Strong-field anomalous transport and burst-like $70\pm 10$ MHz current oscillations in ordered tetrathiafulvalene iodide (TTF-I$_n$, $n\cong 0.7$): mechanism of the proposed electrically driven iodine-sublattice order-disorder transition

Ordered tetrathiafulvalene iodide (TTF-I$_n$, $n\cong 0.7$) is a quasi-one-dimensional organic conductor built from incommensurate tetrathiafulvalene (TTF) and iodine sublattices, in which the iodine ions are expected to be highly mobile along the channels between TTF stacks. In 1981, Latif and Kroemer reported that strong pulsed electric fields (thresholds of order $2\times 10^2$ V/cm) drive this crystal through anomalous transport states: negative-resistance-like switching, hysteresis, long-lived conductance memory, and burst-like current oscillations at a sample-independent frequency of $70\pm 10$ MHz. They postulated an electrically driven order-disorder transition of the iodine sublattice - field-induced loss of transverse iodine order followed by localization of electronic carriers on the TTF stacks - while stating that the origin of the oscillations, direct structural evidence under bias, the iodine ionic mobility, and the bulk-versus-interface localization of the changes were all undetermined. The problem of identifying the mechanism of these field-induced anomalies, and specifically of testing the iodine-sublattice disordering postulate, remains open.

Background

Tetrathiafulvalene (TTF; the older literature spells it tetrathiofulvalene) is an organic electron-donor molecule that forms crystalline charge-transfer salts - solids built from segregated stacks of ionized donor molecules and counter-ions - which conduct electricity preferentially along the stack direction and are therefore called quasi-one-dimensional conductors. Oxidation of TTF by iodine gives the non-stoichiometric mixed-valence salt tetrathiafulvalene iodide, written TTF-I$_n$, whose composition parameter $n$ (iodine atoms per TTF) is not fixed by valence but varies over a finite homogeneity range; the conducting ordered phase has $n\cong 0.7$.

Structurally, ordered monoclinic TTF-I$_n$ consists of two interpenetrating sublattices: an almost tetragonal arrangement of positively charged TTF stacks, and a monoclinic iodine sublattice whose ions sit in channels between the stacks. The two sublattices are incommensurate along the stack direction - their periods do not stand in a ratio of small integers - so the iodine positions are not locked into a fixed registry with the TTF backbone. The equilibrium iodine spacing along a channel (4.6-5.0 Å) only slightly exceeds the ionic diameter (4.32 Å), so direct iodine-iodine core repulsion is weak, and a calculation for the bromide analog found the locking potential exerted by the TTF sublattice on the halogen sublattice to be small (Theodorou, 1979). The dominant ordering interaction is Coulomb repulsion between iodine ions themselves. These facts imply that iodine ions should be unusually mobile along their channels, with their motion limited in real crystals by pinning at impurities and defects, analogous to the pinning of dislocations or magnetic domain walls.

The halogen sublattices of TTF salts do undergo genuine order-disorder transitions. Ordered monoclinic TTF-I$_n$ is stable only for approximately $0.71\lesssim n\lesssim 0.77$; at slightly lower iodine content the stable phase is one in which the halogen ions occupy random positions within the channels - a 'gaslike' disordered phase - while the TTF sublattice is essentially unchanged. The two forms of TTF-I$_n$ were distinguished by their optical properties (Warmack and Callcott, 1976), a temperature-dependent reflectance study of (TTF)I$_{0.71}$ appeared in 1980 (Cao, Yakushi and Kuroda), and a broad specific-heat anomaly associated with ordering of the chlorine sublattice was later observed in (TTF)Cl$_{0.67}$ (Madison et al., 1982). Separately, the dc conductivity of TTF halides had long been anomalous: enormous, erratic hysteresis during temperature cycling (Warmack, Callcott and Watson, 1975), which those authors attributed to changes within the samples. The bromide analog TTF-Br$_{0.79}$ was meanwhile shown to undergo a commensurate Peierls transition - a metal-to-insulator transition driven by a periodic lattice distortion that gaps the electronic band at the Fermi wave vector - near 170 K (Chaikin et al., 1980), the conventional pathway by which these quasi-one-dimensional salts lose metallic conductivity.

In the 1981 experiment, Latif and Kroemer applied short (300 ns to 6.5 µs) voltage pulses between contacts on TTF-I$_n$ ($n\cong 0.7$) needle crystals. Above threshold bias fields of order $2\times 10^2$ V/cm the response became anomalous: the current peaked and then fell with increasing voltage, as if the differential resistance turned negative; the current also decayed during a single constant-voltage pulse; repeated pulsing cumulatively drove the crystal into a low-conductance state that persisted for seconds to days after the field was removed (a memory effect) and could be reversed by pulses of opposite polarity or by aging; and at the highest fields the current broke into burst-like oscillations, several rapidly decaying bursts per pulse, at a frequency of $70\pm 10$ MHz - the only numerical quantity in the entire data set that reproduced from sample to sample. All other numbers (threshold voltages, current amplitudes) fluctuated erratically even for repeated measurements on the same sample. Similar effects were seen in TTF-Br$_n$. Multi-contact voltage probing showed the conductance changes to be localized near one current-carrying contact, but the authors could not decide with certainty between an interface (electrochemical) origin and a bulk origin close to the contacts; silver-paint contacts were shown to react chemically with the crystal, forming a rectifying interface layer (presumably AgI).

The authors postulated a mechanism: the electric field drags the mobile iodine ions along their channels; because different chains are pinned at randomly distributed defects, this motion destroys the transverse (inter-chain) order of the iodine sublattice; once transverse order is lost, the iodine sublattice 'evaporates' into the gaslike disordered phase at unchanged composition (normally stable only at lower $n$); a disordered iodine sublattice localizes the holes that carry conduction along the TTF stacks, lowering the electronic conductivity; and the resulting local field enhancement provides positive feedback driving the transition to completion. On this picture the erratic quantitative irreproducibility is intrinsic, a consequence of random defect pinning. The paper itself, however, states that the origin of the oscillations - in particular the regular, sample-independent frequency - remains unexplained, and identifies as missing: x-ray structure determination under high biased fields on a geometry where the highest-field region lies in the true bulk away from contacts, a direct measurement of the iodine ionic mobility in the presence of the overwhelming electronic conductivity, and a definitive bulk-versus-interface assignment. It is these questions, jointly testing the disordering postulate, that constitute the open problem.

Problem Statement

What is the microscopic origin of the anomalous transport of ordered tetrathiafulvalene iodide (TTF-I$_n$, $n\cong 0.7$) under strong pulsed electric fields - the negative-resistance-like transition to a low-conductance state, the hysteresis and long-lived conductance memory, and the burst-like current oscillations at the sample-independent frequency of $70\pm 10$ MHz - and, in particular, does the iodine sublattice undergo an electrically driven order-disorder transition in the crystal bulk, as postulated by Latif and Kroemer?

The unresolved determinations are: (i) a specified structural test (x-ray diffraction or an equivalent microscopic probe) under high biased fields, on a contact geometry in which the highest-field region lies well away from the contacts within the true bulk of the crystal; (ii) an identified mechanism for the oscillations that accounts for the regular frequency of $70\pm 10$ MHz being reproducible across samples while amplitudes and thresholds fluctuate erratically, together with controls that establish or exclude circuit and contact/measurement artifacts; (iii) a determination of, or a rigorous bound on, the iodine ionic mobility under conditions in which a large static electronic conductivity coexists with the possible ionic current; (iv) a localization of the field-induced changes between bulk volume and contact/interface regions, given the demonstrated electrochemical reactivity of metal contacts toward these salts; (v) an account of the microscopic mechanism underlying the negative-resistance-like switching and the hysteresis and long-lived conductance memory.

Determinations (i)-(v) jointly test one hypothesis - field-driven disordering of the mobile iodine sublattice with resulting localization of the electronic carriers.

References

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