Pressure Dependence of the Structural and Electronic Properties of Organic Superconductors

ABSTRACTThe pressure dependence (≤12 × 102 MPa) of the cell parameters of (TMTSF)2 - C104 and κ - (BEDT-TTF)2 - Cu(NCS)2 has been measured at room temperature. The magnitudes of the principal compressibilities calculated and the bulk modulus (B) do not vary all over the experimental pressure range for (TMTSF)2 - C104, while ki decrease slightly for κ - (ET)2 - Cu(NCS)2 and its Dulk modulus strongly increases. The directions of the principal compressibilities strongly varywith pressure for the former salt; in the latter case, the projections of the directions of the smallest and of the largest principal compressibilities remain parallel and perpendicular to the long molecular axis respectively whereas the intermediate is along the monoclinic b axis.The crystal structures have been refined at 4 × 102 ,6 × 102 and 9.5 × 102 MPa (R = 0.05) for (TMTSF)2 - C104 and at 7 × 102 MPa (R = 0.095) for κ - (ET)2 - Cu(NCS)2. The transfer integrals have been calculated for the two compounds. In (TMTSF)2 - C104, both normal and transverse overlaps strongly increase with pressure; the dimerization is reduced under pressure and the two largest transverse interactions become identical. In κ - (ET)2 - Cu(NCS)2, the intra-dimer interaction strongly increases whereas the variations of the inter-dimer interactions may be very different.