TSeF-TCNQ

Chemical alloying is simply replacing parts of the molecules by isostructural and isoelectronic parts. For example, alloys such as (TTF) . .(TSeF TCNQ [163], (TMTSF), t(TMTTF),C104 [164], and (TMTSF)2(C104), (ReO,), [165] have been obtained. 

Figure 8 Room-temperature conductivity

Figure 9 Longitudinal resistivity ph versus temperature for (TSeT)2Cl at ambient pressure (from Ref. 18), TTF [Pd(dmit)2]2 at 24 kbar (from Ref. 70), and TSeF-TCNQ at 32 kbar (from unpublished results of J. R. Cooper, D. Jerome, and E. M. Engler, 1978).

Figure 10 Normalized room-temperature conductivities versus pressure for single crystals of TSeF-TCNQ. At ambient pressure ob = 800 100 (fl-cm)-1, and cra and (tc are 4 2 (fl-cm)"1. (From Ref. 27.)...

One of the initial motivations for pressure studies was to suppress the CDW transitions in TTF-TCNQ and its derivatives and thereby stabilize a metallic, and possibly superconducting, state at low temperatures [2]. Experiments on TTF-TCNQ and TSeF-TCNQ [27] showed an increase in the CDW or Peierls transition temperatures (Tp) with pressure, as shown in Fig. 12 [80], Later work on materials such as HMTTF-TCNQ showed that the transitions could be suppressed by pressure, but a true metallic state was not obtained up to about 30 kbar [81]. Instead, the ground state was very reminiscent of the semimetallic behavior observed for HMTSF-TCNQ, as shown by the resistivity data in Fig. 13. One possible mechanism for the formation of a semimetallic state is that, as proposed by Weger [82], it arises simply from hybridization of donor and acceptor wave functions. However, diffuse x-ray scattering lines [34] and reasonably sharp conductivity anomalies are often observed, so in many cases incommensurate lattice distortions must play a role. In other words, a semimetallic state can also arise when the Q vector of the CDW does not destroy the whole Fermi surface (FS) but leaves small pockets of holes and electrons. Such a situation is particularly likely in two-chain materials, where the direction of Q is determined not just by the FS nesting properties but by the Coulomb interaction between CDWs on the two chains [10]. 

A Model for the Metal-Insulator Transition in TSeF-TCNQ (Abstract only) 301... 

X-Ray Scattering of TSeF-TCNQ and HMTSel -TCNQ (Abstract only) 361... 

Anisotropy in the Critical Behaviour of TTF-TCNQ and TSeF-TCNQ 469... 

The inverted band picture with hybridized bands appears to apply to TSeF-TCNQ, with both bands ordering at the same temperature, 29°K (Tl). This may be due to a larger tj. than in TTF-TCNQ or to longer correlation lengths and larger distortions on the donor chains (S3) or both. In HMTSF-TCNQ, the value of tj. is even larger, so that it behaves at low temperatures much more like a 3D semimetal (INV 3, S5, Jl). Pressures of the order of a few kbar are sufficient to increase tj. and 3D effects significantly in a number of TCNQ compounds (INV 10). 

A MODEL FOR THE METAL-INSULATOR TRANSITION IN TSeF-TCNQ... 

TSeF-TCNQ differs from its isostructural analog, TTF-TCNQ, in several important ways 1) It has only one... 

Some results of a X-ray diffuse scattering investigation on the charge transfer compounds TSeF-TCNQ and HMTSeF-TCNQ are presented. In the metallic state the results show the formation of a 1-D distortion or a Kohn anomaly in the phonon spectrum. For TSeF-TCNQ this is observaibelow T 238 K and corresponds to a periodicity of (3.15+ 0.05) Ibl along the chain axis. For HMTSeF-TCNQ the distortion was detected even at room temperature and has a period of (2-7+ 0,1) Icl In both materials the distortion has both transverse components. In the insulating state of TSeF-TCNQ below 29 K the 1-D distortion on each stack are correlated to font a 3-D super-lattice. 

In the last few years a large amount of work has been devoted to the search for better organic charge transfer conductors. Most of the efforts have been concentrated on the study of the TTF-TCNQ family /TQ/ and of the selenium analogue family, TSeF-TCNQ /TSe-Q/. 

B. TSeF-TCNQ (x=l). I will relate the lowering of the metal-insulator transition temperature in TSeF-TCNQ in comparison to TTF-TCNQ to their different band structures. In particular the difference will be shown to relate to the magnitude of the overlap between the donor and acceptor wave functions, which will be referred to in this talk as the a axis hybridization. 

C. (TSeF)x(TTF) x(TCNQ) 0contrast effects of donor stack doping in TTF-TCNQ and in TSeF-TCNQ. 

While in TTF-TCNQ effects of donor stack doping (by TSeF) cause lowering of the metal-insulator transition temperature, in TSeF-TCNQ effects of donor stack doping (by TTF) cause an increase of the metal-insulator transition temperature. Moreover the relative lowering caused by doping in TTF-TCNQ... 

Since the TCNQ stack was shown in the previous discussion to drive the metal-insulator transition of TTF-TCNQ it is quite puzzling that the corresponding temperature in TSeF-TCNQ having the ame kind of stack, is lowered. Moreover the relationship between the existence of several phase transitions and the presence of 2 kinds of stacks in TTF-TCNQ makes the existence of a single phase transition in TSeF-TCNQ quite surprising. In the following I will show that the difference between the 2 compounds is caused by a different hybridization between the donor and acceptor electronic wave functions. 

For comparison of the hybridization in TTF-TCNQ and TSeF-TCNQ one should use an experimentally measured quantity sensitive to this parameter like the electron spin relaxation rates. In order to understand how the EPR linewidth has any bearing on the band structure, let us understand what relaxation processes contribute to the measured linewidth. The dominant... 

As to the direction along which the increased dimensionality occurs Examination of the lattice parameters1 of TSeF-TCNQ indicates that they are bigger than in TTF-TCNQ. However, the growth of the a-axis lattice parameter is over-compensated by the respective increase of the van der Waals radius of the selenium in comparison to the sulfur. Therefore the overlap in this direction between the wave functions of the TCNQ and TSeF molecules is larger than the overlap between TCNQ and TTF. 

In conclusion we have shown that the hybridization gap is indeed larger in TSeF-TCNQ than in TTF-TCNQ. However, in order to affect the tendency of the system to undergo a Peierls transition, the hybridization gap should be at... 

Combining the total susceptibilities values for TTF-TCNQ" and TSeF-TCNQ and the measured g-values yield the following conclusion The TCNQ stack... 

Our understanding of the difference between TTF-TCNQ and TSeF-TCNQ will enable us to explain the effects of donor stack doping in these two compounds. Donor stack doping by TTF in TSeF-TCNQ interferes with the hybridization between the donor and acceptor stacks. Therefore the 3 dimensional ordering temperature of the undoped acceptor stack should increase as the hybridization is diminished. The donor stack ordering temperature would be determined on the other hand by competition between the doping effect which will tend to lower it and the diminished hybridization which will tend to increase it. 

ANISOTROPY IN THE CRITICAL BEHAVIOR OF TTF-TCNQ AND TSeF-TCNQ ... 

We present the results of o detailed study of the critical behavior in the a and b axis resistivity of TTF-TCNQ and TSeF-TCNQ. We find that in TTF-TCNQ, df°/dTand df /dT diverge os T - T — 0 with the some critical exponent while in TSeF-TCNQ, dPa/dT and df /dT diverge with different critical exponents. These results ore compored with various models for the origin of the criticol behavior in the resistivity. 

TSeF-TCNQ in the vicinity of the three dimensional ordering temperature T — 28°K. 

The ratio of the transverie to the longitudinal resistivity of TSeF-TCNQ is about... 

In conclusion, our experimental results suggest that the conductivity above T in TSeF-TCNQ is dominated by an anomaly in the electron-phonon scattering associated with the onset of a three dimensional Peierls distortion. VWiile the data in TTF-TCNQ is more complicated, we expect that near T the same... 

Fig. 1 Behavior of the logarithmic derivative of the a-axis resistance in TSeF-TCNQ near the critical temperature.

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