Elastic- and inelastic-neutron-scattering study of tetrathiafulvalenium-tetracyanoquinodimethanide (TTF-TCNQ): New results

Abstract

Several additional studies of tetrathiafulvalenium-tetracyanoquinodimethanide by elastic and inelastic neutron scattering have been performed. The temperature dependence of the lattice parameters $b$ and $asin\beta$ were measured and no anomaly was observed at any of the transition temperatures, $T_1=54\mathrm{}$ K, $T_2=48.5\mathrm{}$ K, or $T_3=38\mathrm{}$ K. The $2k_F$ satellites in the ($h k 0$) zone have been extensively studied. The temperature dependence of their intensity, position along $a^{*}$, and the linewidth along $a^{*}$ are reported. In contrast with the earlier studies on the $2k_F$ satellites possessing a $c^{*}$ component, the linewidths of the ($h k 0$) satellites increase to above 38 K and the satellites are undetectable above 48 K. The $4k_F$ anomaly is shown not to arise from an anomaly in the longitudinal acoustic branch propagating along $b$, the chain direction. The possibility of the $4k_F$ anomaly arising from an optical mode or being totally elastic above 54 K cannot be ruled out. The acoustic phonon branches for $q$ propagating along the $a^{*}$ and $c^{*}$ directions have been measured and the corresponding elastic constants deduced.