Valence dependence of interatomic interactions in RB6

Previous Raman measurements of phonon in hexaborides have shown that bandwidth of low-frequency vibrations of R in divalent hexaborides is very broad in comparison with that in trivalent ones. To investigate this valence dependence, we have calculated interatomic force constants (IFC) from first principles for RB6 (R=Ca, Sr, Ba, Y, and La). The calculated phonon dispersion curves reproduce the valence dependence. From the investigation of calculated IFC, we conclude that the different bandwidth is originated from the flatness of the dispersion curve of R ion vibration produced by IFC between R ions. Thus, the dispersion curve is highly dispersive for the divalent case. Since the IFC in divalent hexaborides are similar to those of dipole-dipole interactions, we suggest that an origin of the difference of IFC is the screening effect of the dipole field by conduction electrons in metallic trivalent hexaborides.