Defect dependence of the dielectric permeability of Qn(TCNQ)2

The conductivity and 9.1 GHz dielectric constant of neutron irradiated Qn(TCNQ)₂ single crystals are strongly affected by defect concentrations less than 1 %. This indicates long range electronic coherence of the order of 100 lattice constants in the non-irradiated material for which the coherence length may still be impurity limited. It is suggested that a collective excitation of electrons is responsible for the temperature dependence of the large dielectric constant.     

Phase transition of [(C6H5)3PCH3]+(TCNQ)-2 and electrical transport properties

The electrical conductivity at 10GHz, the dielectric constant, and the thermoelectric power (TEP) of [(C₆H₅)₃PCH₃]⁺(TCNQ)^-₂, from 230 up to 400 K, have been measured. This organic quasi-one-dimensional solid undergoes a first order phase transition at 314 K. At the transition the conductivity increases by a factor of 2.2 and the activation energy drops to 0.26 from 0.31 eV. At 314 K TEP decreases abruptly from -75 to -60μVK^-1 and remains almost constant for T > 314 K. The dielectric permeability ?₀ is constant and equal to 5 in the low temperature phase, increases abruptly by 7% at the transition, and then depends strongly on temperature in the high temperature phase. Results of the high temperature phase are interpreted in terms of a strongly correlated salt.     

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.     

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.     

The thermoelectric power of MEM(TCNQ)2

MEM(TCNQ)₂ undergoes a first order semiconductor to metal transition at 340.8 K. We have measured the thermoelectric power (TEP) of MEM(TCNQ)₂ in the temperature range above 335 K. Above the transition the TEP is ?65 μV/°K, in the low temperature phase it is strongly temperature dependent and approaches zero near the transition. The indicated loss of spin entropy at the transition is discussed.     

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.     

Spin susceptibility of DMM (TCNQ)2

DMM(TCNQ)₂ crystallizes into both monoclinic and triclinic crystal structures. The TCNQ stacks in the monoclinic structure tetramerized below 260K and the temperature dependence of the susceptibility can be fit to Bulaevskii's theory with an exchange energy of ≈260K. The susceptibility of the triclinic structure is Curie-Weiss from room temperature to 35K where it increases much less rapidly becoming independent of temperature below 6K. This is interpreted in terms of a linear one dimensional antiferromagnetic chain.     

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.     

Magnetic and electric properties of NMeQn(TCNQ)2

It is demonstrated that the magnetic and electric properties of N-methyl-quinolinium (TCNQ)₂ are similar to that found in other well conducting complex TCNQ salts.     

Phase transition in MEM(TCNQ)2 single crystals by infrared reflection spectroscopy

Between 280 and 320 K the MEM⁺ ions in the salt MEM(TCNQ)₂ show an order-disorder phase transition. Moreover MEM(TCNQ)₂ undergoes the phase transition at 335 K. The influence of these two transitions on the i.r. reflectivity spectrum is studied.     

Triplet excitons in (TEA) (TCNQ)2

We report the results of EPR studies on the ionic-radical salt (TEA)⁺ (TCNQ)₂^- composed of an oganic free radical anion and a diamagnetic cation. Between about 40 and 80 K this crystal exhibits the triplet exciton EPR spectrum characteristic of an alternating chain of spins. The triplet spin Hamiltonian parameters are |D| = 44 ± 2 G and |E| = 5.5 ± 1 G. The directions of the zero field splitting principal axes are determined through single crystal rotation studies at 55 K and related to the crystal structure.     

High magnetic field behavior of MEM(TCNQ)2

The charge-transfer complex MEM(TCNQ)₂ is a spin-Peierls system with a non-magnetic, singlet ground state at T=0. We report high-field magnetization data which provide some evidence for a new magnetic spin-Peierls phase at fields above 190 kOe. The experimental results are compared to those for TTFCuBDT.     

Frequency dependence of ionic conductivity and dielectric relaxation studies in Na3H(SO4)2 single crystal

Electrical impedance measurements of Na₃H(SO₄)₂ were performed as a function of both temperature and frequency. The electrical conductivity and dielectric relaxation have been evaluated. The temperature dependence of electrical conductivity reveals that the sample crystals transformed to the fast ionic state in the high temperature phase. The dynamical disordering of hydrogen and sodium atoms and the orientation of SO₄ tetrahedra results in fast ionic conductivity. In addition to the proton conduction, the possibility of a Na⁺ contribution to the conductivity in the high temperature phase is proposed. The frequency dependence of AC conductivity is proportional to ω^s. The value of the exponent, s, lies between 0.85 and 0.46 in the room temperature phase, whereas it remains almost constant, 0.6, in the high-temperature phase. The dielectric dispersion is examined using the modulus formalism. An Arrhenius-type behavior is observed when the crystal undergoes the structural phase transition.     

Anisotropic electrical conductivity,phase transition and thermal hysteresis of a charge-transfer salt dibutylammonium bis-7,7,8,8-tetracyanoquinodimethane DBA（TCNQ）2

This paper reports that a charge-transfer salt dibutylammonium bis-7,7,8,8-tetraeyanoquinodimethane [DBA （TCNQ）2] has been prepared. The temperature dependences of the DC electrical conductivity of the DBA （TCNQ）2 single crystal measured along the crystallographic a, b, and c axes are reported. The crystal shows semicondueting behaviour and the room-temperature conductivities are highly anisotropic （σa = 3.63× 10^-4S/cm, σb = 2.84× 10^-6S/cm, and （σe = 1.82 × 10^-5S/cm）. Particularly, a sharp semiconductor to semiconductor transition has been observed around 270 K on the resistivity curves measured under cooling and heating. In addition, thermal hysteresis phenomena on conductivity and differential scanning calorimetry curves are also reported.     

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.     

Giant dielectric constant in the one-dimensional semiconductor Meφ3As (TCNQ)2

We have measured a large highly anisotropic dielectric constant in methyltriphenylarsonium ditetracyanoquinodimethanide as a function of frequency and temperature. We present a model based upon finite barriers in the conducting strands. Magnetic resonance and electrical conductivity are used in testing the model; good agreement is obtained.     

Proton conduction in (NH4)3 H(SO4)2 single crystals

D.C. electrical conductivity, DTA and coulometric studies on (NH₄)₃ H(SO₄)₂ single crystals are made. Conductivity is markedly anisotropic with maximum along c¹ direction. A sudden jump in the conductivity plot along c¹ direction at 413 K is supported by a large endothermic peak in DTA, confirming the presence of transition at this temperature. The values of activation energy calculated from conductivity measurements indicated that the charge carriers are protons. This was further confirmed by coulometric experiment where the gas evolved was hydrogen, as established by a gas chromatograph and the volume of H₂ released agreed with that expected from electrolysis. The mechanism of protonic conduction in this crystal is discussed.     

Defect concentration dependent phase transition in the organic quasi-one-dimensional conductor N-Propyl-Quinolinium (TCNQ)2

Anomalous changes in magnetic and electrical properties indicative of a phase transition at T_c ～ 220 K is found in the quasi one dimensional complex charge transfer salt N-propyl-quinolinium (TCNQ)₂. A series of samples with defects induced by neutron irradiation is investigated. The physical properties above 220 K are little affected by the radiation damage, however the temperature of the anomaly is shifted to lower temperatures. A dose which is estimated to damage of the order of 1 % of the molecules lowers T_c from 220 K to 150 K.     

Crystal structure,NMR study,dielectric relaxation and AC conductivity of a new compound [Cd3(SCN)2Br6(C2H9N2)2]n

The crystal structure, the ¹³C NMR spectroscopy and the complex impedance have been carried out on [Cd₃(SCN)₂Br₆(C₂H₉N₂)₂]_n. Crystal structure shows a 2D polymeric network built up of two crystallographically independent cadmium atoms with two different octahedral coordinations. This compound exhibits a phase transition at (T=355±2 K) which has been characterized by differential scanning calorimetry (DSC), X-rays powder diffraction, AC conductivity and dielectric measurements. Examination of ¹³C CP/MAS line shapes shows indirect spin–spin coupling (¹⁴N and ¹³C) with a dipolar coupling constant of 1339 Hz. The AC conductivity of this compound has been carried out in the temperature range 325–376 K and the frequency range from 10⁻² Hz to 10 MHz. The impedance data were well fitted to two equivalent electrical circuits. The results of the modulus study reveal the presence of two distinct relaxation processes. One, at low frequency side, is thermally activated due to the ionic conduction of the crystal and the other, at higher frequency side, gradually disappears when temperature reaches 355 K which is attributed to the localized dipoles in the crystal. Moreover, the temperature dependence of DC-conductivity in both phases follows the Arrhenius law and the frequency dependence of σ(ω,T) follows Jonscher's universal law. The near values of activation energies obtained from the conductivity data and impedance confirm that the transport is through the ion hopping mechanism.     

Phonons in the organic conductor TEA(TCNQ)2 studied by neutron inelastic scattering

The lattice dynamics of the one-dimensional organic conductor TEA(TCNQ)₂ is studied by inelastic neutron scattering at temperatures 77, 175 and 295 K. Special attention is paid to the phonons propagating along c¹ which is approximately along the conducting axis. These phonons show a well defined Kohn anomaly at 2k_F as well as gaps which we attribute to both the tetrametic structure of the crystals and to the electron- phonon coupling. The temperature variation of the phonons along c¹ is found to be typical for a system where electron-phonon coupling dominates.