Optical absorption spectra and crystal field analysis of Er³⁺ in the cubic Cs₂NaErCl₆ system

Abstract

Optical absorption spectra are reported for the ⁴ I _15/2 → ⁴ I _13/2, ⁴ I _11/2, ⁴ F _9/2, ⁴ S _3/2, ² H _11/2, ⁴ F _7/2, ⁴ F _5/2, ⁴ F _3/2, ² G _9/2 and ⁴ G _11/2 transition regions of Er³⁺ in the cubic Cs₂NaErCl₆ system. These spectra, obtained under variable temperature conditions (2·6 K to 300 K), are analysed on the basis of a crystal field model in which the Er³⁺ ions are assumed to reside at sites of exact octahedral (O_h) symmetry. This crystal field model is used to calculate energy levels and to calculate the magnetic dipole strengths and intensities of crystal field transition origin lines. Vibronically-induced electric dipole strengths are also calculated using a model in which the ErCl₆ ^3- clusters are treated as isolated chromophoric units (uncoupled to the lattice). This latter model allows calculation of electric dipole intensities for the vibronic lines associated with the v ₃(t _1u), v ₄(t _1u) and v ₆(t _2u) vibrational modes of the ErCl₆ ^3- cluster (the so-called moiety modes). Detailed analysis of the spectra leads to the location and assignment of all crystal field levels split out of the ⁴ I _13/2, ⁴ I _11/2, ⁴ F _9/2, ⁴ S _3/2, ² H _11/2, ⁴ F _7/2, ⁴ F _5/2, ⁴ F _3/2, ² G _9/2 and ⁴ G _11/2 intermediate-coupling states, and of the three lowest-lying crystal field levels split out of the ⁴ I _15/2 ground state. The best fit between the calculated and observed crystal field energy level schemes was achieved using the values of crystal field coefficients B ₀ ⁽⁴⁾ = 1608 cm^-1 and B ₀ ⁽⁶⁾ = -195 cm^-1 (defined according to the unit-tensor formalism). The most intense transitions were observed (and calculated) to be ⁴ I _15/2 → ⁴ G _11/2 > ⁴ I _13/2 > ² H _11/2. The ⁴ I _15/2 → ⁴ I _13/2 transition region is entirely dominated by intense magnetic dipole origin lines, whereas nearly all of the intensity in the ⁴ I _15/2 → ⁴ G _11/2 and ² H _11/2 transition regions is found in vibronic lines associated with the v ₆(85–90 cm^-1), v ₄(107–114 cm^-1) and v ₃(245–260 cm^-1) ErCl₆ ^3- moiety modes.