Pressure investigation of the metal-semiconductor transition in TTF-TCNQ

The Electrical Conductivity of TTF-TCNQ Under Pressure

“…-We have measured the b-axis conductivity of three single crystals of the organic charge transfer salt TTF-TCNQ from 20-300 K under hydrostatic (He gas) pressures of up to 6 kbar. The main reason for this study was to clarify some results reported a year ago [1] which seemed to indicate that there were two transitions below 60 K in this compound and what is more important, that one of them was strongly pressure dependent. Although we have mainly investigated the region below 60 K we will also briefly mention the effect of pressure at higher temperatures in the metallic region.…”

A landau theory for the displacive phase transitions in TTF-TCNQ

“…The Landau theory for this special instability has been given in detail recently [28]. Note that this is only one of several possibilities [29], but it indicates that perhaps in TTF-TCNQ too, the glide-plane symmetry may give rise to a potential instability that may account for the transition observed at 48 K [8], or possibly for the weak first-order transition observed at 38 K [7,10,11].…”

“…This may be due to some compensation between the longitudinal phonon (giving a positive effect) and librations, or intra-molecular distortions (giving a negative effect). The transition at 38 K falls with pressure [10] (d In TL/dP = -2.6 % kbar -'), suggesting above-mentioned libron hardening (disc-brake effect), impeding the additional libration, or sliding motions. The very strong effect of TSeF impurities in suppressing TL [11] is also in line with this interpretation, since the transition is due to a disappearance of xF and appearance of Ef [for pattern (10) so that the a-axis is common to the twins but the c-axis isn't.…”

“…Therefore we may expect the electron-phonon coupling, which is inversely proportional to the force constant, to fall much more rapidly under pressure for the librational modes than for the longitudinal phonon. Thus the transition temperature for the sliding motion of the molecules may be expected to fall with increasing pressure, while the Peierls transition temperature may even rise (due to increased electron-phonon matrix element g with the increased overlap) [10]. Tp (at 54 K) rises slightly under pressure [18,52] (d In Tp/dP = 1.3 % kbar-1) but much less than [24] in (1) TSeF-TCNQ (7 % kbar-').…”

“…These points have local symmetry 2 (as in figure 8, but with TCNQ instead of TTF molecules ; see also figure 10). The nodes of up [for pattern (10), or the extrema of xF for pattern (11) [24], which attributes these reflections to very strong correlations caused by intramolecular electron-electron interactions U. (Large U compared with the electronic bandwidth.)…”