SUPERCONDUCTING AND SEMICONDUCTING MAGNETIC CHARGE-TRANSFER SALTS - (BEDT-TTF)(4)AFE(C(2)O4)(3)CENTER-DOT-C6H5CN (A=H2O, K, NH4)

Three new molecular charge transfer salts of bis(ethylenedithio)tetrat hiafulvalene (BEDT-TTF), (BEDT-TF)(4)AFe(C2O4)(3)C6H5CN (A = H2O, K, N H4), have been prepared, and their crystal structures and physical pro perties determined. The structures of all three salts consist of succe ssive layers of BEDT-TTF and layers of approximately hexagonal geometr y containing alternating A and Fe(C2O4)(3)(3-), with C6H5CN lying with in the hexagonal cavities. When A = K or NH4 the BEDT-TTF layers consi st of dimers (BEDT-TTF)(2)(2+) separated by isolated (BEDT-TTF)(0), th e charge difference being estimated from the C=C and C-S bond lengths. These salts are semiconductors to (sigma similar to 10(-4) S cm(-1), E(A) = 0.14 eV), and their magnetic susceptibilities are dominated by S = (5)/Fe-2(III). The EPR spectra accordingly show only one sharp sig nal. When A = H2O the BEDT-TTF adopt the beta '' packing, and the salt is a superconductor (T-c 7.0(3) K). The magnetic susceptibility above the critical temperature is the sum of a Pauli component (2 x 10(-3) emu mol(-1)) and a Curie-Weiss term. Below the transition the suscepti bility depends on the field penetration according to the London penetr ation depth, increasing with increasing field until the critical field . The Meissner effect is almost complete, indicating absence of pinnin g. The EPR spectra of the A = H2O compound are characterized by two re sonances, one of Dysonian shape due to the conduction electrons on the organic cations and the other of Lorentzian shape arising from the 3d electrons of Fe-III. Electronic band structure calculations suggest t hat the A = K compound is semiconducting (E(g) = 0.3 eV) in good agree ment with that result obtained from electrical measurement and the A = H2O is metallic (W = 1.1 eV) with both electron and hole pockets in t he Fermi surface. Optical reflectivity of the latter gives an electron ic bandwidth of 1.0 eV, fully consistent with the band structure calcu lation.