Raman scattering study of hexaboride crystals

The valence-dependent Raman scattering spectra has been found for the RB₆ crystals (R = Ca²⁺, Yb²⁺, Ce³⁺, Pr³⁺, Gd³⁺, and Dy³⁺). In the divalent crystals of CaB₆ and YbB₆, E_g phonon shows doublet, while the line shape is a broad single peak for the trivalent case. Since both doublet peaks satisfy the selection rule due to the cubic symmetry, the doublet spectra is not explained by the structural phase transition, but by the anisotropic charge distribution on B₆. In the trivalent crystals, the two kind of the extra peaks have been commonly observed at around 200 cm^-1 and at around 1400 cm^-1. The peak intensities of the former decrease with decreasing temperature, and their peak energies increase with increasing lattice constant. These anomalous features are well explained by the vibration of R ion in the oversized cage space formed octahedral B₆ molecule. Thus, this new excitation can be attributed to a local vibration of the R ion, which can be regarded as "rattler."