Determining ethylene group disorder levels inκ−(BEDT−TTF)2Cu[N(CN)2]Br

We present a detailed structural investigation of the organic superconductor �-(BEDTTTF)2Cu[N(CN)2]Br at temperatures T from 9 to 300 K. Anomalies in the T dependence of the lattice parameters are associated with a glass-like transition previously reported at Tg = 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 glass-like transition is not primarily caused by configurational freezing-out of the ethylene groups. Due to their exotic superconducting- and normal-state properties, which resemble those of the high-Tc cuprates, the organic charge-transfer salts κ-(BEDT-TTF)2X, X = Cu(NCS)2, Cu[N(CN)2]Br and Cu[N(CN)2]Cl have been intensively studied in recent years 1,2,3,4,5,6 . Resulting from their layered crystal structure, the electronic properties of these materials are quasi-twodimensional. This leads to electronic ground state properties for the series of materials which are commonly summarized within a conceptual phase diagram, with the antiferromagnetic insulator X = Cu[N(CN)2]Cl and the correlated metals X = Cu[N(CN)2]Br, Cu(NCS)2 on opposite sites of a bandwidth-controlled Mott transition 7 .