Organic Charge-transfer Salts Research Articles

Superconductivity in the Organic Charge Transfer Salts: (TMTSF)2X and (TMTTF)2X

Abstract We report pressure dependent studies of the a-axis resistivity as a function of temperature for several members of the isostructural families of organic charge transfer salts, (TMTSF)2X and (TMTTF) 2X. For a typical (TMTSF)2X material the low temperature metal-insulator transition seen at 1 bar is suppressed above some critical pressure, Pc, where a superconducting transition is observed near 1 K. We find a correlation between Pc and the ambient pressure c lattice parameter which reflects the anion size. The (TMTTF) cX salts exhibit very different ambient pressure behaviour but we find that with the application of sufficiently high pressures (∼30 kbar) their behaviour resembles that seen in the (TMTSF)2X family but at lower pressures. In particular we find evidence of a possible superconducting transition near 4 K in (TMTTF)2Br at 25 kbar. At this pressure the conductivity near 4 K is extremely high with a value approaching 106 (Ωcm)−1 and the resistivity ratio is about 400.

DBTTF-TCNQ: A fractionally-charged organic salt with a mixed-stack crystalline motif

X-Ray diffraction measurements demonstrate that the organic charge-transfer salt formed from the donor(D) DBTTF and the acceptor(A) TCNQ exists as a mixed stack(…DADA…) crystalline array. From the donor and acceptor geometries and solid state infrared spectrum, an unusual charge transfer(for a …DADA… motif) of 0.47e is derived. The Madelung contribution, −2.9 eV/DA pair, has been evaluated and found to be deficient relative to the cost of ionization of the components.

3,4,9,10 - Tetrathioperylene - I 1.28, a new highly conducting, incommensurate organic charge transfer salt

We report here the preparation of TTP - I 1.28 (TTP = 3,4,9,10 - Tetrathioperylene), the crystal structures of TTP and its complex TTP - I 1.28 and their electrical conductivities. The crystal structure of TTP consists of two inequivalent types of segregated stacks with intermolecular planar spacings of 3.43 Å, and 3.47 Å. The crystal structure of TTP - I 1.28 consists of two incommensurate sublattices. The TTP sublattice is built up by segregated stacks of TTP molecules with interplanar spacing of 3.25 Å. The iodine sublattice consists of columns formed mainly by I 3 units. The electrical conductivity is very weakly metallic at room temperature and turns to a semiconducting behavior at 220 K with an activation energy of 15 meV.

A scaling method for low temperature behavior of random antiferromagnetic systems (invited)

Low temperature properties of random antiferromagnetic Heisenberg spin-1/2 systems with a wide distribution of exchange couplings are examined using a scaling technique. The method iteratively discards the irrelevant high energy excitations of strongly coupled clusters, while retaining the low lying states which are reproduced by an equivalent smaller cluster. The method is applied to n-doped semiconductors, a spatially random three dimensional system of spin-1/2 donors with Heisenberg exchange varying exponentially with separation. It shows how the system does not order down to temperatures well below the median nearest neighbor exchange. Comparison is made with the corresponding system in two dimensions and the one-dimensional random antiferromagnetic chain (applicable to certain organic charge transfer salts), as well as with the case of Ising spins in three dimensions.

Effects of neutron irradiation induced defects and chemical impurities on the 'DC' conductivity of TTT2I3

The roles of disorder and impurities are studied in the highly conducting organic charge transfer salt TTT2I3. It is shown that impurities in the conducting TTT chains change the conductivity much more than an enhanced disorder in the iodine chains. In situ low-temperature neutron irradiation measurements indicated that molecular concentrations of defects of the order 10-3 increase the low-temperature (21K) resistivity by a factor of three and increase the temperature of the conductivity maximum by more than 100K. Measurements on crystals containing different amounts of chemical impurities and on TTTI1.5-xBrx alloys confirmed this picture.

A comparison of electron-spin resonance spectra in some organic charge-transfer salts

Studies of multiline ESR spectra in ten organic charge-transfer salts give g-tensors which are characteristic of individual molecular ions containing sulphur, nitrogen and smaller atoms. Such nearly universal g-tensors are not found in three salts with molecules containing selenium. These conclusions are supported by some results at variable temperature and hydrostatic pressure.

Periodic lattice distortions and charge density waves in one- and two-dimensional metals

At low temperatures, low dimensional metals are commonly found to exhibit superlattices incommensurate with the primitive lattice (periodic lattice distortion coupled to charge density waves, PLD — CDW). Transitions to such states are driven by strong electron phonon coupling and favoured by the particular forms of the Fermi surfaces of these metals. Experimental investigations of the structural distortions, and of the changes in electronic structure around the Fermi level produced by the CDW — PLD state, will be discussed for the layered dichalcogenides, the linear chain platinum complex KCP, and the organic charge transfer salts.

Valence Bond Theory of Organic Charge-Transfer Salts

Abstract A valence bond (VB) description is introduced for organic charge-transfer (CT) salts based on mixed stacks… D+γA−γD+γA−γ … of donors (D) and acceptors (A). Extrapolating exact results for finite rings shows that the magnetic gap ΔEm is finite for γ γ c as expected in the paramagnetic ionic limit γ= 1. The small finite ΔEm in the “ionic” TMPD-TCNQ complex indicates incomplete CT, in agreement with the resonant Raman result of γ∼ 0.7. The extension of diagrammatic VB techniques to partly-filled segregated ion-radical stacks like … A −γ A −γ A −γ A −γ or … D +γ D +γ D +γ D +γ… is discussed.

Coulomb effects in the organic charge transfer salt TTT 2I 3

Madelung energy and simple band structure calculations on different models of the TTT 2I 3 complex indicate, that the iodine chains do not play an active role in the conduction process. The relatively small Coulomb coupling of the iodine chains to the crystal obtained by the Madelung calculations rationalizes the structural findings about the ordering of these chains at low temperatures. The Madelung energy difference between the localized model (with TTT o and TTT + ions) and delocalized model (with partly charged TTT 1 2+ stacks) is large enough to stabilize the former.

Neutral-ionic interface in organic charge-transfer salts

Charge transfer (CT) stabilization in linear stacks $\dots{}{D}^{\ensuremath{\gamma}+}{A}^{\ensuremath{\gamma}\ensuremath{-}}{D}^{\ensuremath{\gamma}+}{A}^{\ensuremath{\gamma}\ensuremath{-}}\dots{}$ of $\ensuremath{\pi}$-electron donors ($D$) and acceptors ($A$) involve spin-dependent configuration interactions that are treated exactly in rings of $N=4, 6, 8, 10$ sites, and extrapolated to $N\ensuremath{\rightarrow}\ensuremath{\infty}$, by adapting valence-bond techniques to electron-hole excitations. The ground state CT $\ensuremath{\gamma}(z)$ and the magnetic gap $\frac{\ensuremath{\Delta}{E}_{m}(z)}{\ensuremath{\surd}2|t|}$ to the lowest triplet are computed for arbitrary $z=\frac{\ensuremath{\delta}}{\ensuremath{\surd}2|t|}$, where $\ensuremath{-}2\ensuremath{\delta}$ is the energy for $\mathrm{DA}\ensuremath{\rightarrow}{D}^{+}{A}^{\ensuremath{-}}$, $\ensuremath{-}|t|=〈{D}^{+}{A}^{\ensuremath{-}}|\mathcal{H}|DA〉$ is the Mulliken CT integral, and ${D}^{2+}$, ${A}^{2\ensuremath{-}}$ sites are excluded. The spin degeneracy of ${A}^{\ensuremath{-}}\ensuremath{\sigma}$ and ${D}^{+}\ensuremath{\sigma}$ ion radicals is treated exactly. Instead of the discontinuous change from $\ensuremath{\gamma}=0$ to $\ensuremath{\gamma}=1$ in the limit $|t|\ensuremath{\rightarrow}0$, finite overlap gives a continuous $\ensuremath{\gamma}(z)$ and $\ensuremath{\gamma}({z}_{c})=0.68\ifmmode\pm\else\textpm\fi{}0.01$ at the neutral-ionic interface ${z}_{c}=0.53\ifmmode\pm\else\textpm\fi{}0.01$. The magnetic gap $\ensuremath{\Delta}{E}_{m}$ is finite for $z<{z}_{c}$ and vanishes for $z>{z}_{c}$, where there is a diamagnetic to paramagnetic transition and the ground state switches from $k=0$ to $k=\ensuremath{\pi}$ symmetry. Collective effects due to long-range three-dimensional Coulomb interactions are included in a Hartree approximation and produce a first-order transition, with discontinuous $\ensuremath{\gamma}(z)$, when the critical value $\frac{m}{\ensuremath{\surd}2|t|}=1.4\ifmmode\pm\else\textpm\fi{}0.1$ of the Madelung stabilization $m$ of a dimer is exceeded. The puzzling magnetic gaps in paramagnetic organic CT salts with mixed regular stacks arise naturally for partial CT and $z<{z}_{c}$. Valence-bond analysis of CT excitations models the physical properties of organic complexes with overlapping sites and intermediate $\ensuremath{\gamma}$.

NMP‐TCNQ: A Realization of the Disordered Hubbard Model

AbstractSeveral quantitative models have been proposed for the electronic structure of the organic charge transfer salt NMP‐TCNQ. They range from the periodic Hubbard model that emphasizes the role of electron correlation in the material, to models that emphasize the role of its intrinsic static disorder. A detailed study is made of the dielectric properties of the material, and it is found that the intrinsic static disorder depends on the electronic density of states at the Fermi level. Then, considering the system as a realization of the one‐dimensional disordered Hubbard model and using available experimental data, the intrinsic disorder and other band parameters are calculated.

Quasi-one-dimensional conductors

This review is intended to be an introduction to the physics of quasi-one-dimensional conductors which exhibit a Peirels instability. It is first shown that the Peirels instability occurs because of a fundamental instability in the electronic system. The mean field theory of the Peierls instability for the Fröhlich Hamiltonian of a one-dimensional electronic system which interacts with lattice modes of vibration is then discussed. The properties of the periodic lattice distortions and the accompanying charge density waves which result from the instability are described and the important effects of fluctuations and three dimensionality are considered. The observed properties of the mixed valence planar transition metal compound, K 2[Pt(CN) 4]Br 0.30·3H 2O, and the organic charge transfer salt, tetrathiofulvalinium tetracyanoquinodimethane, are then presented and they are interpreted in terms of the Peierls-Fröhlich model of one-dimensional conductor. Theoretical predictions for one-dimensional electronic systems in, which Coulomb interactions are important are also described and their relevance to tetrathiofulvalinium tetracyanoquinodimethane is considered.

Polycrystalline techniques for conductivity studies of organic charge transfer salts

Polycrystalline compaction conductivity measurements are extensively used to characterize newly synthesized quasi-one-dimensional organic charge transfer salts. This technique, however, can be both quantitatively and qualitatively misleading. Studies of the polycrystalline compaction and single crystal conductivities of a variety of tetracyanoquinodimethan (TCNQ) salts allow us to define the limits of reliability of the polycrystalline compaction technique, and denote its shortcomings. A new polycrystalline technique, the voltage shorted compaction (VSC), is described. The VSC has proven to be a valuable aid in characterizing the electrical properties of new materials where single crystals are difficult to produce.

Intrinsic and extrinsic magnetic susceptibilities of some TCNQ complexes

Measurements of the proton spin-lattice relaxation rate (T1)-1 in the quasi-one-dimensional organic charge transfer salt quinolinium (TCNQ)2 are reported over the magnetic field (H) range 1.2–10 kOe and the temperature (T) range 0.03–55 K. In the low temperature range (defined by gμBH ⪆ kT) the relaxation shows an activation energy type of behavior; T-11 = B exp [-ϵ(H)/kT], B = (15.0 ± 2.5) sec-1, and the activation energy (ϵ) is given by ϵ = AHδ, with δ = 1.65 ± 0.10 and A = (1.76 ± 0.34) x 10-2 K (kOe)-1.65. The mechanism responsible for this behavior is not yet identified.

Surface magnetism in organic charge transfer salts

We have investigated the magnetic properties of the charge transfer salts Qn(TCNQ) 2, Cs 2(TCNQ) 3 and TTF—TCNQ in the form of single crystals and after strong pressing or grinding to a fine powder, which introduces lattice defects and increases the surface area. It is found that for the former two compounds pressing or grinding leads to a non-linear, saturating component in the magnetisation field curves, whereas the effect is absent for TTF—TCNQ. It is suggested that this behaviour could arise from strongly coupled localised spins at the surface, i.e. surface magnetism in these materials.

Electronic spectra of the organic charge transfer salts TTT-I n

Electronic spectra of TTT—I n both in solution and in the solid state were measured between 8000 and 40,000 cm -1. The highly conducting phase TTT—I 1.5 is assigned as (TTT oTTT +) ( T 3) -. A phase TTT—I 2.7 is obtained and its possible electronic structure discussed. Conductivity results on both phases are presented.

Paramagnetic susceptibility of disorderedN−methylphenazinium−tetracyanoquinodimethanide

The organic charge-transfer salt $N\ensuremath{-}\mathrm{methylphenazinium}\ensuremath{-}\mathrm{tetracyanoquinodimethanide}$ (NMP-TCNQ) is described in terms of the one-dimensional disordered Hubbard model. For low temperatures this model reduces, via a perturbation expansion, to that of a disordered Heisenberg antiferromagnet. The magnetic susceptibility of the latter can be adequately described by the disordered classical Heisenberg model. Fitting of susceptibility data by the one-dimensional classical Heisenberg model provides us with values $t=0.055$ eV, ${U}_{\mathrm{eff}}=0.130$ eV, and $\ensuremath{\sigma}=0.136$ eV for the parameters of the original Hubbard Hamiltonian. The low-temperature heat capacity and strong-field magnetization are also analyzed.

Theory of one-dimensional hopping conductivity and diffusion

Hopping motion is considered in a linear chain containing an arbitrary density of particles that are not allowed to hop to occupied sites. The general case of two inequivalent lattice sites $A$ and $B$ is treated. Transition-rate equations are solved by Monte Carlo simulation and, where possible, by analytic techniques. Only for the case of equivalent sites do the results here agree with those recently obtained by Huber from nonlinear differential equations for site occupancy which neglect certain correlations. The conductivity is mean-field-like for equivalent sites, but shows sizeable departure for inequivalent sites, having an activation energy increased over the value. Here mean-field behavior is one where the only effect of forbidden hops to occupied sites is to reduce effective transition rates by a factor $1\ensuremath{-}n$, where $n$ is the average occupation number of the site to which a jump occurs. The velocity correlation function is shown to consist of a $\ensuremath{\delta}$-function part which reproduces the conductivity and a function $\ensuremath{\beta}(t)$ which is negative for times $tg0$. This is qualitatively quite different from the picture given by Huber's equations. Motion of a single distinguishable particle shows an anomalous ${x}^{2}\ensuremath{\propto}{t}^{\frac{1}{2}}$ dependence of the mean square displacement upon time $t$, but the displacement $X$ of all the particles does obey a diffusion relation ${X}^{2}\ensuremath{\propto}t$. This difference is explained in terms of the number of particles which have to be pushed aside in order for a particle to move a distance $x$, and in terms of the ensuing density fluctuation. Differences between time dependences and attempt fequencies as measured by bulk conductivity and microscopic probes such as NMR are noted and discussed in light of data on the one-dimensional superionic conductor $\ensuremath{\beta}$ eucryptite (LiAlSi${\mathrm{O}}_{4}$). Reinterpretation of NMR relaxation data on some of the organic charge-transfer salts is also suggested.

Impurity effects in the organic charge transfer salt Qn(TCNQ)2

Variations in the conductivity of QN(TCNQ)2 depending on sample preparation are correlated with the presence of doubly ionised TCNQ in the samples. Increasing TCNQ2- concentration results in decreasing conductivity and increasing activation energy. The influence of D+ impurities compensating the excess charge is discussed.

Short Notes

X-ray powder diffraction studies of two semiconducting crystal phases in the organic charge transfer salt (DEPE)(TCNQ) sub 4 has revealed almost identical electrical properties to those of metallic crystals. It is hypothesized that the similarity arises from the nature of the measurements themselves, i.e., electrode geometry. The possibility that a separate metallic phase for (DEPE)(TCNQ) sub 4 exists, is not, however, ruled out.