TTF Stacks Research Articles

Temperature-Pressure Phase Diagram in TTF-TCNQ: Strong Suppression of Charge-Density-Wave State under Extremely High Pressure

We have measured the resistivity of the organic conductor TTF-TCNQ under high pressure up to 8.0 GPa using a cubic anvil apparatus. Below 3 GPa, the pressure dependence of the charge-density-wave (CDW) transition temperature, T CDW , is consistent with previous results, except for the observation of the “49 K-transition” at around 1 GPa. Above 3 GPa, T CDW decreases with increasing pressure but the CDW state survives even at 8.0 GPa. At 8.0 GPa, however, the resistivity shows metallic behavior below 25 K after the CDW transition with T CDW = 31 K. The origin of suppression is discussed in terms of the imperfect nesting originating from the increase in the interchain transfer integral, the significant deviation from the third-order commensurability, as well as the bandwidth broadening for both TTF and TCNQ stacks. The complete suppression of the CDW phase is expected near 9 GPa.

‘Core and sheath' structure of a TTF complex forming a square grid

The crystal structure of the pentakis(methoxycarbonyl)cyclopentadienide salt of tetrathiafulvalene is reported. It is found to have a novel two-dimensional structure in the form of a square grid. The grid is formed out of TTF stacks running in orthogonal directions; the TTF stacks form a core structure with the counterions providing a sheath around them. DC conductivity measurements on single crystals reveal the anisotropy expected of this unusual two-dimensional packing motif.

Synthesis, structure and physical properties of tetrathiafulvalenium salts of the ferracarborane complex commo-[3,3′-Fe{1-(C4H3S)-1,2-C2B9H10}2]–

The sandwich complex commo-[3,3′-Fe{1-(C4H3S)-1,2-C2B9H10}2]–1 and its tetrathiafulvalenium salts [ttf]5[Fe(C2B9H10C4H3S)2]22 and [ttf][Fe(C2B9H10C4H3S)2]3 were synthesised in order to study the effect of the thiophene (C4H3S) group on the crystal packing, electrical and magnetic properties of ttf–metallacarborane salts. The crystal structure of 2 features unusual two-dimensional networks of ttf units which extend in the a and c directions. Each ttf sheet contains stacked trimers of ttf units, which propagate along the a direction, and bridging ttf units, which lie between the ttf stacks approximately orthogonally to the ttf units within the trimers. The trimers are linked along the c direction via short S ⋯ S contacts (3.56 A) with these bridging ttf units. In contrast, the crystal structure of compound 3 contains discrete stacked dimers of [ttf]˙+ cations. Whilst crystals of 2 are semiconducting (σ300 K= 2 × 10–3 S cm–1) with an activation energy of 0.22 eV, those of 3 are insulating (σ290 K⩽ 10–7 S cm–1). Both compounds exhibit weak ferromagnetic interactions between the unpaired spins of their [Fe(C2B9H10C4H3S)2]– anions, with small positive Weiss constants of 1.9 and 0.5 K respectively.

A crystal-orbital calculation on three-dimensional crystal of TTF-TCNQ

A crystal-orbital calculation on the three-dimensional TTF-TCNQ crystal has been carried out in the Hartree-Fock all-valence electron approximation using the CRYSTAL92 routine package. The expected quasi-one-dimensional metal nature of the TTF-TCNQ band structure along the direction of TTF and TCNQ infinite stacks alignment has been reproduced. The charge transfer between TTF and TCNQ has been calculated to be of 0.57 e.

Crystal structure and conduction property of the first quasi-one-dimensional conductor of tetrathiafulvalenium salt with mixed inorganic anions, (TTF)(ClO 4) 0.14±0.02(Cl) 0.42±0.08

The first quasi-one-dimensional tetrathiafulvalenium conductor with mixed non-stoichiometric inorganic anions, (TTF)(ClO 4) 0.14±0.02(Cl) 0.42±0.08, was crystallized electrochemically and found to have a structure of a tetragonal system with a = b = 11.213(2) A ̊ , c = 3.600(1) A ̊ , Z = 2 . Crystal structure was analyzed assuming the space group of P4 2/ mnm. TTF radicals form stable segregated eclipsed stacks along the elongated c-axis but anions are fully disordered in the channels surrounded by the TTF stacks. The room temperature conductivity along the TTF stacks is 7–27 S/cm/. The conductor shows a sharp metal-insulator transition at 253±4 K and below this temperature it behaves as a one-dimensional variable range hopping (VRH) conductor down to 70 K. Another non-stoichiometric perchlorate of TTF, (TTF)(ClO 4) 0.62±0.02, was also obtained in polycrystalline form at the same electrochemical preparation and found to have a much lower powder conductivity (7 × 10 −4 S/cm) which suggests a possible Peierls structure at room temperature for this compound.

Atomic-scale imaging of anisotropic organic conductors by scanning probe techniques (STM/AFM)

Monocrystals of the charge transfer complex of tetrathiofulvalence with tetracyanoquinodimethane (TTF/TCNQ) and the fluoranthenyl radical cation salt (FA) 2PF 6 were studied by scanning tunneling microscopy (STM) and atomic force microscopy (AFM) at ambient conditions. Structures observed at the atomic scale in STM and AFM images agree well with the molecular arrangement of corresponding crystallographic planes. Patterns revealed in STM and AFM images were assigned to TTF and TCNQ stacks of the (001) surface. Comparison of results provides complementary electronic and atomic structure information. Features observed in STM images of different surfaces of (FA) 2PF 6 correlate to charge density distribution assigned to fluoranthene molecules of the (011) plane forming stacks in the crystallographic a -direction. Dimerization of (FA) molecules observed in STM images has been found to be more pronounced than expected from X-ray data. Patterns corresponding to PF - 6 anions were observed only at certain tunneling conditions.

Charge Transfer Complexes Based on the Twin-TCNQ-Type Acceptor 11,11,12,12,13,13,14,14-Octacyano-1,4:5,8-anthradiquinotetramethane (OCNAQ)

Abstract Charge transfer complexes of OCNAQ with pyrene, PT (phenothiazine), TMTTF (tetramethyltetrathiafulvalene), TTF (5,6 : 11,12-bis(epidithio)naphthacene), and TTF (tetrathiafulvalene) have been prepared. The pyrene and PT complexes are electrically insulating and the ground state is neutral. In most of the other complexes OCNAQ is fully charged. One of the TTT complexes, (TTT)2OCNAQ(DMF), where DMF is N,N-dimethylformamide, has been found to show metallic conductivity. A structural study has revealed that short S···N contacts exist between TTT and OCNAQ, in addition to one-dimensional TTT stacks. This transverse interaction has been demonstrated by the small anisotropy of the conductivity. The 2 : 1 TTF complex is also metallic down to 43 K, at which point a metal-to-insulator transition occurs. A structure analysis and thermoelectric power data suggest that the charge conduction in this crystal takes place not only through the TTF stacks but also through a two-dimensional network of the OCNAQ molecules.

A new class of conducting organo-metallic salts: TTF radical cation salts with metal halides

A new series of semi-conducting organo-metallic salts derived from tetrathiafulvalene (TTF) has been prepared by the electro-oxidation technique: (TTF) (MnCl 3 −) −0.75 ( σ RT ⋍ 15 Ω −1 cm −1), ( TTF) ( MnCl 4 −−) ⋍0.25 ( σ RT ⋍ 0.2 Ω −1 cm −1 ) and (TTF)(CoCl 4 −−) ⋍0.25 ( σ RT ⋍ 0.2 Ω − cm − ). Preliminary X-ray work has shown that the first compound presents both regular TTF stacks and parallel inorganic chains. All of these compounds are strong paramagnets exhibiting antiferromagnetic interactions at low temperatures.

The evolution of nonlinear electrical transport effects in TTF-TCNQ as driven through CDW commensurability

The b-axis conductivity of the one-dimensional two-chain organic conductor TTF-TCNQ was measured as a function of electric field E for temperatures T ≲ 30 K and pressures P ≳ 24 kbars. At P = 1 bar, TTF-TCNQ undergoes three successive phase transitions : the high temperature T H (incommensurate Peierls) transition involving the electron-like TCNQ l stacks, the lower two transitions (T M,T L) involving the hole-like TTF stacks. We have found at P = 1 bar deviations from Ohm's law, with a threshold field E T, in the neighbourhood and below T H. E T reaches a minimum just below T H before rapidly increasing below T M. At 4 and 9 kbars T M = T L , and hence, the range of T where there is a single incommensurate CDW is large. In this P range, nonlinear effects are observed and the T dependence of E T is less weak below its minimum value than at 1 bar. As the CDW is driven through third-order commensurability at ∼ 19 kbars, we find at constant T/T H a peak in E T and a minimum in the excess current at 30 V/cm. These results are discussed in terms of the depinning of the CDW by the electric field, taking into account the interactions between the two types of chains and the possibility of discommensuration transport near commensurability.

Ab initio selfconsistent field linear combination of atomic orbitals band structures of infinite TCNQ and TTF stacks

Ab initio SCF LCAO crystal orbital calculations have been performed for infinite neutral TCNQ and TTF stacks. The width of the conduction band of poly(TCNQ) is 1.17 eV and the width of the valence band of poly(TTF) is 0.30 eV. The positions of the conduction band of poly(TCNQ) and of the valence band of poly(TTF) favor more charge transfer than the LEMO and HOMO levels of a single TCNQ and TTF molecule, respectively.

All-valence-electron band structures of infinite TCNQ and TTF stacks

MINDO/3 crystal orbital (CO) calculations have been performed for infinite neutral TCNQ and TTF stacks. The width of the conduction band of poly(TCNQ) (0.556 eV) and of the valence band of poly(TTF) (0.524 eV) agrees well with the previous MINDO/2 value of poly(TCNQ) and with the estimated value of poly(TTF). The band structures show a strong dependence on the stacking distance in the case of poly(TCNQ) and a much less strong dependence in the case of poly(TTF).

Model for the low-temperature phases and the two phase transitions of tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ)

The evidence concerning the 38- and 53-K phase transitions in tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) and the phases below each is critically reviewed. It is concluded that the 53-K transition is on the TCNQ stacks, that the TTF stacks show negligible long-range order. A Ginzburg-Landau model is constructed that assumes that isolated TCNQ stacks have a much stronger tendency to a Peierls transition and much larger amplitude charge-density waves than do the TTF stacks. Coulomb interactions between charge-density waves are assumed to be the dominant interstack interaction. The model shows how the TTF and TCNQ stacks are largely decoupled from one another and how the development of TTF long-range order can change the superlattice cell parameter along $\stackrel{\ensuremath{\rightarrow}}{\mathrm{a}}$ from $2a$ at 53 K to larger values at lower temperatures. The model also has implications for the electric and magnetic behavior.

Study of tetrathiafulvalene (TTF) dimers and trimers in eclipsed and slipped geometries

Self-consistent-field-$X\ensuremath{\alpha}$-scattered-wave molecular-orbital (MO) calculations have been performed on dimers of tetrathiafulvalene (TTF) in eclipsed (${D}_{2h}$) and slipped (${C}_{2h}$) geometries. In both cases the MO's of the dimer are formed to a good approximation simply by taking bonding and antibonding combinations of the MO's of the monomer. The interaction leads to a splitting of about 0.5 eV for the ${D}_{2h}$ dimer. Owing to the slipped geometry the corresponding splitting is only 0.12 eV for the ${C}_{2h}$ dimer. These splittings provide estimates of the bandwidths for infinite TTF stacks in the conducting salts TTF ${\mathrm{Br}}_{0.71\ensuremath{-}0.76}$ and TTF-TCNQ (tetracyanoquinodimethane), respectively. Extended H\uckel (EH) calculations for the TTF molecule and both dimers are in good agreement with the $X\ensuremath{\alpha}$ results; the EH splittings being 0.67 and 0.15 eV for the ${D}_{2h}$ and ${C}_{2h}$ dimers, respectively. EH calculations on TTF trimers yield bandwidths of 0.99 and 0.20 eV for the eclipsed and slipped geometries, respectively, and also indicate that for qualitative considerations results obtained for the dimers are sufficient. It is suggested that eclipsed stacks of TTF molecules can be treated by normal band-structure techniques using the monomer MO's as fixed ($k$-independent) basis functions to generate Bloch functions. For the slipped geometry found in TTF-TCNQ, however, the small bandwidth found here and also in previous studies makes it doubtful whether band theory is appropriate.

On the crystallography of the TTF-TCNQ salt at reduced temperatures

A structure determination of the one-dimensionally conducting salt tetrathiofulvalinium-tetracyanoquinodimethanide (TTF-TCNQ) at 100°K, and additional measurements at lower temperatures, show that no major change in crystal structure occurs between room temperature and 40°K. At 100°K the interplanar spacings are reduced from 3.47 to 3.43 and from 3.17 to 3.11 Å for the TTF and TCNQ stacks, respectively. The bond lengths at 100°K agree better with the values listed for salts of TCNQ -0.5, than with those for TCNQ -1. On cooling a broadening of the b∗ axis reflections is observed, which would be compatible with one-dimensional distortions of the crystal lattice.