A novel paramagnetic molecular superconductor formed by bis(ethylenedithio) tetrathiafulvalene, tris(oxalato) ferrate(III) anions and bromobenzene as guest molecule:: ET4[(H3O) Fe(C2O4)3]•C6H5Br

The new paramagnetic molecular superconductor ET4[(H3O) Fe(C2O4)(3)]center dot C6H5Br (1) (T-c = 4.0 K) contains layers of superconducting ET donors alternating with paramagnetic hexagonal layers formed by (H3O)(+), [Fe(C2O4)(3)](3-) and guest C6H5Br molecules located in the hexagonal cavities. Conductivity measurements show metallic behavior from room temperature with a minimum in the resistivity at ca. 50 K followed by a smooth increase and a sharp drop in the resistivity with an onset at 4.0 K and a zero resistance at ca. 1.0 K. Magnetoresistance measurements indicate that H-c1 is about 7 mT and that H-c2 is very anisotropic (H-c2 perpendicular to >= 5.5 T and H-c2 parallel to approximate to 0.5 T). Magnetic susceptibility measurements show the expected paramagnetic behavior of the high spin S 5/2 Fe(III) ions and an extra temperature independent paramagnetism (Pauli-type) typical of metallic systems. Low field DC and AC magnetic measurements show the expected Meissner effect below ca. 3.5 K and confirm the value of H-c1 obtained from magnetoresistance measurements. X and Q-band ESR spectra show a dysonian narrow line coming from the conducting sublattice plus 5 lines from the [Fe(C2O4)3]32 anion, as expected for an S = 5/2 ground spin state with low ZFS. Heat capacity measurements also confirm the superconducting transition at low temperatures and show a broad transition at higher temperatures that can be attributed to an order-disorder transition of some of the terminal ethylene groups of the ET molecules.