Bulk Kosterlitz–Thouless Type Molecular Superconductor β″-(BEDT-TTF)2[(H2O)(NH4)2Cr(C2O4)3]·18-crown‑6

A new molecular superconductor, β″-(BEDT-TTF)2­[(H2O)­(NH4)2Cr­(C2O4)3]·18-crown-6, has been synthesized from the organic donor molecule BEDT-TTF with the anion Cr­(C2O4)3 3–. The crystal structure consists of conducting organic layers of BEDT-TTF molecules which adopt the β″ packing motif (layer A), layers of NH4 + and Λ-Cr­(C2O4)3 3– (layer B), layers of (H2O)­(NH4)­18-crown-6 (layer C), and layers of NH4 + and Δ-Cr­(C2O4)3 3– (layer D) which produce a superstructure with a repeating pattern of ABCDABCDA. As a result of this packing arrangement, this is the 2D superconductor with the widest gap between conducting layers where only a single donor packing motif is present (β″). Superconducting critical temperatures at ambient pressure observed by electrical transport and magnetic measurements are 4.0–4.9 and 2.5 K, respectively. The strong 2D nature of this system, the broad transition to T zero at 1.8K, and the transition of α of V ∝ I α from 1 to 3 on I–V curves strongly suggest that the superconducting transition is very close to a Kosterlitz–Thouless transition. The magnetic field dependence of the superconducting critical temperature parallel to the conducting plane gives an upper critical field μ0 H c2∥ > 8 T, which is over the calculated Pauli–Clogston limit for this material.