X-ray study of the anion ordering transition in di (tetramethyltetraselenafulvalen)-ium perchlorate (TMTSF)2ClO4 : quenching and irradiation effects

Abstract

We report the results of an X-ray diffraction study of the phase transition of (TMTSF) 2 ClO 4 at 24 K. The influence of the cooling rate is evaluated by its effect on the intensity and width of the superstructure reflections appearing at the transition. We also report important irradiation effects on this anion ordering transition, previously unobserved in other compounds of the (TMTSF) 2 X family. The superstructure reflection intensities are strongly reduced and the transition temperature is lowered upon X-ray irradiation. Partial annealing of the irradiation induced disorder is observed. The intensities of 220 superstructure reflections were measured and a preliminary analysis shows that anion ordering accounts for most of the intensity. We conclude that distortions of the TMTSF backbone at the transition are minor (in contrast to the features previously reported for (TMTSF, 2 ReO 4 ). Presentation des resultats d'une etude de diffraction RX de la transition de phase observee a 24 K. L'irradiation reduit rapidement l'intensite des reflexions de surstructure et abaisse la temperature de transition. Un phenomene de recuit partiel du desordre induit par l'irradiation est observe