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.     

2-3-Benzacridinium (TCNQ)2: A small bandgap semiconductor

Single crystal conductivity and thermoelectric power experiments on 2-3-BAd(TCNQ)₂ are interpreted in terms of a highly anisotropic band semiconductor model. This behaviour is contrasted to that found in Ad(TCNQ)₂ and is claimed to be due to the difference in the dipole moments of the donors.     

Band filling and disorder in molecular conductors

We have systematically probed the roles of band filling and disorder in molecular conductors utilizing the (NMP)_x(Phen)_1-x(TCNQ)(0.5 < × ≤ 1.0) system. Conductivity results show a semiconducting behavior with charge carriers activated to extended states with a large strongly temperature dependent mobility. The energy gap is found to decrease with decreasing band filling, varying as x². The inconsistency of these results with various disorder model is indicated.     

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.     

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.     

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.     

Static dielectric constant of disordered organic quasi one-dimensional conductors: Localization by defects

We have measured the low temperature dielectric constant ? of two similar quasi one-dimensional organic conductors, N-Me-iso Qn(TCNQ)₂ and Qn(TCNQ)₂. For N-Me-iso Qn(TCNQ)₂ below 10 K, ? is independent of temperature and is frequency independent in the range 5 × 10⁵ Hz to 9 × 10⁹ Hz, within the 50% experimental uncertainty. Thus we believe the low temperature microwave dielectric constant to be a good approximation of the static value in this salt. For Qn(TCNQ)₂ at low temperatures, the relation ? ∝ (c+c₀)^-2 holds, where c is the defect concentration and c₀ is an effective defect concentration of the nominally pure material. This relation is predicted by the model of interrupted metallic strands with energy spacings larger than kT, and it indicates that electrons are strongly localized by defects along the conducting chains.     

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'.     

Effect of pressure on the spin-Peierls temperature

Experiments have shown that a hydrostatic pressure increases the spin-Peierls transition temperature (T_c) in MEM-(TCNQ)₂ while a lowering is observed in TTF-CuBDT. A mean field theory, involving an anharmonic coupling between the distortion and the strain and a possibility for both structural and spin-Peierls instabilities is presented. From this theory, we conclude that, depending on the sign of the coupling energy between the dimerization soft mode and the strain, T_c can increase or decrease with pressure. Without this anharmonic coupling, T_c decreases with pressure.     

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.     

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.     

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)].     

Phase phonons and intramolecular electron-phonon coupling in the organic linear chain semiconductor TEA(TCNQ)2

The vibrational excitations responsible for the remarkable series of infrared absorption bands observed in the organic linear chain semiconductor TEA(TCNQ)₂ are identified to be the phase phonons which result from the coupling of the conduction electron molecular orbital to the totally symmetric vibrations of the TCNQ molecule.     

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.     

An interpretation of the 2kF and 4kF x-ray scattering for tetrathiafulvalene tetracyanoquinodimethane (TTF-TCNQ)

A simple interpretation is given of the 2k_F and 4k_F X-ray scatterings observed for TTF-TCNQ. It is suggested that the TCNQ chains have a simple tight-binding band, and that Coulomb interactions are important for the TTF chains. The charge transfer between the chains then gives rise to the condition k_F(TTF) = 2k_F(TCNQ). Therefore the X-ray scattering is interpreted as arising from Peierls instabilities on both types of chain.     

First-principle study of electronic structure and stability of Sn0.5Sb0.5O2

A theoretical study on Sb-doped SnO₂ has been carried out by means of periodic density functional theory (DFT) at generalized gradient approximation (GGA) level. Stability and conductivity analyses were performed based on the formation energy and electronic structures. The results show that Sn_0.5Sb_0.5O₂ solid solution is stable because the formation energy of Sn_0.5Sb_0.5O₂ is −0.06 eV. The calculated energy band structure and density of states showed that the band gap of SnO₂ narrowed due to the presence of the Sb impurity energy levels in the bottom of the conduction band, namely there is Sb 5s distribution of electronic states from the Fermi level to the bottom of conduction band after the doping of antimony. The studies provide a theoretical basis to the development and application of Sn_1−xSb_xO₂ solid solution electrode.     

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.