Tuning of electronic structures of quasi-one-dimensional halogen-bridged Ni-Pd mixed-metal complexes, [Ni _1-xPd _x(chxn) ₂X]X ₂ (X = Cl, Br) with strong electron correlation

This article describes the tuning of the electronic structures in a series of quasi-one-dimensional halogen-bridged Ni-Pd mixed-metal complexes, [Ni _1-xPd _x(chxn) ₂X]X ₂ (chxn = 1R,2R-diaminocyclohexane; X, Y = Cl or Br). In these compounds, planar [M(chxn) ₂] units are bridged by halogen ions (X), forming linear chain M-X-M structures. Therefore, their electronic structures are 1D and are composed of hybridization between the d _z2 orbitals of the metal ions and pz orbitals of the bridging halogens. In this system, electron-correlation on the Ni sites and electron-phonon interaction on the Pd sites should compete with each other. Their electronic structures have been investigated by means of the single-crystal reflectance, ESR and IR spectroscopies, electrical conductivities, and X-ray diffuse scattering. Moreover, their local electronic structures have been directly observed by scanning tunneling microscopy (STM) using single-crystals at room temperature and under ambient pressure. As a result, while the oxidation state of the Ni sites is +3 in all mixing ratios in these compounds due to strong electron-correlation, the conversion from the Pd ^III Mott-Hubbard states to the Pd ^II-Pd ^IV mixed-valence states is observed around x ≈ 0:90 for Br and x ≈ 0:85 for Cl. Interestingly, the spin-soliton in Ni _0.05Pd _0.95(chxn) ₂Br ₃ was directly observed by STM for the first time.