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.     

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.     

Far infrared polarization measurements of MEM(TCNQ)2 at room temperature and 4.2 K

Polarization measurements have been made in the far infrared absorption spectra on single crystals of MEM(TCNQ)₂ at 1.5, 4.2 and 298 K. We demonstrated experimentally that the direction of polarization of phonons at 126 and 156 cm⁻¹ below spin-Peierls transition are parallel to the c-axis. Secondly, we measured the absorption spectra of MEMI powder at 1.5 and 4.2 K and found that there is no absorption in the range from 130 to 170 cm⁻¹. This indicates that there is no intramolecular vibration of MEM in this region. It supports the assignment of 156 cm⁻¹ absorption as phonon. Thirdly, comparing the FIR absorption spectra at 4.2 and 298 K with linearly polarized radiation field parallel to the c-axis, we found that the height of the strong absorption band observed around 86–180 cm⁻¹ region at 4.2 K becomes lower with the raising of temperature to 298 K. We attributed this band to the pinned CDW. The lowest edge of the CDW is 70 cm⁻¹.     

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'.     

Configurational entropy in the organic conductor quinolinium (tetracyanoquinodimethane)2

The thermoelectric power (TEP) measurements as a function of temperature and concentration of defects are reported on neutron irradiated organic quasi-one-dimensional conductor Qn(TCNQ)₂. The temperature dependence of the TEP suggests a freezing out of the entropy due to short range interactions, while the concentration dependence is shown to be due to the change in configurational entropy.     

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.     

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.     

THERMOELECTRIC POWER OF A LOCAL PAIR MODEL AND ITS APPLICABILITY TO YBa2Cu3O7

A formula of thermoelectric power (TEP) for the local pair model has been derived. It has been found that from T_c to about 125K the TEP data of YBa₂Cu₃O₇ (Y-123) can be described by this formula. We have also found that the formula of the diffusion thermoelec-tricity in the free electron approximation can be used to express the TEP data of Y-123 in the region of 220 to about 300 K. It seems that a general transition occurs in the intermediate temperature region, that is from the regime in which the local pair subsystem is dominating to the regime in which the itinerant carrier subsystem is dominating.     

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.     

Microscopic interactions in CuGeO 3 and organic Spin-Peierls systems deduced from their pretransitional lattice fluctuations

CuGeO₃ exhibits a Spin-Peierls (SP) transition, at T _SP = 14.3 K, which is announced above 19 K by an important regime of one-dimensional (1D) pretransitional lattice fluctuations which can be detected until about 40 K using X-ray diffuse scattering investigations. A quantitative analysis of this scattering shows that in this 1D direction the correlation length follows the “universal” behaviour expected for the thermal fluctuations of a real order parameter which characterizes the lattice dimerization. This allows to define a 1D mean-field temperature, T _SP ^MF , of about 60 K and invalidates any mean field scenario for the SP transition of CuGeO₃. As T _SP ^MF is as high as 4 T _SP we propose that the 3D-SP order is achieved by the interchain coupling between 1D solitons which form below about 16-20 K. CuGeO₃ being in the non-adiabatic regime, it is also suggested that the observed pretransitional fluctuations of CuGeO₃ originate from the X-ray scattering on a very broad damped critical response of lower frequency than the “critical” phonon modes. From the quantitative analysis of the 1D fluctuations we also estimate the microscopic parameters of the SP chain. These parameters allow to locate CuGeO₃ close to the quantum critical boundary separating the gapped SP ground state to the ungapped anti-ferromagnetic ground state. The vicinity of a quantum critical point emphasizes the role of the quantum and non-adiabatic fluctuations and the importance of the interchain coupling in the physics of CuGeO₃. Finally we compare these findings with those obtained for the organic SP systems (BCPTTF)₂PF₆, (TMTTF)₂PF₆ and MEM(TCNQ)₂. From a similar analysis of the pretransitional lattice fluctuations it is found that (BCPTTF)₂PF₆ and (TMTTF)₂PF₆ are located on the SP gapped classical-quantum boundary and are in the adiabatic regime where the fluctuations lead to the formation of a pseudo-gap in the spin degrees of freedom. Differently, we place MEM(TCNQ)₂ inside the SP quantum phase around the crossover line between the adiabatic and non-adiabatic regimes. Received 13 September 2000 and Received in final form 6 February 2001     

Dielectric permeability of the quasi-one-dimensional charge transfer salts: Qn(TCNQ)2 and NMeAd(TCNQ)2

The dielectric constant and conductivity of the high conductivity organic charge transfer comples salts Qn(TCNQ)₂ and NMeAd(TCNQ)₂ are measured at 9.1 GHz in the temperature range 4–320 K. The large increase with temperature of the dielectric constant is at variance with theoretical models describing the systems as metals or semiconductors at high temperatures. A model of localised states requires unreasonably large dipole moments in the excited state to fit with observations.     

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.     

Two dimensional ordering and fluctuations in alpha(')-NaV2O5

X-ray diffraction measurements of Bragg and diffuse scattering associated with charge ordering in the inorganic compound alpha(')- NaV2O5 show a continuous phase transition at a temperature of about 33.1 K. Many of this material's properties suggest a spin-Peierls transition, as established in CuGeO3 and MEM(TCNQ)(2). We compare the order parameter as well as fluctuations in the order parameter in these materials, and conclude that alpha(')- NaV2O5 is dissimilar, and that the transition is two dimensional in nature, due to charge ordering associated with the fractional average valence at the vanadium site within the orthorhombic a- b plane.     

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.     

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.     

Low concentration of the superconducting phase and thermoelectric power in 90 K superconductors

The thermoelectrical power (TEP) and the electrical resistivity behaviour of three 90 K superconductors viz, Y₁Ba₂Cu₃O_7−x , Sm₁Ba₂Cu₃O_7−x and Gd₁Ba₂Cu₃O_7−x , after the specimens were quenched from the sintering temperature (920°C) to 77 K, are reported. Interestingly the Y₁₂₃ specimen, which has the presence of trace amount of the orthorhombic phase in an otherwise tetragonal phase and does not show a superconducting transition down to 77 K shows zero TEP around 82 K, theT _c for the well oxygenated specimen. The Sm and Gd specimens on the other hand show completely tetragonal structure, semiconducting behaviour in resistivity and no zero TEP up to 77 K. It is argued that the critical concentration of the superconducting phase necessary to make the TEP zero is much smaller than that required for zero resistivity.     

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.     

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.     

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.