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.