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

The real (ε₁) and imaginary (ε₂) parts of the dielectric function, together with the energy-loss function, -Im[1^ε(E)], of freshly cleaved crystals of NiCl₂, NiBr₂, and NiI₂ have been obtained in the region 2-31 eV by Kramers-Kronig analysis of near-normal-incidence reflectance spectra. The spectra of NiI₂ have been obtained over the entire region at 300 and 30 K, and those of NiCl₂ and NiBr₂ 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 NiX₂ (X=Cl,Br,I) were analyzed in terms of the known band structures of NiCl₂ and NiBr₂. Nearly all of the main spectral features beyond the energy gap can be identified in terms of direct interband transitions at the symmetry points Γ, Z, F, and L and along symmetry lines of the Brillouin zone. The energy gap is assigned to Γ₃^-→Γ₁⁺ transitions in NiCl₂ (8.72 eV), NiBr₂ (7.90 eV), and NiI₂ (6.26 eV). Finally, the interpretation of the satellite exciton at 5.65 eV in NiI₂ (30 K) and at 6.50 eV in NiBr₂ (30 K) is discussed in terms of the triplet exciton state predicted by Onodera and Toyozawa.