Abstract
The structural disorder in calcium-deficient hydroxyapatite (Ca-def HAp) was investigated by extended X-ray absorption fine structure spectroscopy (EXAFS) and X-ray absorption near edge structure spectroscopy (XANES). Ca-def HAp was prepared by hydrolysis of α-tricalcium phosphate. The Ca/P molar ratio in Ca-def HAp was 1.54. In order to synthesize Ca-def HAp with various Ca/P molar ratios, a hydrothermal treatment in Ca(NO3)2 aqueous solution for Ca-def HAp was carried out. The Ca/P molar ratio in the sample obtained by the hydrothermal treatment increased linearly with increasing reaction time. By hydrothermal treatment for 13 days, the Ca/P molar ration in the sample increased up to 1.65 which almost stoichiometric ratio. Ultraviolet-Visible (UV-VIS) absorption at 280nm appeared in Ca-def HAp and the peak intensities decreased with increasing Ca/P molar ratio in Ca-def HAps. The UV-VIS absorption, therefore, could be identified as intrinsic peak due to Ca vacancies in the HAp lattice. The fact also would suggest that Ca ions in the solution were taken into the vacancies in the lattice during the hydrothermal treatments. Intensity of background subtracted and normalized XANES spectra of Ca-def HAp was weaker than that of stoichiometric HAp The reason would be why the distortions are introduced by loosing of structure resulting from deficiency of Ca ion in HAp lattice. From EXAFS analysis of both Ca-def HAp and stoichiometric HAp, it was revealed that the coordination number of Ca ion in Ca-def HAp was lower than stoichiometric HAp. In addition, the Ca-O distance in Ca-def HAp became to be slightly longer than that of stoichiometric HAp Therefore, it could be concluded that the structural disorder resulting from deficiency of Ca ions will caused the extension of Ca-O distance in HAp structure.
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