Colour centres in plasma-sprayed hydroxyapatite.
Very pure hydroxyapatite is diamagnetic and shows no electron paramagnetic resonance (EPR) spectrum. Plasma-sprayed coatings made with such a pure hydroxyapatite contain paramagnetic point-defects. Irradiated deposits show a strong EPR absorption assigned to an O2- anion adjacent to a calcium vacancy, with anisotropic parameters, g1 = 2.004, g2 = 2.013 and g3 = 2.038. The EPR spectra and colour fade away following thermal annealing. These features are discussed in terms of point-defects in the hydroxyapatite crystal structure.
Relationship between the colour change of hydroxyapatite and the trace element manganese.
Hydroxyapatite containing the trace element manganese turns blue after sintering at high temperature in an oxidizing atmosphere. This phenomenon is related to the oxidation of manganese ions in the special crystal structure of hydroxyapatite. Combination of the crystal structure with electron spin resonance spectra gives an explanation for this oxidation of Mn2+ ions at Ca(l) sites into Mn5+ ions and the formation of MnO4(3-) ions at PO4(3-) sites, corresponding to the colour change of hydroxyapatite.
SR topography of plastically deformed Fe-Si bicrystals - observation in situ and post mortem
SR reflection topography was applied to study the interaction of slip dislocations with the grain boundaries in Fe-4at%Si bicrystals deformed by compression. Three experimental settings with either a white or monochromatic beam were used and the corresponding orientation contrast analysed. Dislocation transmission through the grain boundary was observed in bicrystals with favourable geometry of the slip systems.