STRUCTURAL DISORDER IN HYDROXYAPATITE

Four mineral apatites (three samples of Totenkopf-hydroxyapatite, one sample of Durango-fluorapatite) and a synthetic fluorapatite were analysed as a function of temperature by X-ray and neutron single crystal diffraction mainly to quantify the disorder contribution to the atomic Debye Waller factor (DWF). The anisotropic displacement parameters (ADP's) obtained at RT for hydroxyapatites with different temperature treatment exhibit marked differences up to 75%. The ADP's of all atoms decrease systematically with increasing temperature treatment. Possible processes for such a global structure change are discussed. The anharmonic terms of the atomic DWF are not influenced by the temperature treatments. Especially the oxygen atoms of the PO4-groups show highly remarkable anharmonic contributions indicating preferred directions of displacements caused by the disorder of the channel ions. The resulting perturbation of local symmetry suggests that P63/m describes only a time/space average. As expected the fluorapatites with their ordered F arrangement show appreciable lower third and fourth order terms. The dichroitic behaviour of the OH-ions was determined up to the temperature of 500-degrees-C by single crystal IR-spectroscopy on Totenkopf-hydroxyapatite. In the studied temperature range the two IR-spectroscopically different OH-ions both maintain their orientation parallel to the channel axis. Neutron diffraction studies were performed on the mineral hydroxyapatite at several temperatures (25-degrees-C, 400-degrees-C and 800-degrees-C) to determine precisely the location of the OH-ion. At 800-degrees-C the position of the OH-ion along the channel is no longer well defined. The temperature dependency of the fourth order terms in the atomic DWF of the framework oxygen points to a subtle structural phase transition below 400-degrees-C.