Similarities and differences in the structural and electronic properties of .kappa.-phase organic conducting and superconducting salts

The similarities and differences in the crystal and band electronic structures of four {kappa}-phase tetrathiafulvalene-based organic conducting salts {kappa}-(BEDT-TTF){sub 2}X (X{sup {minus}} = Cu(NCS){sub 2}{sup {minus}}, I{sub 3}{sup {minus}}), {kappa}-(MDT-TTF){sub 2}AuI{sub 2}, and {kappa}-(BMDT-TTF){sub 2}AU(CN){sub 2} have been examined. Donor dimers have a bond-over-ring arrangement in the superconducting salts {kappa}-(BEDT-TTF){sub 2}X (X{sup {minus}} = Cu(NCS){sub 2}{sup {minus}}, I{sub 3}{sup {minus}}) and {kappa}-(MDT-TTF){sub 2}AuI{sub 2} but a bond-over-bond arrangement in the nonsuperconducting salt {kappa}-(BEDT-TTF){sub 2}Au(CN){sub 2}, so that the intradimer spacing is considerably larger in the nonsuperconducting than in the superconducting {kappa}-phases. The terminal ethylene groups of BEDT-TTF have a staggered arrangement in the high-T{sub c} salts {kappa}-(BEDT-TTF){sub 2}Cu(NCS){sub 2} (T{sub c} = 10.4 K) and {beta}*-(BEDT-TTF){sub 2}I{sub 3} (T{sub c} = {approximately} 8 K) but are eclipsed in the low-T{sub c} salts {kappa}(BEDT-TTF){sub 2}I{sub 3} (T{sub c} = 3.6 K), {beta}-(BEDT-TTF){sub 2}AuI{sub 2} (T{sub c} = 5.0 K), and {beta}-(BEDT-TTF){sub 2}IBr{sub 2} (T{sub c} = 2.8 K). All the Fermi surfaces of the {kappa}-phase can be described in terms of overlapping distorted circles and hence are essentially two-dimensional in nature. However, the Fermi surface of the nonsuperconducting {kappa}-phase, {kappa}-(BMDT-TTF){sub 2}Au(CN){sub 2} exhibits a partial nesting in contrast to the casemore » of the superconducting {kappa}-phases, which is probably responsible for the metal-insulator transition at {approximately} 60 K. 26 refs., 8 figs., 2 tabs.« less