Structural determination of the lowest temperature modulated phase of tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) : another investigation

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Abstract
This X-ray structural study deals with an improved structural determination of the lowest temperature 2 kF modulated phase at normal pressure (locking phase) of the quasi-one- dimensional conductor tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ). The refine- ment of a semi-rigid molecular displacement model is carried out. Two opposite distortional modes are obtained for the TTF and TCNQ stacks. The TTF molecules do not exhibit any significant intramolecular deformations and slide upon their mean molecular plane, whereas the TCNQ molecules undergo a large out-of-plane intramolecular distortion which involves a substontial displacement of the quinoid ring perpendicularly to the mean molecular plane. Furthermore, an analysis of the symmetry of the translational modes reveals two distinct polarization modes within the TTF sheets parallel to be (b, c) plane, and only one within all the TCNQ sheets. The Charge Density Wave (CDW) ordering in the locking phase is deduced from this structural study and its nucleation just before the first-order locking transition is suggested for