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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.