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 (⁸S_7/2) of the Eu²⁺ ions and ⁵D₀-⁷F_J 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.