The behavior of the phase transitions is mapped out as a function of the dihedral angle (θ) or the transfer integral $\mathrm{}\left(t\right)\mathrm{}$ between donor columns for θ-type BEDT-TTF [BEDT-TTF: bis(ethylenedithio)tetrathiafulvalene, abbreviated as ET] salts involving the series $\theta -(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}-\mathrm{T}\mathrm{T}\mathrm{F}{)}_2{\mathrm{MM}}^{\prime }(\mathrm{SCN}{)}_4$ [$M=\mathrm{T}\mathrm{l},\mathrm{R}\mathrm{b},\mathrm{C}\mathrm{s},$ $M^{\prime }=\mathrm{C}\mathrm{o},\mathrm{Z}\mathrm{n}:$ abbreviated as $\theta {-MM}^{\prime }$], which we have recently prepared. The electronic correlation parameter $\mathrm{}(U/t)\mathrm{}$ increases by increasing the dihedral angle (θ). In the phase diagram of θ-ET salts the ground state varies from insulating, to superconducting, to metallic with decreasing θ, namely, $U/t.\mathrm{}$ The series $\theta {-MM}^{\prime }$ is located at the center of the phase diagram where metallic, paramagnetic insulating, and singlet states are observed at low temperatures. With applied pressure, the metal-insulator transition temperature rises because the dihedral angle increases, which is related to the enhancement of the electronic correlation.

• Received 25 June 1997

DOI:https://doi.org/10.1103/PhysRevB.57.12023