Tuning the electronic structure from charge-transfer insulator to Mott-Hubbard and Peierls insulators in one-dimensional halogen-bridged mixed-metal compounds

The electronic structures of one-dimensional (1D) halogen-bridged mixed-metal compounds, [Ni_1−xPd_x(chxn)₂X]X₂ (chxn= cyclohexanediamine; X=Cl,Br; 0⩽x⩽1), are investigated through optical and magnetic measurements. The results reveal that the system changes from charge-transfer (CT) insulator to Mott-Hubbard (MH) insulator at around x∼0.3, and from MH insulator to Peierls insulator at x∼0.9 with increasing x. From the analysis of optical conductivity spectra using the 1D two-band extended Peierls-Hubbard model, electronic parameters such as Coulomb repulsion energy, pd hybridization, CT energy, and electron-lattice interaction are determined.