New superconducting charge-transfer salts (BEDT-TTF)4[A·M(C2O4)3]·C6H5NO2 (A = H3O or NH4, M = Cr or Fe, BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene)

Abstract

The syntheses, crystal structures, and physical properties of two new crystalline charge-transfer salts of BEDT-TTF, bis(ethylenedithio)tetrathiafulvalene, containing tris(oxalato)metallate(III) anions of 3d elements are reported. Electrochemical oxidation of BEDT-TTF in the presence of (NH4)3[Fe(C2O4)3]·3H2O or (NH4)3[Cr(C2O4)3]·3H2O in C6H5NO2, yields crystals of β′′-(BEDT-TTF)4[A·Fe(C2O4)3]·C6H5NO2[1] or β′′-(BEDT-TTF)4[A·Cr(C2O4)3]·C6H5NO2[2] (A = H3O+ or NH4+). The crystal structure of [1] has been solved at 120 K in the monoclinic space group C2/c, and that of [2] in the same space group at 298 and 120 K. For [1], a = 10.273 Å, b = 19.949 Å, c = 35.030 Å, β = 92.97°, V = 7169.6(2) Å3, Z = 8. For [2], at 298 K: a = 10.304 Å, b = 20.091 Å, c = 35.251 Å, β = 92.70°, V = 7289.3(2) Å3, Z = 8, and at 120 K a = 10.283 Å, b = 19.917 Å, c = 34.939 Å, β = 93.30°, V = 7144.4(1) Å3, Z = 8. The crystal structures of both compounds consist of alternating layers of BEDT-TTF cations and layers containing [M(C2O4)3]3−, H3O+ or NH4+, and PhNO2. The BEDT-TTF molecules are arranged in the β′′ packing motif and the tris(oxalato)metallate(III) ions form the well-known honeycomb motif found in many molecular based magnets. SQUID magnetometry, Raman spectroscopy and electron paramagnetic resonance (EPR) measurements were performed on crystals of [1]. SQUID magnetometry, single-crystal four-probe conductivity measurements, Raman spectroscopy, EPR and polarised infrared reflectance were performed on crystals of [2]. Both compounds have metal to superconducting transitions with Tc = 6.2 K for [1] and for [2], Tc = 5.8 K.