Ionic channel structures in [(M⁺)_x([18]crown-6)][Ni(dmit)₂]₂ molecular conductors

The [(M⁺)_x([18]crown-6)] supramolecular cations (SC⁺), in which M⁺ and [18]crown-6 are alkali metal ions (M⁺ = Li⁺, Na⁺, and Cs⁺) and 1,4,7,10,13, 16-hexaoxacyclooctadecane, respectively, form ionic channel structures through the regular stacks of [18]crown-6 in [Ni(dmit)₂]-based molecular conductors (dmit^2- = 2-thioxo-1,3-dithiole-4,5-dithiolate). In addition to the [Ni(dmit)₂] salts that have the ionic channel structures (these salts are abbreviated as type I salts), Li⁺ and Na⁺ form dimerized [(M⁺)₂([18]crown-6)₂] units in the crystals (type II salts). The K⁺ and Rb⁺ are coordinated tightly into the [18]crown-6 cavity to form typical disk-shape SC⁺ units in the corresponding [Ni(dmit)₂] salts (type III salts). The type I, II, and III salts have typical stoichiometries of [(M⁺)_x([18]crown-6)]-[Ni(dmit)₂]₂, [(M⁺)([18]crown-6)(H₂O)_x-(CH ₃CN)_1.5-x][Ni(dmit)₂]₃ (x=1 for Li⁺ or 0.5 for Na⁺), and [M⁺([18]crown-6)][Ni(dmit)₂]₃, respectively; the salts of the same type are isostructural. In agreement with the trimer structures of [Ni(dmit)₂] in the type II and III salts, they exhibit semiconducting behavior with electrical conductivities at 300 K (σ_{300 K}) of 0.01-0.1 S cm^-1. Type I salts contain a regular stack of partially oxidized [Ni(dmit)₂] units, which form a quasi one-dimensional metallic band within the tight-binding approximation regime. The electrical conductivities at 300 K are 10-30 S cm^-1, and an almost temperature-independent conductivity was observed at higher temperatures. However, the one-dimensional electronic structures in these salts are strongly influenced by the static and dynamic structures of the coexisting ionic channel. The Na⁺ salt is a semiconductor, whose magnetic behavior is described by the disordered one-dimensional antiferromagnetic chain. On the other hand, the Cs⁺ salt is a exhibits metallic properties with 2k_F instability at room temperature. The Li⁺ salt shows a gradual transition from the high-temperature metallic phase to the low-temperature one-dimensional antiferromagnetic semiconductor phase, which was associated with the freezing of Li⁺ motion at lower temperatures. The preferential crystallization of type I salts was possible by controlling the equilibrium constant (K_c) of the complex formation between M⁺ ions and the [18]crown-6 molecule. The ionic channel structures were obtained when the K_c was low in the electrocrystallization solution, while type II or III salts were formed in the high K_c region.