TMTSF DMTCNQ

HMTSF-TCNQ, which is believed to have strong coupling between chains, does not show ESR signal at room temperature [37]. The linewidth is presumably on the order of 4000 Oe. On the contrary, TMTSF-DMTCNQ, a Se compound with a narrow line ( 70 Oe at 300 K), has been taken as evidence of weak interchain coupling [38]. 

Figure 22 Logarithm of the normalized resistance log(R/R0) versus the absorbed ionizing dose for samples of the organic charge-transfer salt TMTSF-DMTCNQ 1, electron irradiations 2, x-ray irradiations. (Adapted from Ref. 89.)...

An important precursor to the Bechgaard salts, TMTSF-DMTCNQ [2], also showed evidence for a low-temperature semimetallic state at pressures below ca. 12 kbar [83], but in view of the similarities with the Bechgaard salts, it is probable that a fully metallic state is formed at higher pressures [2]. 

Because of the softness of organic metals one expects them to show interesting behavior under applied pressures. This had been demonstrated earlier by Jerome and co-workers on several compounds and in the case of TMTSF-DMTCNQ (DMTCNQ = dimethyltetracyanoqui-nodimethane) a pressure of 10 kbar transforms it abruptly from a Peierls semiconductor with Tm = 50 K to a metal at all temperatures (91). When the temperature-dependent resistance of the (TMTSF)2X family became known, the very low transition temperatures in some of the compounds suggested that these salts would easily become metallic, and maybe even superconducting, under pressure. 

However the elaboration of a new charge-transfer conductor, TMTSF-DMTCNQ, and the use of high pressure have provided a clue for the discovery... 

Figure 2 Temperature dependence of the longitudinal resistivity of TMTSF-DMTCNQ under 13 kbar in zero magnetic field and in a transverse field of 75 kOe, after [20],...

Results are described for three new highly conducting organic solids, all based on substituted TCNQ-derivatives TMTSF-2,5-dimethyl-TCNQ /TMTSF-DMTCNQ/ 1 1 shows "normal" conductivity vs.temperature behaviour, i.e. metallic conductivity at higher temperatures followed by a metal-semiconductor transition as the temperature is decreased below 5o K. 

The organic solid TMTSF-DMTCNQ /6/ /Fig. 1/ has a room-temperature doncudtivity of about 5oo (-Cl cmf1, and the temperature dependence of the conductivity shows "normal" behaviour i.e. the conductivity increases /by a factor of approximately lo/ tc a sharp maximum - in this case at 50 K. Below this temperature the material undergoes the characteristic metal-semiconductor transition. 

TMTSF-DMTCNQ deserves comparison with the conducting form of the paren. t system TMTSF-TCNQ /4/. The temperature dependences of the conductivities of both systems are very alike, but in TMTSF-TCNQ the maximum is situated at 71 K. 

The crystal structures of the two systems are also very alike. Both are triclinic with comparable unit-cell parameters and with the "herring-bone" structure known from TTF-TCNQ /8/. The most pronounced crystallographic difference between the two systems is a rather elongated /6.1 %/ b-axis in TMTSF-DMTCNQ. This is to be expected because the b-direc-tion is across the width of the molecules where the additional methyl-groups tend to push the chains apart thereb. y probably reducing the interchain coupling. 

TTF-MTCNQ /9/ /Fig.2/ and TTF-DETCNQ /10//Fig. 3/ both have room-temperature conductivities of the same order of magnitude as TMTSF-DMTCNQ, but the increase in conductivity with decreasing temperature is merely 20 and 50 %, respectively, in the broad maxims at 210 and 180 K. In TTF-DETCNQ a sharp decline in conductivity occurs at 110 K, while TTF-MTCNQ gradually becomes insulating. As crystal data are only available at present for TTF-DETCNQ, the discussion will be restricted to this material. 

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