Optical properties of $R{\mathrm{Ni}}_2{\mathrm{B}}_2\mathrm{C}$ $(R=\mathrm{Y},\mathrm{T}\mathrm{b},\mathrm{E}\mathrm{r},\mathrm{D}\mathrm{y})$

The optical properties of single crystals of $R{\mathrm{Ni}}_2{\mathrm{B}}_2\mathrm{C}$ $(R=\mathrm{Y},\mathrm{}\mathrm{T}\mathrm{b},\mathrm{}\mathrm{Er},$ and Dy) were measured between 1.7 and 5.2 eV at room temperature using a spectroscopic ellipsometer. The spectra for all compounds are similar and the peak positions appear in a similar energy region. The similarity of the spectra of $R{\mathrm{Ni}}_2{\mathrm{B}}_2\mathrm{C}$ $(R=\mathrm{T}\mathrm{b},\mathrm{}\mathrm{E}\mathrm{r},\mathrm{}\mathrm{Dy})$ to that of ${\mathrm{YNi}}_2{\mathrm{B}}_2\mathrm{C}$ indicates that the rare-earth $4f$ states are not actively involved in the optical transitions.