STRAIN INDEX, LATTICE SOFTNESS AND SUPERCONDUCTIVITY OF ORGANIC DONOR-MOLECULE SALTS - CRYSTAL AND ELECTRONIC-STRUCTURES OF 3 ISOSTRUCTURAL SALTS KAPPA-(BEDT-TTF)2CU[N(CN)2]CL, KAPPA-(BEDT-TTF)2CU[N(CN)2]BR, KAPPA-(BEDT-TTF)2CU[N(CN)2]I

The recently discovered kappa-phase salts kappa-(BEDT-TTF)2]Cu[N(CN)2]X (X = Cl, Br, I) are isostructural but differ in their superconducting properties: the Br-salt is an ambient-pressure superconductor with T(c) = 11.6 K, the Cl-salt becomes superconducting (T(c) = 12.8 K) under a slight applied pressure of 0.3 kbar, and the I-salt does not exhibit superconductivity under an applied-pressure of up to 5 kbar. We examine the structural and electronic properties of these three salts on the basis of their crystal structures determined at 127 K by single crystal X-ray diffraction measurements. The band electronic structures of the three salts are virtually the same and do not account for the differences in the conduction properties. The differences in the three salts with regard to superconductivity are examined from the viewpoint of lattice softness by calculating strain indices for short intermolecular contacts. The random potential arising from ethylene group conformational disorder prevents superconductivity in the iodide salt.