Thermoelectric power of ETPP (TCNQ)2 and DEPE (TCNQ)4

Results of measurements of thermoelectric power of two complexes of TCNQ, namely, ethyltriphenylphosphonium (TCNQ)₂ and 1,2 Di (N-ethyl-4-pyridinium) ethylene (TCNQ)₄ in the temperature range 100–370 K are presented. Over a certain temperature range, thermoelectric power remains independent of temperature suggesting that the most likely mechanism of charge transfer is hopping.     

Infrared reflectivity of MTPA (TCNQ)2 single crystals

Polarized infrared reflectivity of single crystal MTPA (TCNQ)₂ was measured in the 40–4000 cm^?1 region and evaluated to obtain the dielectric function and conductivity. For E ⊥ b polarization, a very strong coupling between TCNQ intramolecular vibrations and electronic motion is observed. The bare electronic absorption is modelled by a sum of two classical oscillators.     

Specific heat and magnetic susceptibility at low temperature of two conductive TCNQ salts

The specific heat measurements between 1.4 and 4.4 K of acridinium (TCNQ)₂ and quinolinium (TCNQ)₂ salts show up a linear component; moreover, in presence of a strong magnetic field (H = 40kG), an artificial Schottky anomaly is revealed. Magnetic susceptibility experiments confirm the simultaneous existence of charge carriers, in a partially filled energy band, and localized paramagnetic centers. A standard energy band model is proposed to interpret these two properties.     

Two band transport and disorder effects in a series of organic alloys: (TTF1-xTSeFx)-TCNQ (tetrathiafulvalene-tetraselenafulvalene-tetracyanoquinodimethane)

We have measured the thermoelectric power of the organic solid solutions (TTF_1-xTSeF_x)-TCNQ from 300 to 20 K. We find that in TTF- TCNQ the conductivity is dominated by the TCNQ chain down to 58 K and the TTF chain dominates between 58 and 38 K. In TSeF-TCNQ the cation and anion conductivities are comparable at all temperatures. We also find that the disorder leads to band tailing and a finite density of localized states within the Peierls gap.     

Intrinsic and extrinsic magnetic susceptibilities of some TCNQ complexes

The magnetic susceptibilities (χ) of quinolinium·(TCNQ)₂, N-methyl phenazinium·TCNQ and Li·TCNQ were measured from 2 to 300 K and are discussed in connection with the low-temperature specific heats (C) measured by other authors, χ is decomposed into three parts: χ_d the temperature-independent part, χ_c, Curie-Weiss type paramagnetism, and χ_p, the remainder. Correspondingly, C is composed of three terms, γT, H/T² and αT³. The electronic state of these substances is discussed in terms of each type of susceptibility.The model, on which the above separation of χ and C is based, defines two types of electrons: localized electrons associated with a magnetic moment and band electrons. Though this model is useful phenomenologically, it is shown that the analysis of χ on the basis of this model indicates less band electrons and more localized electrons or stronger magnetic interactions than does that of C.     

Direct evidence for controlled band filling in the mixed crystal NMPxPhen1−xTCNQ

Raman spectra of the segregated stack mixed crystal NMP_xPhen_1?xTNCQ and the mixed stack compound Phen TCNQ are presented. By means of the relation between charge transfer and line shift, we give for the first time a direct proof for controlled band filling in the mixed crystal series, whereas for Phen TCNQ a zero charge transfer is obtained. Raman spectra of Phenazine and the related NMP⁺ have also been measured and used for comparison.     

Some magnetic properties of Mn(TCNQ)2

The EPR spectra of polycrystalline Mn(TCNQ)₂·3H₂O and Mn(TCNQ?d₄)₂ have been studied as a function of temperature from 1.5 K to 375 K. At very low temperatures the line width indicates an exchange interaction similar to that of other manganese salts. At 77 K and above the line is narrowed and shifted most likely through interaction with the electronic motion. The bulk susceptibility was measured at room temperature. The observed μ_eff=4.66 implies an antiferromagnetic coupling of the manganese ions.     

Peierls transitions in semimetallic one dimensional charge transfer salts

Peierls transitions in one dimensional charge transfer salts in which both donor and acceptor molecules have even valency such as TTF-TCNQ have been studied. Transitions involving macroscopic occupation of phonon states of wavevector $\text{k}{}_{\mathrm{<mtext>F</mtext>}}\text{(TTF)}\text{+ k}{}_{\mathrm{<mtext>F</mtext>}}\text{(TCNQ)}\text{=}\text{1}\text{2}\text{G}$ and |k_F(TTF) ? k_F (TCNQ)| can occur as well as transitions of wavevector 2k_F(TTF) [and 2k_F(TCNQ)].     

Spectroscopic investigation of pressure-induced phase transitions in TCNQ complex salts

In most of the TCNQ complex salts, conduction electrons are localized on specific TCNQ sites, so that these salts show nonmetalic behavior. The caesium salt, Cs₂(TCNQ)₃, is one of the 2:3 complex salts. In the crystal, TCNQ molecules form trimeric units, which consist of two TCNQ radical anion sandwiching a neutral TCNQ along the column. The rubidium salt, Rb₂(TCNQ)₃, also has a similar crystal structure to Cs₂(TCNQ)₃. We measured infrared absorption (IR) and Raman spectra for these salts under high pressure by using a diamond anvil cell. In the case of IR spectra, Cs₂(TCNQ)₃ showed a spectral change probably due to a pressure-induced phase transition. Similar feature was not clearly observed in the Rb₂(TCNQ)₃. On the other hand, the Raman spectra, Cs₂(TCNQ)₃ showed two phase transition at 2.5 and 4.1 GPa in the compression stage. The change from localization phase to delocalization phase occurred at latter transition with large hysteresis. Similar phase transition occurred at 3.2 GPa in the Rb₂(TCNQ)₃. The reason for the difference in transition pressure is that the ion radius of Rb⁺ is smaller than that of Cs⁺, because a small ion radius of the counter ion probably favors the charge localization-delocalization transition of the TCNQ column.     

Stabilization variation of organic conductor surfaces induced by π—π stacking interactions

The structures and stabilization of three crystal surfaces of TCNQ-based charge transfer complexes (CTCs) including PrQ(TCNQ)₂, MPM(TCNQ)₂, and MEM(TCNQ)₂, have been investigated by scanning tunneling microscopy (STM). The three bulk-truncated surfaces are all ac-surface, which are terminated with TCNQ molecular arrays. On the ac-surface of PrQ(TCNQ)₂, the TCNQ molecules form a tetramer structure with a wavelike row behavior and a γ angle of about 18° between adjacent molecules. Moreover, the dimer structures are resolved on both ac-surfaces of MPM(TCNQ)₂ and MEM(TCNQ)₂. In addition, the tetramer structure is the most stable structure, while the dimer structures are unstable and easily subject to the STM tip disturbance, which results in changeable unit cells. The main reasons for the surface stabilization variation among the three ac-surfaces are provided by using the 'π-atom model'.     

Low temperature EPR study of the “impurity” resonances in (TCNQ)- and (TCNQ)-2 salts

The so-called “impurity” resonances in polycrystalline Li⁺ (TCNQ)^-, Cu⁺ (TCNQ)^- and Cu⁺ (TCNQ)^-₂ have been studied at liquid helium-temperatures. The Li⁺ (TCNQ)^- resonance is partially resolved at low microwave powers with g_⊥ = 2.0025 ± 0.0005 and g_∥ = 2.008 ± 0.001. Itis found that the (TCNQ) and (TCNQ)^-₂ complexes give characteristics spectra. The possible origin of these resonances is discussed.     

Temperature and frequency dependence of the nuclear relaxation rate in Qn(TCNQ)2

The magnitude, frequency dependence and temperature dependence of the proton relaxation rates in Qn(TCNQ)₂ are explained in terms of a theory which treats the short wavelength response as coherent and one-dimensional, while the long wavelength response is dominated by anisotropic (intrachain vs interchain) diffusion. The diffusive character of the response near q = 0 is consistent with recent attempts to understand the electronic properties of Qn(TCNQ)₂ in terms of weak localization of states in one-dimension. Analysis of the data leads to the conclusion that the effective screened Coulomb interaction is relatively weak.     

Anomalous optical response with variation in band filling in linear chain conductors

We report the polarized infrared reflectance of ($\text{N}$-methylphenazinium)_x (phenazine)_1-x(tetracyanoquinodimethanide), (NMP)_x(Phen)_1-x(TCNQ) for 0.5≤x≤1.0. We demonstrate that the “Drude edge” in TCNQ salts is related to the conduction electron charge density. An anomalous rapid red shift of the plasma frequency, observed for $\text{x ～}\text{2}\text{3}$, may signal the emptying of the conduction electrons from half of the chains and/or a crossover from small U/W (coulomb repulsion/bandwidth) to large U/W behavior.     

Comment on the 4kF instability of quasi-one-dimensional systems

It will be shown that the 4k_F-instability, as observed in some TCNQ compounds, may be understood as a second-order effect of the electron gas. Moreover, the presented approach suggests a further instability at 6k_F.     

Temperature dependent microwave dielectric constant of TTF-TCNQ and related one-dimensional conductors

The results of an experimental study of the low temperature (4.2–40 K) complex dielectric constant of the organic conductors (TTF)(TCNQ), (TMTTF)(TCNQ), (DSeDTF)(TCNQ), (TSeF)(TCNQ), and the alloy (TTF)_0.97(TSeF)_0.03(TCNQ) are described. The similar features observed in these different systems suggest that a common mechanism is involved.     

Model for conduction in TCNQ salts

We have extended our model for conductivity, σ, and its temperature, T, dependence to a group of molecular conductors including (Qn) (TCNQ)₂, (Adz) (TCNQ)₂ and (Adn) (TCNQ)₂. We have parametrically fit and then quantitatively calculated σ(T) for each of these materials as a product of an activated carrier concentration (600K, 450K, and 350K respectively) and a strongly T-dependent mobility determined by known electron-phonon coupling to the molecular vibrations of TCNQ.     

Observation of quasi-universal magnetic behavior in a random exchange Heisenberg antiferromagnetic chain: Neutron irradiated quinolinium (TCNQ)2

Low field electron spin resonance measurements of the magnetic susceptibility (χ) and absorption linewidth over the temperature range 0.04 – 300 K are reported for quinolinium (TCNQ)₂ into which increased amounts of disorder have been introduced by fast neutron irradiation. It is found that below 20 K, χ = AT^-α; A increases linearly with the irradiation dose, but $\text{α (? 0.8)}$ is almost independent of it, in agreement with the quasi universal behavior predicted by recent renormalization calculations for a random exchange Heisenberg antiferromagnetic chain. Measurements of the g-shift at 4.2 K range indicate that all of χ is associated with TCNQ chains. These results are discussed in terms of the renormalization calculation of Soos and Bondeson.     

Surface magnetism in organic charge transfer salts

We have investigated the magnetic properties of the charge transfer salts Qn(TCNQ)₂, Cs₂(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.     

Characterization of bronze Roman coins of the fifth century called <Emphasis Type="Italic">nummi</Emphasis> through different analytical techniques

Mixing of different organic charge-transfer-complexes (CT-complexes) might allow the adjustment of the optical and morphological properties of the resulting material system. In this work, a study of two CT-complexes, mixed by thermal coevaporation at different concentrations by substituting only the acceptor molecules, is presented. Electron diffraction patterns, which were collected on samples of the ternary system of the prototypical CT-complexes DBTTF-TCNQ and DBTTF-F₄TCNQ do not show any indication of a mixed crystalline phase or novel crystalline order. X-ray diffraction measurements additionally confirm the phase separation in the ternary system. However, upon mixing of the complexes, the degree of crystallinity of the individual phases is reduced. This effect correlates with the mixing ratio of the CT-complexes in the ternary compound. Furthermore, we do not observe a shift or the appearance of new peaks in the infrared spectra of (DBTTF-TCNQ) _x :(DBTTF-F₄TCNQ)_1?x . Hence, there is no indication for a pronounced or novel chemical interaction between the individual CT-complexes in the mixed compound.     

Isotope effect on the Peierls transition temperature of TTF-TCNQ

Conductivity measurements on isotopically substituted single crystals of TTF-TCNQ show a relatively large shift in the phase transition temperatures at 52.5 K (T₁) and 37.7 K (T₃). Deuteration of both TTF and TCNQ chains increases T₁ by 0.75 ± 0.15 K, N¹⁵ substitution increases T₁ by 0.6±0.1 K while deuteration of the TTF chains apparently has no significant effect. Different possible interpretations of these results are briefly discussed.