Reflectance and thermoreflectance studies of ${\mathrm{CrCl}}_3$, ${\mathrm{CrBr}}_3$, ${\mathrm{NiCl}}_2$, and ${\mathrm{NiBr}}_2$ crystals

Reflectance spectra at 300 and 80 K and thermoreflectance spectra at 80 and 30 K are shown for ${\mathrm{CrCl}}_3$, ${\mathrm{CrBr}}_3$, ${\mathrm{NiCl}}_2$, and ${\mathrm{NiBr}}_2$ in the energy range 2-10 eV. A general consistency was found between the two types of data. The temperature-shift coefficients of the various peaks have been found to be positive in nickel halides, while in chromium compounds were negative as usual. Through the combined analysis of the two types of experiments, it has been possible to distinguish the transitions of "charge transfer" character from those of excitonic and band-to-band origin. A discussion of the various possible charge-transfer processes has also been made.