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-R₁,R₂-DCNQI [R₁=R ₂=Br (abbreviated as DBr) or R₁=R₂=CH ₃ (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 (ṽ_i) of infrared absorption bands of R ₁,R₂-DCNQI and its Li and Ba complexes are linearly correlated with the degrees of charge transfer (ρ) (ρ=- 0.5 and -1.0e for the Li and Ba complexes, respectively). The ṽ_i-ρ relationships indicate that the ρ 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 ṽ_i-ρ 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 Cu ²⁺) 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⁺⋯Cu ¹⁺⋯)_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.