HMTTF

Therefore, HMTTF-TCNQF4 has no partially full bands and hence is not a metallic conductor. 

Figure 6.42 Crystal structure of HMTTF-TCNQ. (a) Projection on the plane perpendicular to the stacking axis and (b) projection on a plane containing the stacking axis. (After Greene et ai, 1976.)...

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]. 

Figure 13 Longitudinal resistivity of HMTTF-TCNQ at various pressures, compared with HMTSF-TCNQ. (From Ref. 81.)...

Fig. 3 gives the structures of (i) six classes of good one-electron donors [TTF and friends HMTTF and friends TTT and friends BEDT-TTF perylene, and MPc (metal phthalocyanines)] and (ii) three classes of good one-electron acceptors (TCNQ, TNAP, and DCNQI). All these donors and acceptors are flat, or almost perfectly flat, molecules this requirement seems almost self-evident if one wants to form a tight crystal lattice with good intermolecular overlap. It is conceivable that this requirement could be relaxed, but a good example of this has not been found. It is known that there are small non-planarities (e.g. in BEDT-TTF) but it is not clear whether this is critical for the solid-state properties. 

Q- TTF-TCNQ (above 58 K) TMTSF-TCNQ (black form) NMP-TCNQ TSF-TCNQ (above 40 K) HMTSF-TNAP TMTTF-TCNQ HMTTF-TCNQ... 

---