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

Abstract

The new paramagnetic molecular superconductor ET₄[(H₃O)Fe(C₂O₄)₃]·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⊥ ≥ 5.5 T and H_c2|| ≈ 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]³⁻ 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.