EPR studies of a new family of organic conductors: Dimethyl(ethylenedithio)diselenadithiafulvalene compounds (DMET${)}_2$X [X=${\mathrm{PF}}_6$, ${\mathrm{BF}}_4$, Au(CN${)}_2$, ${\mathrm{I}}_3$, and ${\mathrm{AuBr}}_2$]

A new family of organic conductors dimethyl(ethylenedithio)diselenadithiafulvalene compounds (DMET${)}_2$X [X=${\mathrm{PF}}_6$, ${\mathrm{BF}}_4$, Au(CN${)}_2$, ${\mathrm{I}}_3$, and ${\mathrm{AuBr}}_2$] has been investigated by means of EPR. In spite of the large variety of conductive behaviors ranging from insulator to metal, the spin susceptibility at higher temperatures was found to be quite similar in magnitude and in temperature dependence for all the salts (except the ${\mathrm{AuBr}}_2$ salt). Below 20–25 K, the first three salts are found to undergo phase transitions; the transitions in the ${\mathrm{BF}}_4$ and Au(CN${)}_2$ salts are clearly magnetic, i.e., an antiferromagnetic or spin-density-wave transition. In the ${\mathrm{PF}}_6$ salt, EPR properties suggest a possible spin-Peierls transition. The ${\mathrm{I}}_3$ and ${\mathrm{AuBr}}_2$ salts have no EPR anomaly in the measured temperature region. The difference in the EPR linewidth among these salts is well understood as a result of the variation of dimensionality, in the framework of the Elliott mechanism. The DMET family contains all the electronic features observed in the tetramethyltetrathiafulvalene, tetramethyl- tetraselenafulvalene and bis(ethylenedithio)-tetrathiafulvalene families.