Magnetic circularly polarized emission and magnetic circular dichroism study of the ⁷F_J↔⁵D₄ transitions in crystalline Cs₂NaTbCl₆

Abstract

Absorption and magnetic circular dichroism (MCD) spectra are reported for the ⁷ F ₆→⁵ D ₄ transition of Tb³⁺ in crystalline Cs₂NaTbCl₆. These spectra are interpreted in terms of a crystal-field analysis of Tb³⁺ situated in an octahedral site with six Cl^- nearest neighbours. The extensive, well-resolved vibrational structure in the absorption/MCD spectra is assigned, and these assignments (based on Γ point vibrational modes) are in agreement with previous assignments made for similar elpasolite-hexachloride systems. Total emission and magnetic circularly polarized emission (MCPE) spectra are also reported for the ⁷ F ₆, ⁷ F ₅, ⁷ F ₄, and ⁷ F ₃←⁵ D ₄ transitions of Tb³⁺ in crystalline Cs₂NaTbCl₆. These spectra are interpreted in terms of a crystal-field analysis of Tb³⁺ ions located at octahedral sites in the crystal system. All crystal-field calculations were carried out using a six-parameter weak-field model, and an optimum fit of the spectroscopic observables was achieved with the following parameter set: ζ = 1840 cm^-1, F ₂ = 417 cm^-1, F ₄ = 57·60 cm^-1, F ₆ = 6·30 cm^-1, A ₄ = + 1900 cm^-1, and A ₆ = - 200 cm^-1. In emission, a large number of magnetic-dipole transitions between (octahedral) crystal-field sub-levels of the various ⁷ F _J (J = 6, 5, 4, and 3) and ⁵ D ₄ states were observed and assigned. The transitions of ΔJ = odd free-ion parentage consisted entirely of pure magnetic-dipole lines with no appearance of vibronically induced electric-dipole components. On the other hand, transitions of ΔJ = even free-ion parentage exhibited both pure magnetic-dipole lines and vibronically induced electric-dipole components with extensive vibrational structure.