Determining ethylene group disorder levels in {kappa}-(BEDT-TTF){sub 2}Cu[N(China){sub 2}]Br

We present a detailed structural investigation of the organic superconductor {kappa}-(BEDT-TTF){sub 2}Cu[N(China){sub 2}]Br at temperatures T from 9 to 300 K. Anomalies in the T dependence of the lattice parameters are associated with a glasslike transition previously reported at T{sub g}=77 K. From structure refinements at 9, 100, and 300 K, the orthorhombic crystalline symmetry, space group Pnma, is established at all temperatures. Further, we extract the T dependence of the occupation factor of the eclipsed conformation of the terminal ethylene groups of the BEDT-TTF molecule. At 300 K, we find 67(2)%, with an increase to 97(3)% at 9 K. We conclude that the glasslike transition is not primarily caused by configurational freezing-out of the ethylene groups.