Spectra and energy levels of Er3+(4f11) in NaBi(WO4)2

Absorption and fluorescence spectra of Er3+(4f(11)) in crystals of NaBi(WO4)(2) (NBW) are reported at temperatures between 15 K and room temperature. The absorption spectra include the details of the crystal-field splitting of 11 multiplet manifolds, L-2S+1(J) of Er3+(4f(11)), spanning the wavelength range between 350 nm and 1550 nm. The crystal-field splitting of the ground-state I-4(15/2), is obtained from an analysis of the fluorescence spectrum, S-4(3/2)-->I-4(15/2). Spectra are characterized by inhomogeneous broadening due to the disordered crystal structure in which different valency cations, Na+ and Bi3+, statistically fill the S-4 symmetry sites. The Er3+ ions likely replace the Bi3+ ions in these sites. A quasi-center model has been chosen to interpret the crystal-field splitting of each manifold, using D-2d rather than S-4 symmetry as the site for the rare-earth ion in the lattice. To test the feasibility of the model, the splitting of the energy levels of Nd3+ in NBW was carried out first and compared with experimental levels reported in literature. A least-squares fitting analysis between 26 calculated-to-observed energy (Stark) levels gave a root-mean-square (rms) deviation of 8 cm(-1) for the I-4(J) and F-4(3/2) multiplet manifolds of Nd3+ in NBW. Using the phenomenological lattice-sum parameters, A(nm), obtained from the analysis of the Nd3+ energy levels, we predicted an initial set of crystal-field parameters, B-nm, for Er3+. With only a modest fitting of the multiplet centroids, these B-nm predict the observed splitting in the Er3+ spectra remarkably well. In a fitting of the energy levels in which both the B-nm and centroids are allowed to vary, we obtained a rms deviation of 6 cm(-1) for 57 calculated-to-observed Stark levels. The results suggest that the quasi-center model has merit when used to calculate the crystal-field splitting of the energy levels of the trivalent rare-earth ions in crystal hosts having a disordered structure. (C) 2003 American Institute of Physics.