Emission spectra of Cs 2NaTbCl 6 and Cs 2NaYCl 6: Tb 3+

Abstract

The emission spectra of microcrystalline Cs 2NaTbCl 6 and Cs 2Na(Y 0.99Tb 0.01)Cl 6 have been measured at room temperature and at 77 K. The crystal structures of these compounds are face-centered cubic and the terbium (III) ions lie at sites of octahedral (O h) symmetry surrounded by six chloride ions. Emission is observed from both the 5D 3 and 5D 4 excited states of Tb 3+. Assignments have been made for nearly all of the magnetic-dipole transitions split out of the Tb 3+ 7F 6, 7F 5, 7F 4, 7F 3, 7F 2, 7F 1 ← 5D 4 and 7F 4, 7F 2 ← 5D 3 transitions. These assignments are based on the calculated transition energies and relative magnetic-dipole strengths and intensities obtained from a weak-field crystal-field analysis of octahedral TbCl 6 3− units. Magnetic-dipole lines dominate the spectra for transitions of Δ J = ±1 free-ion parentage, whereas both magnetic-dipole lines and vibronically induced electric-dipole lines contribute significantly to the emission intensities of the Δ J = 0, ±2 transitions. The crystal-field sub-levels of both 5D 3 and 5D 4 appear to reach a Boltzmann thermal equilibrium prior to emission. Emission from 5D 3 is partially quenched in going from low temperature to high temperature and in going from Cs 2NaYCl 6: Tb 3+ (1%) to Cs 2NaTbCl 6. This study has led to the identification and assignment of nearly all of the pure magnetic-dipole transitions split out of the Tb 3+ 7F 6, 7F 5, 7F 4, 7F 3, 7F 2, 7F 1 ← 5D 4 and 7F 4, 7F 2 ← 5D 3 transitions in crystal-line Cs 2NaTbCl 6. The assignments were based on calculated transition energies and relative magnetic-dipole strengths (and intensities) obtained from a (weak-field) crystal-field analysis of octahedral (O h) TbCl 6 3− clusters. Excellent agreement between the calculated and observed relative intensities of the magnetic-dipole lines was achieved by assuming a Boltzmann equilibrated set of crystal-field sub-levels for both the 5D 4 and 5D 3 emitting states. Furthermore, the experimental results suggest that 5D 4 ← 5D 3 relaxation is temperature-dependent. The energy levels calculated and displayed in table 1 appear to be qualitatively correct and are in semiquantitative agreement with the emission results (as interpreted in section 4). Calculated and observed transition energies for the assigned magnetic-dipole transitions generally agree to within 0.2%. One of the most remarkable features of the emission spectra obtained on Cs 2NaTbCl 6 is the absence of any vibrational structure in the Δ J = ± 1 transitions ( 7F 6, 7F 3 ← 5D 4 and 7F 4, 7F 2 ← 5D 3), and the presence of extensive vibrational structure in the Δ J = O, ±2 transitions ( 7F 6, 7F 4, 7F 2 ← 5D 4). If other than OO vibronic transitions do contribute to the Δ J = ±1 emissions, their intensities must be at least two or three orders-of-magnitude weaker than the OO magnetic-dipole lines. Vibronically induced electric-dipole transitions appear, however, to make substantial contributions to the 7F 6, 7F 4, 7F 2 ← 5D 4 emission spectra. A clear-cut theoretical explanation for the absence of vibrational structure in the Δ J = ±1 transitions is not readily apparent. We are presently examining this problem in greater detail.