VIBRATIONAL STUDIES ON ELECTRONIC-STRUCTURES IN METALLIC AND INSULATING PHASES OF THE CU COMPLEXES OF SUBSTITUTED DICYANOQUINONEDIIMINES (DCNQI) - A COMPARISON WITH THE CASES OF THE LI AND BA COMPLEXES

Electronic structures in metallic and insulating phases of the Li, Cu, and Ba complexes of 2,5-R(1),R(2)-DCNQI [R(1)=R(2)=Br (abbreviated as DBr) or R(1)=R(2)=CH3 (abbreviated as DMe); DCNQI=N,N'-dicyanoquinonediimine; 2,5- is usually omitted] have been studied by observing temperature dependencies of their infrared absorption bands between 295 and 23 K. At room temperature, the wave numbers (nu(i)) Of infrared absorption bands of R(1),R(2)-DCNQI and its Li and Ba complexes are linearly correlated with the degrees of charge transfer (rho) (rho=-0.5 and -1.0e for the Li and Ba complexes, respectively). The nu(l)-rho relationships indicate that the rho value for the Cu complexes is -0.67e. This result is consistent with the previously established view that the Cu cations in the Cu complexes at room temperature are in a mixed-valence state of Cu-1.33+. In the infrared spectrum of Cu(DBr-DCNQI)(2) at room temperature, no electron-molecular vibration (EMV) coupling bands are observed. Below the metal-insulator (M-I) transition temperature (T-MI), EMV bands grow continuously and the ordinary infrared bands observed at room temperature gradually split into three bands with decreasing temperature. Similarly, the infrared bands of Li(DBr-DCNQI)(2) split into two bands. These splittings are due to an inhomogeneous charge distribution in the DCNQI columns produced by the freezing of charge-density wave (CDW). The peak-to-peak amplitudes of CDWs in the DCNQI columns estimated by use of the nu(l)-rho relationships are 0.08+/-0.04 and 0.40+/-0.04e, respectively, for the Li and Cu complexes of DBr-DCNQI. The state-of the frozen CDW is inferred from the number of split bands. Based on the observed continuous change of the infrared spectra of Cu(DBr-DCNQI)(2) and the discontinuous changes of other quantities such as x-ray satellite reflections, lattice parameters, and magnetic susceptibilities, the M-I transition in Cu(DBr-DCNQI)(2) may be described as follows: (1) above T-MI the charges on Cu cations (two Cu1+'s: one Cu2+) are dynamically averaged to +1.33e through the Cu...N=C bridge. (2) At T-MI the charges abruptly localize in the order of (Cu1+...Cu2+...Cu1+...)(n). At the same time, the CDWs begin to be frozen in the DCNQI columns. (3) As temperature decreases below T-MI, the order of the frozen CDW develops gradually. In contrast to these changes in Cu(DBr-DCNQI)(2), neither EMV bands nor band splittings are observed in the infrared spectra of Cu(DMe-DCNQI)(2) at low temperatures. Instead, almost all bands show negative absorption lobes on their low-wave number sides and become asymmetric. This asymmetrization is due to interactions between the vibrational levels and low-lying continuous electronic levels responsible for a broad band observed in the 1600-800 cm(-1) region.