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 4a-->×3.40b-->×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 a--> direction becomes incommensurate and displays an abrupt decrease from its low-temperature value of 4a--> and gradually decreases with increasing temperature to 2a--> near 51°K.