Fundamental reflectivity and electronic structure of NiBr2and NiCl2insulators

The fundamental reflectivity of Ni${\mathrm{Br}}_2$ and Ni${\mathrm{Cl}}_2$ has been measured over the energy range 2-11 eV from 300 to 30 K with the use of synchrotron radiation. The imaginary part of the dielectric constant ${\epsilon }_2$ has been determined at 30 K by means of the Kramers-Kronig technique. The structure in the complex optical spectra of nickel halides is interpreted in terms of charge-transfer transitions, orbital promotions, excitons, and direct allowed interband transitions at the symmetry points $\Gamma$, $Z$, and $F$, and along symmetry lines $\Lambda$, $B$, and $\Gamma -L$ of the Brillouin zone. The energy gap is assigned to ${\Gamma }_3^{-}\to {\Gamma }_1^{+}$ transitions at the zone center, both in Ni${\mathrm{Br}}_2$ (7.90 eV) and Ni${\mathrm{Cl}}_2$ (8.70 eV). Finally, the interpretation of the satellite exciton at 6.5 eV in Ni${\mathrm{Br}}_2$ (30 K) is discussed.