Interconfigurational and intraconfigurational transitions of Yb2+ and Yb3+ ions in hydroxyapatite: A cathodoluminescence study

Abstract We present a cathodoluminescence (CL) study of the interconfigurational transitions of Yb 2+ and intraconfigurational transitions of Yb 3+ dopant ions in hydroxyapatite (HAp) powders. Our results demonstrate that strong electric fields present in the HAp lattice induce manifold levels in the electronic configuration of the ytterbium ions, due to splitting of the 4 f 13 5 d 1 orbitals. CL spectra of Yb 2+ display a series of sharp peaks centered at 3.27, 2.98, 2.85, 2.53, 2.27, 2.09 and 1.63 eV, corresponding to transitions between multiple levels produced by a trigonal distortion of the regular octahedral crystal field of the Yb 2+ 4 f 13 5 d 1 configuration. CL spectra of Yb 3+ display four emissions centered at 1.17, 1.21, 1.24 and 1.27 eV, generated by intraconfigurational transitions between the 2 F 5/2 and 2 F 7/2 states of Yb 3+ ions. Two types of samples were synthesized at different pH values, resulting in variations of the valence of the ytterbium ions, such that the ratios of Yb 2+ /Yb 3+ were 0.31 and 0.55 in calcined powders synthesized at pH values of 6 and 4, respectively, as determined from X-ray photoelectron spectroscopy. Infrared CL images of the two samples show an inhomogeneous spatial distribution of the Yb 3+ dopant in the powders. Thermal treatment of the samples, at 873 K in an oxygen atmosphere, result in quenching of the Yb 2+ luminescence due to oxidation of the Yb 2+ into Yb 3+ .