Neutron-Scattering Study of the 38- and 54-K Phase Transitions in Deuterated Tetrathiafulvalene- Tetracyanoquinodimethane (TTF-TCNQ)

Abstract

Neutron-elastic-scattering experiments performed on deuterated TTF-TCNQ single crystals reveal two structural transformations at 38 \ifmmode\pm\else\textpm\fi{} 0.5 K and at 54 \ifmmode\pm\else\textpm\fi{} 1.0 K. Below 38 K, a $4\stackrel{\ensuremath{\rightarrow}}{\mathrm{a}}\ifmmode\times\else\texttimes\fi{}3.4\stackrel{\ensuremath{\rightarrow}}{\mathrm{b}}\ifmmode\times\else\texttimes\fi{}\stackrel{\ensuremath{\rightarrow}}{\mathrm{c}}$ modulation relative to the underlying undistorted lattice is established. At 38 K the modulation period in the $\stackrel{\ensuremath{\rightarrow}}{\mathrm{a}}$ direction becomes incommensurate and displays an abrupt decrease from its low-temperature value of $4\stackrel{\ensuremath{\rightarrow}}{\mathrm{a}}$ and gradually decreases with increasing temperature to $2\stackrel{\ensuremath{\rightarrow}}{\mathrm{a}}$ near 51 K.