Ultraviolet reflectance and electronic states of layered nickel halides studied with synchrotron radiation

Abstract

The real (${\ensuremath{\epsilon}}_{1}$) and imaginary (${\ensuremath{\epsilon}}_{2}$) parts of the dielectric function, together with the energy-loss function, $\ensuremath{-}\mathrm{Im}[\frac{1}{\stackrel{^}{\ensuremath{\epsilon}}(E)}]$, of freshly cleaved crystals of Ni${\mathrm{Cl}}_{2}$, Ni${\mathrm{Br}}_{2}$, and Ni${\mathrm{I}}_{2}$ have been obtained in the region 2-31 eV by Kramers-Kronig analysis of near-normal-incidence reflectance spectra. The spectra of Ni${\mathrm{I}}_{2}$ have been obtained over the entire region at 300 and 30 K, and those of Ni${\mathrm{Cl}}_{2}$ and Ni${\mathrm{Br}}_{2}$ have been measured at 30 K only from 2 to 11 eV. The results can be described in terms of charge-transfer transitions, interband transitions, and plasma oscillations. The low-temperature spectra of the materials reveal the presence of exciton structures. Plasma resonance effects have been identified in the high-energy region (15-20 eV). The optical spectra of $\mathrm{Ni}{X}_{2} (X=\mathrm{C}\mathrm{l},\mathrm{B}\mathrm{r},\mathrm{I})$ were analyzed in terms of the known band structures of Ni${\mathrm{Cl}}_{2}$ and Ni${\mathrm{Br}}_{2}$. Nearly all of the main spectral features beyond the energy gap can be identified in terms of direct interband transitions at the symmetry points $\ensuremath{\Gamma}$, $Z$, $F$, and $L$ and along symmetry lines of the Brillouin zone. The energy gap is assigned to ${\ensuremath{\Gamma}}_{3}^{\ensuremath{-}}\ensuremath{\rightarrow}{\ensuremath{\Gamma}}_{1}^{+}$ transitions in Ni${\mathrm{Cl}}_{2}$ (8.72 eV), Ni${\mathrm{Br}}_{2}$ (7.90 eV), and Ni${\mathrm{I}}_{2}$ (6.26 eV). Finally, the interpretation of the satellite exciton at 5.65 eV in Ni${\mathrm{I}}_{2}$ (30 K) and at 6.50 eV in Ni${\mathrm{Br}}_{2}$ (30 K) is discussed in terms of the triplet exciton state predicted by Onodera and Toyozawa.