Energy levels and crystal field parameters for Nd3+ ions in BaY2F8, LiKYF5, and K2YF5 single crystals

The available experimental energy levels of Nd3+ ions doped into single crystals of BaY2F8, LiKYF5, and K2YF5, 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, Bkq, 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 C2 site symmetry to higher orthorhombic D2 and tetragonal D4 symmetry used in CF calculations for BaY2F8. Similarly, for triclinic C1 site symmetry in LiKYF5 and K2YF5 approximation to monoclinic C2 and orthorhombic D2 symmetry for LiKYF5, whereas the monoclinic Cs symmetry for K2YF5, are considered. It is shown that the C2v approximation used previously for K2YF5:Nd3+ is not suitable. SPM enables to calculate for the unapproximated and idealized polyhedrons YF8 in a given ion–host system of the combined coordination factors Sgk,q expressed in the modified crystallographic axis system CAS* and approximated symmetry adapted axis systems, respectively. The quantities Sgk,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 BaY2F8:Nd3+. The present considerations enable also clarification of some ambiguous points occurring in previous studies.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► The experimental energy levels of Nd3+ ions at low site symmetry are reanalyzed. ► A combined approach is utilized for analysis of experimental spectra. ► The improved and more reliable sets of crystal-field parameters are generated. ► The low symmetry aspects inherent in the CF Hamiltonian are elucidated.