Neutron diffraction study of the compressibility of TTF-TCNQ under hydrostatic pressure

Abstract

Nous présentons des mesures de compressibilité de TTF-TCNQ deutéré par diffraction de neutrons sous hautes pressions hydrostatiques jusqu'à ~ 20 kbar à la température ambiante. Les valeurs observées sont respectivement suivant les axes a, b et c, ka = 2,7, kb = 4,7 et kc = 3,2 x 10-12 cm2/dyne. Ces résultats indiquent l'existence d'une contribution importante de type électrostatique (coulomb) à l'interaction interchaînes. Nous suggérons que la cohésion des cristaux de TTF-TCNQ provient de l'empilement de dipôles moléculaires fluctuants. Abstract.-The linear axial compressibilities of a deuterated sample of TTF-TCNQ (tetrathiafulvalene-tetracyanoquinodimethane) have been measured by a neutron diffraction technique under hydrostatic pressures up to ~ 20 kbar at room temperature. The observed compressibilities along the crystallographic axes are, respectively, ka = 2.7, kb = 4.7 and kc = 3.2 x 10-12 cm2/dyne. These results indicate a strong Coulomb type interchain interaction. It is suggested that the dominant part of the crystal cohesion results from a stacking of strongly dynamically polarized TTF and TCNQ molecules acting as effective molecular dipoles.

Linear temperature dependence of the constant volume resistivity of TTF-TCNQ

“…This estimate is consistent with the small differences in a and c axis compressibilities [13]. Only the c axis is affected by tilt angle changes, and if the slightly larger c axis compressibility is attributed to such an effect, the increases in tilt angles are less than 0.5°a t 20 kbar.…”

“…-We have looked for changes in the angles of tilt of the TTF and TCNQ molecules with respect to the stacking axis, under pressure, by analysing intensity changes in the Bragg peaks measured in high pressure neutron diffraction experiments on deuterated TTF-TCNQ. Experimental data were taken from experiments previously analysed to determine the compressibility [13], together with similar experiments to follow Bragg peaks not previously measured. Best determinations were made from comparison of the intensity ratios of peaks which are relatively insensitive to changes in lattice parameter, but change rapidly with changes in either or both of the tilt angles.…”

“…Derivation of constant volume resistivity. -X-ray measurements of the thermal expansion show increases of 1.1 %, 2.3 % and 0.8 % in the a, b and c axes between 60 and 295 K ~[I6], and"high pressure neutron diffraction measurements give the room temperature compressibilities as 0.27 %, 0.47 % and 0.32 %/kbar for the a, b and c axes [13]. Whilst there is no simple correspondence between the thermal expansion and compressibility, we follow Cooper [12] who chose to keep the stacking axis cell length, b to its value at 60 K, since most of the thermal expansion and compressibility is in this direction.…”

Crystal stability and optical properties of organic chain compounds

“…In the case of TTF-TCNQ this means that, roughly, C > 2eV, which just compensates the "Madelung defect" on one side, and is consistent with the condition for the ω −t mode softening (6) on the other. As was already pointed out, the dominant contribution to C is expected to come from the nearest-neighbor interaction V sr between acceptor and donor pairs, i. e. from the a direction which is indeed the direction of smallest compressibility in TTF-TCNQ [4]. This interchain coupling is expected to be responsible for the fractional charge transfer in other OCCs as well.…”

Deformation Potential Approach to the Estimation of the Peierls Phase Transition Temperature

“…The experimental value of K is 2.2 x 10" J m-2 [12] and the cross section S perpendicular to each chain calculated from the crystal structure of TTF-TCNQ [l] is 5.51 x Finally, by virtue of the above quantities and (23), we obtain Tp = 46 K. From the above calculation, it can be seen that the theoretically estimated PPT temperature of TTF-TCNQ is in very good agreement with the experimentally observed one [5] though we ignore the contributions of electron-electron interaction to the metal-insulator phase transition in our model. This indicates that the metal-insulator phase transition is dominated by the interaction of electrons with phonons in 1D organic charge-transfer crystals.…”