Elastic-neutron-scattering study of the phase transitions in tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ)

Elastic-neutron-scattering experiments performed on deuterated tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) single crystals reveal two structural phase transformations at 38°K and at 54 ± 1.0°K. Below 38°K, a $4\overrightarrow{\mathrm{a}}\times 3.40\overrightarrow{\mathrm{b}}\times \overrightarrow{\mathrm{c}}$ modulation relative to the underlying undistorted lattice is established. The 38ÅK transition is shown to be first order with a hysteresis of ∼ 1°K. At this transition, the modulation period in the $\overrightarrow{\mathrm{a}}$ direction becomes incommensurate and displays an abrupt decrease from its low-temperature value of $4\overrightarrow{\mathrm{a}}$ and gradually decreases with increasing temperature to $2\overrightarrow{\mathrm{a}}$ near 51°K.