Low-dimensional Molecular Conductors Research Articles

Charge Fluctuations in the Dimer-Mott Insulating State of (rac-DM-EDT-TTF)2PF6

Low-dimensional molecular conductors are known to display various insulating ground states that result from significant electronic correlations, depending on the degree of dimerization. Of particular interest are intermediate systems where the interplay between dimer-Mott and charge-ordered phases is observed. Here we report an optical study of (rac-DM-EDT-TTF)2PF6, a weakly dimerized organic conductor that is based on the chiral DM-EDT-TTF donor molecule. Variable-temperature infrared reflectance spectra clearly demonstrate a quasi-one-dimensional response together with a metal–insulator phase transition at 110 K. Both the broadening of the charge-sensitive vibrational ν2 mode observed in Raman spectra and the strong EMV-activated ν3 mode in the reflectance spectra polarized in the stack direction suggest the charge fluctuations to develop in the insulating state. Significant modification of the hydrogen-bonding type interaction between the conducting DM-EDT-TTF layer and the PF6 anions is detected below 110 K. We suggest that the low-temperature insulating state in (rac-DM-EDT-TTF)2PF6 should be regarded as a coexistence of the dimer-Mott state and fluctuating charge-order.

New low-dimensional molecular conductors based on bis(ethylenedithio)tetrathiafulvalene radical cation salts with octahedral metal complex anion [ReIVCl6]2–

Three new radical cation salts of bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) with the magnetic metal complex anion [ReIVCl6]2–, viz., β-(BEDT-TTF)2[ReCl6] (1), α´-(BEDT TTF)4[ReCl6]•(DCE)2 (2), and δ-(BEDT-TTF)4[ReCl6]1.33•(CB)0.67 (3) (DCE is dichloroethane, CB is chlorobenzene), were synthesized for the first time. The X-ray diffraction analyses of the obtained crystals at room and low temperatures showed that all of them had the sandwich structure characteristic of alternating anionic and cationic layers, which is characteristic of conducting radical cation salts. The structural phase transition accompanied by unit cell tripling and decreasing the symmetry from monoclinic to triclinic in the low-temperature phase occurs in the crystals of salt 2. The transition is related to modulated displacements of both [ReIVCl6]2– anions and BEDT-TTF radical cations. A specific feature of the crystals of salt 3 is a strong positional disorder in the anionic layer, which is expressed as a random distribution of [ReIVCl6]2– anions and CB solvent molecules in a ratio of 2 : 1 along the channels in the ordered cationic framework.

Mott metal-insulator transition induced by utilizing a glasslike structural ordering in low-dimensional molecular conductors

We utilize a glasslike structural transition in order to induce a Mott metal-insulator transition in the quasi-two-dimensional organic charge-transfer salt $\ensuremath{\kappa}\ensuremath{-}{\text{(BEDT-TTF)}}_{2}\mathrm{Cu}[\mathrm{N}{\text{(CN)}}_{2}\mathrm{Br}]$. In this material, the terminal ethylene groups of the BEDT-TTF molecules can adopt two different structural orientations within the crystal structure, namely eclipsed (E) and staggered (S) with the relative orientation of the outer C--C bonds being parallel and canted, respectively. These two conformations are thermally disordered at room temperature and undergo a glasslike ordering transition at ${T}_{g}\ensuremath{\sim}75$ K. When cooling through ${T}_{g}$, a small fraction that depends on the cooling rate remains frozen in the S configuration, which is of slightly higher energy, corresponding to a controllable degree of structural disorder. We demonstrate that, when thermally coupled to a low-temperature heat bath, a pulsed heating current through the sample causes a very fast relaxation with cooling rates at ${T}_{g}$ of the order of several $1000 \mathrm{K}/\mathrm{min}$. The freezing of the structural degrees of freedom causes a decrease of the electronic bandwidth $W$ with increasing cooling rate, and hence a Mott metal-insulator transition as the system crosses the critical ratio ${(W/U)}_{c}$ of bandwidth to on-site Coulomb repulsion $U$. Due to the glassy character of the transition, the effect is persistent below ${T}_{g}$ and can be reversibly repeated by melting the frozen configuration upon warming above ${T}_{g}$. Both by exploiting the characteristics of slowly changing relaxation times close to this temperature and by controlling the heating power, the materials can be fine-tuned across the Mott transition. A simple model allows for an estimate of the energy difference between the E and S state as well as the accompanying degree of frozen disorder in the population of the two orientations.

Pseudopolymorphism, superstructure, and phase transitions in the crystals of (BEDT-TTF4[MNOX5]2-x G x molecular conductor family, where M=Os, Ru; X=Cl, Br; G is the solvent molecule

We analyze the specific features of the structure and properties of crystals that belong to the family of low-dimensional molecular conductors based on BEDT-TTF (ET) radical cation salts with [MNOX5]2s- (M=Os, Ru; X=Cl, Br) mononitrosyl metal complexes as anions. It is shown that when the synthesis components are changed single crystals with α’, β, δ, and κ-types of the conducting layers form. All radical cation salts of this family can be considered as pseudopolymorphous phases with a general formula of (ET)4[MNOX5]2-xGx (G is a guest solvent molecule, BN or NB). The studied crystals display a range of remarkable structural features such as conformational and charge ordering in the conducting ET layers as well as commensurate and incommensurate structural modulations mainly related to the positional ordering of the components of complex anion layers.

Counterion dimerisation effects in the two-chain compound (Per)2[Co(mnt)2]: structure and anomalous pressure dependence of the electrical transport properties

The crystal structure of (Per)2[Co(mnt)2], was investigated by single crystal X-ray diffraction. Its refinement shows a strong chemical dimerization of the anions [Co(mnt)2]22−. This is responsible for a unit cell doubling along the staking axis b when compared with other members of the α-(Per)2[M(mnt)2] family of the low dimensional molecular conductors, with M = Ni, Cu, Au, Pt or Pd. The dimerized [Co(mnt)2]22− stacks appear however as poorly correlated in the ac plane. The electrical conductivity of this compound measured in single crystals exhibits anomalous pressure dependence, where under increasing hydrostatic pressure the conductivity decreases while the metal insulator transition temperature is enhanced. This effect is ascribed to the dimeric nature of the counterions which imposes a sliding of the stacked perylene molecules along alternated directions in their planes.

Structural features of low-dimensional molecular conductors—Representatives of new hybrid polyfunctional materials: Review

The crystal structures of the family of low-dimensional molecular conductors based on radical cation salts of different organic π donors with photochromic and magnetic metal complexes as anions have been considered. This class of supramolecular systems demonstrates a large variety of structural types and a wide range of transport properties. The specificity of the structure and properties of such hybrid materials is illustrated by several examples. The crystallochemical analysis of the conductors considered indicates the possibility of purposeful control of their transport properties via changing of the charge, sizes, shape, and symmetry of the anionic block components. The specificity of the crystal structure and properties of some organic conductors shows that such systems can be used as model systems in the study of new physical phenomena related to electron correlation and effects of charge ordering.

Structure and Physical Properties of Low-Dimensional Molecular Conductors, [PXX][FeIII(Pc)(CN)2] and [PXX][CoIII(Pc)(CN)2] (PXX = peri-xanthenoxanthene, Pc = phthalocyaninato)

Abstract A novel low-dimensional molecular conductor, [PXX][FeIII(Pc)(CN)2], has been synthesized. This salt contains the magnetic FeIII ion (S = 1/2), and is isomorphous with [PXX][CoIII(Pc)(CN)2] which includes the non-magnetic CoIII ion. In both salts, the [MIII(Pc)(CN)2] (M = Fe or Co) units form a two-leg ladder chain. The two salts exhibit a similar temperature dependence of the thermoelectric power and a similar reflectance spectrum. The FeIII salt shows semiconducting behavior in its electrical resistivity over the temperature range measured, while the isomorphous CoIII salt exhibits metallic behavior in its resistivity above 100 K. The difference in the transport properties between the two salts suggests that the conduction electrons in the FeIII salt are seriously scattered by the local magnetic moment. Spontaneous magnetization is observed below 8 K in the FeIII salt. Upon applying a magnetic field, the resistivity of the FeIII salt drastically decreases below 50 K. The decrease in the resistivity is highly anisotropic to the field orientation. The field orientation dependence is highly consistent with the g-tensor anisotropy in the [FeIII(Pc)(CN)2] unit, suggesting that the negative magnetoresistance originates from the large π–d interaction self-contained in the [FeIII(Pc)(CN)2] unit.

Electronic structure of transition metal complex-based molecular metals and superconductors

The electronic structure of some transition metal complex-based molecular solids is analyzed in order to discuss why the use of transition metal complexes can provide interesting new low-dimensional molecular conductors. The molecular solids considered include several charge transfer salts of both M (dmit) 2 (MNi, Pd, Pt) acceptors and M (dddt) 2 (MNi, Pd, Pt) donors.

The 1,2,3,5-ditelluradiazolyl [HCN2Te2] species. Theoretical characterizations of the cation, radical, and radical dimers

Dithia- and diselena-diazolyl radicals (HCN2E2 E = S, Se) and dimers are important building blocks in the design of low-dimensional molecular conductors. Research on the tellurium-based analogues is much rarer. This work reports the molecular and electronic structures of the cation, radical, and radical dimers of 1,2,3,5-ditelluradiazolyl using ab initio theory including electron correlation by Møller–Plesset perturbation theory up to partial fourth order (MP4SDQ). A face-to-face C2v dimer is predicted to be bound with respect to two radicals by approximately 18 kcal/mol. A C2h, dimer also has been studied and is ca. 2 kcal/mol less stable than the C2v conformer. Relaxing symmetry constraints on the dimers led to more energetically stable structures at the Hartree–Fock level but the C2v structure remains the most stable at a level of theory including electron correlation effects. The results for the Te compounds along with our earlier research on the S and Se analogues provide predictions for the geometries, vibrational frequencies, and ionization potentials for the Te species to assist in future experiments. Key words: tellurium, ab initio, ditelluradiazolyl, dimers, binding.

Low-dimensional molecular conductors (Per)2M(mnt)2, Per = perylene, mnt = maleonitrile dithiolate, M = copper or nickel: low- and high-conductivity phases

Low-dimensional molecular conductors (Per)2M(mnt)2, Per = perylene, mnt = maleonitrile dithiolate, M = copper or nickel: low- and high-conductivity phases