Toward systematic understanding of diversity of electronic properties in low-dimensional molecular solids.

The preparation, crystal structures, and electric and magnetic properties of (BETS)2MX4 molecular conductors (BETS = bis(ethylenedithio)tetraselenafulvalene; M = Mn2+, Fe3+, Co2+, Ni2+, Cu2+; X = Cl, Br, CN) are reported. Resistivity measurements down to 2 K reveal the coexistence of the BETS π conduction electrons and the localized magnetic moments of the anions in the κ-(BETS)2FeCl4 and κ-(BETS)4(CoCl4)(C2H3Cl3) salts. Another FeCl4 phase, λ-(BETS)2FeCl4, undergoes a sharp metal-insulator (MI) transition around 8 K. At the same temperature, a magnetic transition of the FeCl4-anions takes place cooperatively. A superconducting transition is observed at 4.6 K in λ-(BETS)2(FeCl4)0.5(GaCl4)0.5, where half of the anion sites are occupied by magnetic ions. The crystals prepared from 1,1,2-trichloroethane solutions with the NiCl42- and MnCl42- anions exhibit the behavior of a semimetal down to ≈100 K. The (BETS)4(Cu2Cl6) salt remains metallic down to 4.2 K. ESR studies show that the Fe3+ ions in κ- and λ-(BETS...

New BETS Conductors with Magnetic Anions (BETS = bis(ethylenedithio)tetraselenafulvalene)

We synthesized a new porous coordination polymer Cu[Cu(pdt)2], which shows relatively high electrical conductivity (6 × 10−4 S cm−1 at 300 K) by the introduction of electron donors and acceptors as building units. This compound is applicable as a porous electrode with high power density. In addition, this compound forms a triangular spin lattice and shows spin frustration.

Electroconductive porous coordination polymer Cu[Cu(pdt)2] composed of donor and acceptor building units.

Since the discovery of the first organic superconductors (TMTSF)2X (TMTSF ) tetramethyltetraselenafulvalene (see Figure 1); X ) PF6, ClO4, ...) about a quarter century ago,1 extremely large progress has been achieved in the field of physics and chemistry of molecular conductors. Contrary to the physicists interested in the unified interpretation on the common nature of various conductors, chemists are usually interested in the individuality of each molecular conductor. Consequently, for most of the chemists in this field, the largest concern has been the development of a new type of molecular conductors. When the metallic states of TMTSF conductors were found to be stable down to about 10 K,2 many chemists noticed the possibility of the existence of two-dimensional (2-D) organic conductors with intermolecular 2-D networks of peripheral chalcogen atoms of π molecules,3 although all the molecular metals developed in the 1970s were believed to be 1-D systems because face-to-face stacking of planar π molecules had been believed to be essential to construct the π conduction band. Thus, the discovery of Bechgaard salts pushed us to evaluate the dimensionality of the organic conductor. To take a step forward, we adopted a simple extended Hückel tightbinding band calculation at that time, which provided a very useful tool to treat semiquantitatively the molecular design of organic conductors. From the viewpoint of molecular design, the extended-Hückel tight-binding band calculation can be regarded as a transformation of the anisotropy of intermolecular interactions in real space into the anisotropy of the system in the momentum space. By adopting the extended Hückel approximation (t∝S), both anisotropy of intermolecular short contacts and anisotropy of frontier molecular orbitals forming the conduction band are included simultaneously, where t and S are the transfer integral and the intermolecular overlap integral of the frontier orbitals, respectively. Despite of the close 2-D molecular arrangement, the tightbinding band calculation suggested that TMTSF2X was an essentially 1-D system owing to the anisotropy of the π frontier orbitals.4 Nevertheless, as expected, the extended Hückel tight-binding band examinations of a series of molecular conductors with the networks of peripheral chalcogen atoms gave us an important hint to develop new organic metals with stable 2-D cylindrical Fermi surfaces on the basis of multi-chalcogen π donor molecules. Then, by the observations of Shubnikov-de Haas (SdH) and de Haas-van Alphen (dHvA) oscillations in the subsequently developed BEDT-TTF ()bis(ethylenedithio)tetrathiafulvalene) superconductors with â and κ type * Corresponding author. Tel: +81-564-55-7410. Fax: +81-564-542254. E-mail: hayao@ims.ac.jp. 5265 Chem. Rev. 2004, 104, 5265−5288

Organic metals and superconductors based on BETS (BETS = bis(ethylenedithio)tetraselenafulvalene).

The absence of resistivity saturation in many strongly correlated metals, including the high-temperature superconductors, is critically examined from the viewpoint of optical conductivity measurements. Coherent quasiparticle conductivity, in the form of a Drude peak centred at zero frequency, is found to disappear as the mean free path (at ω = 0) becomes comparable with the interatomic spacing. This basic loss of coherence at the so-called Mott–Ioffe–Regel (MIR) limit suggests that the universality of the MIR criterion is preserved even in the presence of strong electron correlations. We argue that the shedding of spectral weight at low frequencies, induced by strong correlation effects, is the primary origin of the extended positive slope of the resistivity to high temperatures observed in all so-called ‘bad metals’. Moreover, in common with those metals which exhibit resistivity saturation at high temperatures, the scattering rate itself, as extracted from optical spectra, saturates at a value consisten...

Universality of the Mott–Ioffe–Regel limit in metals

The unique molecular conductors with p\ensuremath{\pi}-d mixing band structures (${\mathit{R}}_{1}$,${\mathit{R}}_{2\mathrm{\ensuremath{-}}}$N,N'-dicyanoquinonediimine${)}_{2}$Cu [(${\mathit{R}}_{1}$,${\mathit{R}}_{2\mathrm{\ensuremath{-}}}$DCNQI${)}_{2}$Cu] (${\mathit{R}}_{1}$,${\mathit{R}}_{2}$=${\mathrm{CH}}_{3}$,${\mathrm{CH}}_{3}$O,Cl,Br) were examined. General features of the phase diagram of the DCNQI-Cu system were depicted. A region that is related to the existence of anomalously heavy-metal electrons has been found. The ${\mathit{T}}^{2}$ dependence of the low-temperature resistivity of the alloyed system (${\mathrm{DMe}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{MeBr}}_{\mathit{x}\mathrm{\ensuremath{-}}}$DCNQI${)}_{2}$Cu (where Me=${\mathrm{CH}}_{3}$) suggests a large enhancement of the electron mass at the critical situation where the system begins to exhibit a characteristic metal instability. The mixed valency of Cu (${\mathrm{Cu}}^{+1.3}$) in (DMe-DCNQI${)}_{2}$Cu was confirmed by ir experiments performed on neutral DMe-DCNQI crystals and (DMe-DCNQI${)}_{2}$M (M=Li, Ba, Cu).

Mixed valency of Cu, electron-mass enhancement, and three-dimensional arrangement of magnetic sites in the organic conductors (R1,R2-N,N'-dicyanoquinonediimine)2Cu (where R1,R2=CH3,CH3O,Cl,Br).

A. Miyamoto

Papers

Mixed valency of Cu, electron-mass enhancement, and three-dimensional arrangement of magnetic sites in the organic conductors (R1,R2-N,N'-dicyanoquinonediimine)2Cu (where R1,R2=CH3,CH3O,Cl,Br).