T_{\text{c}} of 11 K Identified for the Third Polymorph of the (BEDT-TTF)2Ag(CF3)4(TCE) Organic Superconductor

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The crystal structure of the third polymorph of the organic superconductor (BEDT-TTF)2Ag(CF3)4(TCE), where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene and TCE is 1,1,2-trichloroethane, has been investigated. The crystal consists of alternating stacks of two types of donor sheets, \kappa- and \alpha^{\prime}-types. Although this structure is similar to the previously solved two-layered high-T_{\text{c}} phase of (BEDT-TTF)2Ag(CF3)4(TCE), the axis perpendicular to the conducting layer of the unit cell of the new phase is twice as large as that of the two-layered phase; thus, four layers form the repeating unit. Superconductivity is attributed to the \kappa-type conducting layer, because the \alpha^{\prime}-type conducting layer is in an acentric charge-ordered state. The onset superconducting transition temperatures are approximately 11.0 and 9.5 K for the four-layered and two-layered phases, respectively.
2012-02-15