Energy levels and crystal field parameters for Nd 3+ ions in BaY 2F 8, LiKYF 5, and K 2YF 5 single crystals

Abstract

The available experimental energy levels of Nd 3+ ions doped into single crystals of BaY 2F 8, LiKYF 5, and K 2YF 5, which exhibit low site symmetry, are reanalyzed. A combined approach based on the ascent/descent in symmetry (ADS) method, the superposition model (SPM) analysis, and the pseudosymmetry axes method (PAM) is utilized to extract the crystal field (CF) parameters, B kq , from experimental spectra. Corresponding sets of the free-ion parameters are also fitted. The crystallographic data are used to establish the axis systems most appropriate for approximation of the actual monoclinic C 2 site symmetry to higher orthorhombic D 2 and tetragonal D 4 symmetry used in CF calculations for BaY 2F 8. Similarly, for triclinic C 1 site symmetry in LiKYF 5 and K 2YF 5 approximation to monoclinic C 2 and orthorhombic D 2 symmetry for LiKYF 5, whereas the monoclinic C s symmetry for K 2YF 5, are considered. It is shown that the C 2v approximation used previously for K 2YF 5:Nd 3+ is not suitable. SPM enables to calculate for the unapproximated and idealized polyhedrons YF 8 in a given ion–host system of the combined coordination factors Sg k,q expressed in the modified crystallographic axis system CAS* and approximated symmetry adapted axis systems, respectively. The quantities Sg k,q serve as input for PAM calculations for independent determination of the axis system appropriate for higher symmetry approximations. The pseudosymmetry axes represent the axis system that reflects most closely the approximated higher symmetry of the nearest ligands in a paramagnetic complex embodied in the 4th-rank CF parameters. The combined ADS/SPM/PAM approach provides sets of starting CF parameters (CFPs) in well-defined axis systems. Multiple fittings starting from different points in the CF parameter space yield converging solutions, thus increasing the reliability of the final optimized solutions, which may be then considered as the global minima. The present CF analysis, unlike the previous ones, takes into account the low symmetry aspects inherent in the CF Hamiltonian and optical spectra. The improved and more reliable sets of CFPs are generated as well as modified assignments of the CF energy levels are proposed for BaY 2F 8:Nd 3+. The present considerations enable also clarification of some ambiguous points occurring in previous studies.