Synthesis, structure, luminescence, and color features of the Eu- and Cu-doped calcium apatite

Europium-substituted copper-doped calcium hydroxyapatites with common chemical composition Ca10-xEux(PO4)6O2H2-x-y-δCuy (x = 0.23-0.42; y = 0-0.27) were synthesized via solid state reaction. Obtained materials were analyzed by powder XRD, UV-Vis, Raman, luminescence spectroscopy, and colorimetry. Crystal structure characterization (including unit cell parameters, atomic positions, and position occupancies) was provided using the Rietveld refinement method in the P63/m space group. It was established that europium ions occupied the Ca(2) position only and the copper ions were located at the (0,0,0) position inside the hexagonal channel. Europium-substituted copper-doped apatites exhibit light pink color in comparison with the bright red-violet europium-free copper-doped calcium hydroxyapatite. Adversely to the europium-free samples europium-substituted, samples show the visible orange-red luminescence. Versatile analysis of the obtained materials showed that they are promising to be used as the low-toxic inorganic pigments with additional luminescent properties.