Control of Electronic State by Dihedral Angle in θ-type Bis(ethylenedithio)tetraselenafulvalene Salts

Abstract

Five θ-type BETS salts [BETS = bis(ethylenedithio)tetraselenafulvalene], θ-BETS2RbCo(SCN)4, θ-BETS2RbZn(SCN)4, θ-BETS2CsCo(SCN)4, θ-BETS2CsZn(SCN)4, and θ-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 θ-type BETS salts are much lower than those of isostructural BEDT-TTF salts, due to the smaller U/W (U, intramolecular Coulomb repulsion energy; W, bandwidth) of Se-atoms-introduced BETS salts. The electronic phase diagram of θ-type BETS salts indicate that the dihedral angles (θ) between BETS columns in the donor sheet control the electronic states; with decreasing dihedral angle (θ), 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 θ-type BETS salts as well as θ-type BEDT-TTF salts.