Distribution of Eu2+ and Eu3+ Ions in Hydroxyapatite: A Cathodoluminescence and Raman Study.

We present a cathodoluminescence study of the spatial distribution of Eu2+ and Eu3+ dopants in hydroxyapatite powders. The results demonstrate that the distribution of europium ions in the hydroxyapatite lattice depends on their valence state. Monochromatic cathodoluminescence images from prismatic powders show that although the Eu2+ is distributed homogeneously in the entire powder volume, the Eu3+ is present mainly at the powder edges. The luminescence spectrum of the Eu2+ ions displayed a wide and strong blue emission centered at 420 nm, while the luminescence spectrum of the Eu3+ ions displayed several orange-red emissions covering the range from 575 to 725 nm. These emissions correspond to transitions between levels 4f65d1-4f7 (8S7/2) of the Eu2+ ions and 5D0-7FJ levels of the Eu3+ ions. Micro Raman measurements reveal that europium doping generates two phonon signals with frequencies of 555 and 660 cm-1, both of which have not been reported earlier. The powders were synthesized by the combustion synthesis method, maintaining constant the concentration of the europium salt used, and varying the pH of the precursor solutions to modify the concentration ratio of Eu2+ with respect to Eu3+. X-ray photoelectron spectroscopy measurements were used to determine values of 0.32 and 0.55 for the ratio Eu2+/Eu3+ in samples synthesized at pH values of 6 and 4, respectively. Thermal treatments of the samples, at 873 K in an oxygen atmosphere, resulted in a strong quenching of the Eu2+ luminescence due to oxidation of the Eu2+ ions into Eu3+, as well as probable elimination of calcium vacancy defects by annealing.