Crystal-field analysis for RE 3+ ions in laser materials: I. Absorption spectra and energy levels calculations for Nd 3+ and Pr 3+ ions in ABCO 4 crystals

Abstract

Low temperature polarized absorption spectra are analyzed to achieve assignments of energy levels for Nd 3+ ions at tetragonal C 4v symmetry sites in SrLaAlO 4:Nd 3+, SrLaGaO 4:Nd 3+, and CaNdAlO 4 as well as Pr 3+ ions at C 4v sites in SrLaAlO 4:Pr 3+ crystals. The C 4v selection rules for polarized electric dipole transitions are strictly obeyed for Pr 3+ ion doped into SrLaAlO 4. However, for Nd 3+ ions in the same crystal neither the C 4v polarization rules nor the C 4 ones are strictly confirmed. This is explained in terms of structural disorder. The experimental energy levels for Pr 3+ and Nd 3+ ions in these crystals were analyzed in terms of the free-ion parameters and the crystal field (CF) ones, B kq , assuming C 4v site symmetry and using the starting values of B kq ’s obtained from superposition model (SPM) analysis. A good agreement between the calculated and experimental energy levels was obtained with rms deviations in the range from 20.5 to 23.6 cm −1 for Nd 3+-doped crystals and 19.8 cm −1 for SrLaAlO 4:Pr 3+. Using the CF parameters B kq optimized in fittings of the calculated and experimental energy levels, the model parameters, i.e. the intrinsic parameters B ¯ k and the power law exponents t k , were determined and compared with those for other similar systems hitherto studied. The obtained values of the intrinsic parameters may serve as a starting point for SPM analysis of CF parameters for other systems, particularly those exhibiting low symmetry of metal centers. As an example of such systems, we consider Nd 3+ ions at triclinic C 1 symmetry sites in BaLaAlO 4:Nd 3+ crystals. Low temperature polarized absorption spectra for this system is also analyzed yielding assignments of energy levels. The results of preliminary low symmetry CF analysis carried out for BaLaAlO 4:Nd 3+ are presented.