Large Ground-State and Excited-State Crystal Field Splitting of 8-fold-Coordinate Cm³⁺ in [Y(H₂O)₈]Cl₃·15-crown-5

The optical spectra of Cm3+ incorporated into the crystalline host structure of [Y(H2O)8]Cl3·15-crown-5 (1) is investigated by using laser spectroscopic methods at temperatures between 20 and 293 K. The coordination geometry of the [Y(H2O)8]3+ entity in 1 is a distorted bicapped trigonal prism with ∼C2 point symmetry, as confirmed by single-crystal X-ray diffraction at 200 K. The crystal-field splitting of the 8S‘7/2 ground state and the 6D‘7/2 and 6P‘5/2 excited states of the hydrated Cm3+ ion are measured by high-resolution fluorescence emission and excitation spectroscopy at various temperatures. The transitions between the ground state and the respective lowest crystal-field levels of the excited states exhibit narrow fluorescence lines, resolving the four crystal-field levels of the ground state as sharp, well-resolved lines at about 0, 10, 19, and 35 cm-1. The total splittings of the 6D‘7/2 and 6P‘5/2 states are 670 and 170 cm-1, respectively. Thermal population of the ground-state crystal-field levels is observed and quantified in the excitation spectra in the temperature range of 20−70 K. All spectroscopic results are consistent with the presence of one unique [Cm(H2O)8]3+ site. The ground-state splitting of Cm3+ in 1, 35 cm-1, is comparable to that of Cm3+ in solid ThO2, 36 cm-1, which shows the strongest crystal field for Cm3+ reported so far. For this reason the present results are different than the findings for Ln3+ aqua ions, which show rather weak crystal field strengths.