Analysis of spectral data and comparative energy level parametrizations for Ln 3+ in cubic elpasolite crystals

Abstract

Comparative analyses have been made of the 4f n lanthanide ion (Ln 3+) energy level structures in a variety of cubic elpasolite crystals. The systems can be grouped into two sets, namely (i) Ln 3+ in the neat compounds: Cs 2NaLnCl 6 (LnCeYb, except for Nd, Pm and Sm), Cs 2LiErCl 6 Cs 2NaLnBr 6 (LnPr, Ho, Tm), Cs 2KPrF 6 and Rb 2NaEuF 6; and (ii) Ln 3+ diluted into elpasolite host crystals: Cs 2NaYCl 6:Ln 3+ (LnPr, Sm, Eu), Cs 2NaGdCl 6:Ln 3+ (LnHo, Nd) and Cs 2KYF 6:Eu 3+. New energy level data obtained from low temperature absorption, emission and electronic Raman studies are presented for these systems to provide more accurate and extensive data sets. The new energy level data were analysed in terms of a 4f n “free-ion” Hamiltonian and a parametric crystal field Hamiltonian model, with octahedral ( O h) site symmetry for the Ln 3+ ion. The empirical energy level data were fitted to the parametrized total Hamiltonian and the resulting “best-fit” parameter sets are presented and discussed. Examination of the parameter values obtained from least-squares fittings reveals significant differences in the crystal field perturbations experienced by the 4f electrons of Ln 3+ at octahedral sites: the fluoride ligands interact with the 4f electrons somewhat more strongly than do the bromide ligands. The phenomenological (fitted) crystal field parameters of Cs 2NaLnCl 6 do not exhibit a clear trend across the series of lanthanide elements, partly because of the uncertainty in determination due to the term dependence. The magnitude of the B k 0 ( k = 4,6) parameters follow different patterns in the LaCl 3:Ln 3+, LaF 3:Ln 3+ and LiYF 4:Ln 3+ systems. However, the magnitudes of the “free-ion” parameters resulting from free variation in the elpasolite system data fits exhibit clearer trends and the spin-orbit coupling constant is found to fit well to a second-order polynomial in atomic number. The standard deviations of the data fits as defined herein are between 0 and 43 cm −1, so that the energy level schemes of the Ln 3+ ions in elpasolite lattices are reasonably well reproduced.