CHARGE-TRANSFER TYPE EXCITATIONS IN LAYERED CUPRIC HALIDES

Optical conductivity spectra and their polarization dependence have been investigated for the layered cupric halides A2CuCl4 (A = CnH2n+1NH3), which form CuCl4-layers analogous to the CuO2-sheets of the cuprate superconductors. In the visible region (2-4 eV), we found two distinct electronic transitions localized on the CuCl4-layer, whose peak energies shift to lower energy (ca. 0.8 eV) by substitution of chlorine with bromine. We ascribed these transitions to charge-transfer (CT) excitations from the ligand (Cl or Br) to metal (Cu) sites.