's indicate enhanced static disorder associated with the F, O3, and tetrahedral sites.