Optical properties of layered transition-metal iodides

Low-temperature reflectance data from layered single crystals of ${\mathrm{FeI}}_2$, ${\mathrm{CoI}}_2$, and ${\mathrm{NiI}}_2$ have been obtained in the ultraviolet region of the spectrum up to 31 eV with synchrotron radiation, and the high-frequency dielectric tensor elements ${\epsilon }_{\mathrm{xx}}^T\mathrm{}\left(E\right)\mathrm{}$ have been deduced by Kramers-Kronig analysis. The spectra can be described in terms of single-particle (charge transfer, direct interband transitions, and excitons) and collective excitations (valence-electron plasma oscillations). The presence of collective effects is indicated by a plasma resonance which corresponds roughly to an electron density of 12 per molecule. Following recent calculations for the band structures of transition-metal chlorides and ${\mathrm{NiBr}}_2$ and basing our analysis on the experimental data, we found it possible to interpret the low-lying transitions satisfactorily. In particular, the direct energy gap is assigned to ${\Gamma }_3^{-}\to {\Gamma }_1^{+}$ transitions in ${\mathrm{FeI}}_2$ (6 eV), ${\mathrm{CoI}}_2$ (6.15.eV), and ${\mathrm{NiI}}_2$ (6.26 eV). Furthermore, the interpretation of the satellite exciton located at the low-energy side of the $\Gamma$ doublet in iodide compounds is attempted in terms of the Onodera-Toyozawa theory by considering the exchange interaction of the triplet-exciton electron in the itinerant $4s$ states with localized magnetic moment of the metal $3d$ electrons.