Optical absorption spectra and crystal field analysis of Er3+in the cubic Cs2NaErCl6system

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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 (Oh) 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.