Rare-Earth Ions in the Alkali Halides. I. Emission Spectra ofSm2+-Vacancy Complex

Measurements have been made of the crystal-field splitting of the ${}_{}{}^7{}_{}{}^{}F_J^{}{}_{}{}^{}$ energy levels of the $4f^6$ configuration of the ${\mathrm{Sm}}^{2+}$ ion which is dissolved in a number of alkali-halide lattices. Crystal-field term assignments were made from the results of polarized-emission measurements. Observed crystal-field splittings are compared to those predicted from point-charge models. Contributions to the total crystal field are considered to include that due to the presence of a nearest-neighbor cation vacancy as well as that due to displacements, from normal lattice positions, of the nearest-neighbor halide ions and the ${\mathrm{Sm}}^{2+}$ ion. The splitting is shown to arise in part from a displacement of the ${\mathrm{Sm}}^{2+}$ and the cation vacancy toward each other. The extent of the validity of the use of a point-charge model for this case is discussed.