High-frequency dielectric constant of layered Fe, Co, and Ni dihalides

The high-frequency dielectric constants of layered Fe, Co, and Ni dihalides are obtained in the region 2-31 eV by Kramers-Kronig analysis of near-normal-incidence reflectance spectra measured with synchrotron radiation. The primary data may be described unambiguously in terms of single-particle excitations, as charge-transfer and interband transitions, and plasma resonance effects. Consideration of the sum rules involving the imaginary part of the dielectric function, Neff(E), and εeff(E) (the effective electron number and dielectric constant) was taken in order to permit comparison between experimental and theoretical results. A general good agreement was found with the extended-shell-model predictions based on transferable sets of polarizabilities for both anions and cations. The role of static dipoles is finally discussed in connection with the sizable deviations from the isotropic Clausius-Mossotti relation.