I3 Salt Research Articles

Infrared and Raman Studies of Charge Ordering in Organic Conductors, BEDT-TTF Salts with Quarter-Filled Bands

This paper reviews charge ordering in the organic conductors, β″-(BEDT-TTF) (TCNQ), θ-(BEDT-TTF)2X, and α-(BEDT-TTF)2X. Here, BEDT-TTF and TCNQ represent bis(ethylenedithio)tetrathiafulvalene and 7,7,8,8-tetracyanoquinodimethane, respectively. These compounds, all of which have a quarter-filled band, were evaluated using infrared and Raman spectroscopy in addition to optical conductivity measurements. It was found that β″-(BEDT-TTF)(TCNQ) changes continuously from a uniform metal to a charge-ordered metal with increasing temperature. Although charge disproportionation was clearly observed, long-range charge order is not realized. Among six θ-type salts, four compounds with a narrow band show the metal-insulator transition. However, they maintain a large amplitude of charge order (Δρ~0.6) in both metallic and insulating phases. In the X = CsZn(SCN)4 salt with intermediate bandwidth, the amplitude of charge order is very small (Δρ < 0.07) over the whole temperature range. However, fluctuation of charge order is indicated in the Raman spectrum and optical conductivity. No indication of the fluctuation of charge order is found in the wide band X = I3 salt. In α-(BEDT-TTF)2I3 the amplitude of charge order changes discontinuously from small amplitude at high temperature to large amplitude (Δρmax~0.6) at low temperature. The long-range charge-ordered state shows ferroelectric polarization with fast optical response. The fluctuation of multiple stripes occurs in the high-temperature metallic phase. Among α-(BEDT-TTF)2MHg(SCN)4 (X = NH4, K, Rb, Tl), the fluctuation of charge order is indicated only in the X = NH4 salt. α′-(BEDT-TTF)2IBr2 shows successive phase transitions to the ferroelectric state keeping a large amplitude of charge order (Δρmax~0.8) over the whole temperature range. It was found that the amplitude and fluctuation of charge order in these compounds is enhanced as the kinetic energy (bandwidth) decreases.

Pressure Effect on BDA-TTP Conductors

The pressure-induced conducting behavior of the charge-transfer salts (BDA-TTP)2X [BDA-TTP=2,5-bis(1,3-dithian-2-ylidene)-1,3,4,6-tetrathiapentalene; X=ClO4·H2O, ClO4, BF4·H2O, BF4, and I3] has been investigated. With increasing pressure, the transition temperature to insulator observed in the ClO4·H2O, ClO4, BF4·H2O, and BF4 salts increases. On the other hand, the resistance of the I3 salt as a function of temperature at 10.0 kbar shows a drop attributable to a superconducting transition with an onset at 4.5 K. The superconducting transition temperature (Tc) can reach 8.2 K by applying a pressure of 10.3 kbar, but gradually decreases with further increases in the pressure.

Synthesis, Properties, and Structures of 1,2 : 5,6-Bis(ethylenedithio)pyracylene and Its Radical Cation Salts

Abstract 1,2 : 5,6-Bis(ethylenedithio)pyracylene (2) was synthesized as an example of molecular design for a polyaromatic condensed-type donor with two ethylenedithio groups at the periphery. The redox potential of 2 indicates that this heterocycle is a reversible two-stage redox system with low oxidation potentials, which enhances the donor ability. This donor was readily oxidized to afford single crystal radical cation salts with hexafluorophosphate (PF6−) and triiodide (I3−) by electrocrystallization. The crystal structures of the radical cation salts, (2)3(PF6)2 and (2)2I3, show segregated stacks of donors and anions. The I3 salt presents a two-dimensional sheet along the ab plane similar to I3 salt of bis(ethylenedithio)tetrathiafulvalene.

New Series of Organic Metal. Radical Salts of Unsymmetrical Donors Containing Pyrazino Group

Abstract The I3, ClO4, and PF6 salts of unsymmetrical donors containing fused pyrazino groups (DMPDSDTF and DM(DMP)DSDTF) were prepared electrochemically and their electrical conductivities were measured. The I3 salt of DM(DMP)DSDTF exhibited a metallic behavior down to about 120 K. The radical salts of unsymmetrical donors showed much larger conductivities than those of the corresponding symmetrical donors containing pyrazino groups.

High Tc Superconducting States in Organic Metals, β-(BEDT-TTF)2X

Superconductivity in a series of isostructural organic metals, β-(BEDT-TTF)2X, where X=I3, I2Br, IBr2, (I3)1-x(IBr2)x, (IBr2)1-x(I2Br)x and (I2Br)1-x(I3)x, has been studied. The entire scheme of phase diagram connecting the two superconductors, one (I3 salt) with high- and low-Tc states and the other (IBr2 salt) with intermediate Tc, is described. Through the observation of stepwise anomaly in dR/dT, it was found that the suppression of the incommensurate lattice modulation in the I3 salt can be realized by alloying with IBr2 salt. The effect of alloying on the superconductivity was found to show marked differences in the vicinity of each different superconducting states. Strong correlation between the superconductivity and residual conductivity was found, leading to a systematic understanding of the whole scheme in terms of Anderson localization. Alloying and proton irradiation were found to show completely different effects on the pressure dependence of Tc, indicating that the mechanism of Tc reduction in the latter is not the same.

Superconductivity in β-(BEDT-TTF)2AuI2

The superconducting properties in β-(BEDT-TTF)2AuI2 is experimentally Investigated by the conductivity and the low field magnetization measurement. The temperature dependence of Hc2 shows the paramagnetic limited behaviour at low temperatures indicating the spin singlet superconductivity. Two kinds of Tc are obtained and ascribed to the impurity effect due to sample preparation process. Samples which become partially superconducting are studied, and we speculate the nonsuperconducting part is of layered structure. Low Tc and high Tc states are discussed in contrast to the case in the I3 salt. Possible contribution of phonon mode related to the superlattice formation is considered.

129I Mössbauer Spectroscopic Studies of Tetrathiafulvalene–Iodine Charge-Transfer Salts

Abstract The Mössbauer effect of 129I has been applied to investigate tetrathiafulvalene–iodine (TTF–In) charge-transfer salts at 16 K. The electrically conductive TTF–I0.71 salt shows a broad single-line with the isomer shift of −0.36 mm s−1, suggesting that the iodine atoms on the average are not completely ionic (I−1.0) but are negatively charged by about 0.9 (I−0.9) in the low temperature phase. This is due to the weak interactions of the iodine atoms with the sulfur atoms of the TTF molecules. This fact is consistent with the crystallographic data and the electron spin resonance spectra of the salt. This salt is recognized as a partially oxidized mixed-valence compound. The spectrum of the TTF–I3 salt consists of two sets of quadrupole octets, corresponding to the central and terminal iodine atoms of the slightly distorted I3− ion. The charge density localized on each iodine atom of the I3− ion is close to that of the isolated I3− ion. That is, the salt is regarded as a simple ionic compound, for which a high electrical conductivity can not be expected. It is confirmed that a small amount of the TTF–I2 salt coexists in the TTF–I0.71 phase obtained from the solution of excess TTF and iodine. In the salt the iodine atoms exist as polyiodides consisting predominantly of I3− units.

Polarised reflectance spectra of di(bis(ethylenedithiolo)tetrathiafulvalene)dibromoiodate(I), β-(BEDT-TTF)2IBr2

Polarised reflectance spectra of beta -(BEDT-TTF)2IBr2 are observed for single crystals prepared by slow diffusion and an electrochemical crystallisation method. The spectra are found to be essentially identical to each other and also very similar to those observed for the isomorphic compound beta -(BEDT-TTF)2I3 in the near-infrared region. The band width of the IBr2 salt evaluated in terms of the Drude model and the tight-binding approximation is found to be 0.63 eV which is almost equivalent to that of the I3 salt.