Anion Ordering and Optical Properties of the Quasi-One-Dimensional Organic Conductor (ChTM-TTP)₂GaCl₄

Abstract

Low-temperature crystal structure and optical properties of the quasi-one-dimensional organic conductor (ChTM-TTP)₂GaCl₄ (ChTM-TTP: 2-[4,5-(1,2-cyclohexylenedithio)-1,3-dithiol-2-ylidene]-5-[4,5-bis(methylthio)-1,3-dithiol-2-ylidene]-1,3,4,6-tetrathiapentalene) have been investigated. As the temperature lowers, the disordered anion transforms to an ordered state with a superstructure wave vector of q=(1/2,0,1/2). The peak intensities of the superlattice gradually increase in the metallic region, and are saturated below 150 K; this indicates that the charge localization below 150 K is independent of the evolution of the superlattice. The reflectance spectra of (ChTM-TTP)₂X suggest the existence of moderate interchain interactions. The transverse interactions are more pronounced in the GaCl₄ salt (t_⊥/t_|| ≈ 1/7) than the AuBr₂ and Au(CN)₂ salts. The low-temperature optical reflectance of the GaCl₄ salt shows no significant change, indicating that (ChTM-TTP)₂GaCl₄ goes to an insulating state with a small charge gap, which is consistent with the transport property. The low-frequency peak in the chain-axis optical conductivity is attributed to a correlation gap, though the expected on-site Coulomb repulsion U is relatively small.