Competition between organic superconductivity and a displacive structural modulation in the molecular stacks in bis(ethylenedithio) tetrathiafulvalene perrhenate, (BEDT-TTF)2ReO4

The pressure-induced organic superconductor bis(ethylenedithio) tetrathiafulvalene perrhenate, (BEDT-TTF${)}_2$ ${\mathrm{ReO}}_4$, is shown to undergo a first-order structural phase transition with an associated metal-to-insulator transition near T=77 K at ambient pressure. Superstructure reflections with reduced wave vectors q=((1/2),0,(1/2)) are found using single-crystal x-ray diffraction. Evidence is presented that these are not associated with a change of the anion ordering but result from a displacive structural modulation, which most probabably involves both the ${\mathrm{ReO}}_4$ ${\mathrm{}}^{\mathrm{-}}$ anion and BEDT-TTF molecular sublattices.