Neutron and x-ray diffraction evidence for a structural phase transition in the sulfur-based ambient-pressure organic superconductor bis(ethylenedithio)tetrathiafulvalene triiodide

A low-temperature structural phase transition in the ambient-pressure organic superconductor, bis(ethylenedithio)tetrathiafulvalene triiodide, (BEDT-TTF)₂{I}₃$, abbreviated ${(ET)}₂I₃, has been observed from single-crystal x-ray diffraction and time-of-flight neutron diffraction data. Superlattice peaks with indices of (hkl)±q, where $q=(0.08, 0.27, 0.205)$, appear when crystals are cooled below {~}200 K. This is the first example of an incommensurate modulated superstructure in the class of sulfur- and selenium-containing organic superconductors of which (ET)₂{I}₃$ is the first ambient-pressure S-based derivative.