Control of electronic state by dihedral angle in θ-type bis(ethylenedithio)tetraselenafulvalene salts

Five theta -type BETS salts [BETS = bis(ethylenedithio)tetraselenafulvalene], theta -BETS2RbCo-(SCN)(4), theta -BETS2RbZn(SCN)(4), theta -BETS2CsCo(SCN)(4), theta -BETS2CsZn(SCN)(4), and theta -BETS2Ag-(CN)(2), have been newly prepared by the electrocrystallization method. The crystal structure analyses indicate that the former four salts are isostructural to BEDT-TTF analogues. [BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene] However, the metal-insulator transition temperatures of theta -type BETS salts are much lower than those of isostructural BEDT-TTF salts, due to the smaller U/M (U, intramolecular Coulomb repulsion energy; W,bandwidth) of Se-atoms-introduced BETS salts. The electronic phase diagram of theta -type BETS salts indicate that the dihedral angles (theta) between BETS columns in the donor sheet control the electronic states; with decreasing dihedral angle (theta), the transverse interaction (t) increases, so that the larger bandwidth (W = 8t) makes the metal-insulator transition temperatures lower in the BETS salts. This phase diagram is universal for theta -type BETS salts as well as theta -type BEDT-TTF salts.