ESR study of two phases of di[bis(ethylenedithio)tetrathiafulvalene]triiodide [(BEDT-TTF)2I3]

The sulfur-based organic metal di[bis(ethylenedithio)tetrathiafulvalene]triiodide [or (BEDT-TTF${)}_2$ ${\mathrm{I}}_3$, abbreviated (ET${)}_2$ ${\mathrm{I}}_3$] occurs in at least two crystalline phases. One phase (denoted as β) exhibits ambient-pressure superconductivity, while the second (denoted α) is characterized by an abrupt decrease in conductivity below 135 K. Electron-spin-resonance data are reported for both phases. The spin susceptibility of the superconducting β phase is constant below 100 K as expected for a metal, while that of the α phase decreases by nearly 2 orders of magnitude below 136 K. The microwave conductivity of the β phase agrees with the published dc conductivity. The superconducting transition temperature, $T_c$, and its pressure dependence are reported for β-[(ET${)}_2$ ${\mathrm{I}}_3$].