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

To resolve a superstructure formation previously reported for the organic superconductor κ-(BEDT-TTF)2Cu[N(CN)2]Br, we present synchrotron x-ray diffraction experiments carried out at the MAGS beamline at BESSY, Helmholtz-Centre Berlin. Surprisingly, in our low temperature (28K) experiments, when searching k-space at (h03.5), h=7, 8 and (h00.5), 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)2Cu[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.