Searching for crystallographic superstructures in -(BEDT-TTF)2Cu[N(CN)2]Br

Abstract

To resolve a superstructure formation previously reported for the organic superconductor κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Br, we present synchrotron x-ray diffraction experiments carried out at the MAGS beamline at BESSY, HZB. Surprisingly, in our low temperature (28K) experiments, when searching kspace at (h 0 3.5), h = 7, 8 and (h 0 0.5), h = 5, 7, for none of these spots we could detect scattering intensity associated to a superstructure formation, in contradiction to previous reports. Our data suggest that details of the structural properties of κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Br-such as superstructure formation-sensitively depend on sample handling, e.g., cooling rates (in our case 4K/min), or thermal cycling. A direct relationship between superstructure formation and terminal ethylene group ordering cannot be verified, disproving proposals put forth previously.

Relaxation dynamics in the one-dimensional organic charge-transfer salt δ−(EDT−TTF−CONMe2)2Br

“…It should be noted that the origin of the glasslike transition in the mentioned systems [31] is still controversial. Recent synchrotron x-ray diffraction experiments [33,48] established the level of residual ethylene group disorder in -Br. From this the authors concluded that the glasslike transition is not primarily caused by the configurational freezing-out of the ET-endgroup motions, but rather other structural and electronic degrees of freedom have to be taken into account.…”