Papanearchou, Nearchos I.

The problem of the carbonate substitution in bioapatites is studied via x-ray and neutron powder diffraction. A series of samples of low temperature carbonate hydroxyapatites with varying carbonate content was synthesized. The carbonate solubility in the hydroxyapatite system was studied with x-ray diffraction. The detailed crystal structure parameters of the synthetic carbonate hydroxyapatites were studied as a function of temperature and carbonate content from the analysis of Rietveld refinements of neutron powder diffraction data. The phosphate tetrahedron is distorted by the carbonate substitution. The tetrahedral bond lengths P-O1 and P-O2 decrease by 2--3%. The atomic displacement parameters (ADPs) of the tetrahedral P, O1, O2, and O3 sites reveal a static disorder in the carbonate hydroxyapatite specimens. The results of the present comparison are consistent with the mechanism of carbonate substitution on the mirror plane of the phosphate tetrahedron.

The effect of simulated body fluid (SBF) on the crystal structure and surface structure properties of Ferrimagnetic Bioglass Ceramics (FBC) as a function of the time of immersion in SBF was studied. The materials examined were prepared with a systematic variation of the composition of the oxides in the system [0.45(CaO, P2O5) (0.52-x)SiO2 xFe2O3 0.03Na2O] and the heat treatment temperature. X-ray powder diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive x-ray spectroscopy (EDX), neutron powder diffraction (NPD) and Rietveld refinement methods were used in our studies. The in vitro experiments demonstrated the bioactivity of the FBC through formation of hydroxyapatite (HAp). It varies with the composition of the FBC, annealing temperature, and time of immersion of the samples in SBF in a non-systematic way. SEM and EDX studies showed that pores appear on the surface of some specimens, while a surface layer of Si, P, and Ca partially covers the Fe, O dendrites of the original samples. Formation and size of pores was determined by the specific processing parameters of the samples. Quantitative phase analysis of the neutron diffraction patterns revealed a phase transition of the calcium phosphate from hexagonal to monoclinic, in weight percentages depended on the composition of the as-prepared FBC and time of immersion in SBF. The weight percentage of the ferrimagnetic phase [Fe3O4] is less affected by immersion in SBF. Inclusion of magnetic scattering of the neutrons in the Rietveld refinement process improved the quality of the fit between calculated and experimental diffraction patterns, resulting in a reduction of the weighted pattern residuals Rwp from 0.15 to 0.07.