Analysis of the EXAFS spectrum of hydroxyapatite

An analysis of the EXAFS spectrum of hydroxyapatite, Ca 5 (PO 4 ) 3 OH, is described using a spherical-wave theory. Structural information on the local atomic environment surrounding the calcium ion is presented out to a distance of 0.6 nm. Phase shifts were calculated using the ab initio method described previously. Local atomic potentials were constructed using the muffin-tin approximation, choosing a local atomic arrangement consistent with the known crystal structure of hydroxyapatite. The accuracy of the phase shifts and the calculation procedure were checked using the EXAFS spectrum of calcium oxide. X-ray crystallography provided the initial EXAFS parameters used in simulating the spectrum of hydroxyapatite. The inter-atomic distance, r j , and the Debye-Waller terms, sigma j 2 , were refined to obtain the best agreement with experiment. To analyse the EXAFS fully, multiple scattering contributions needed to be included.