β″-(ET)3(MnCl4)(1,1,2-C2H3Cl3) (ET = bis(ethylenedithio)tetrathiafulvalene); a pressure-sensitive new molecular conductor with localized spins

Abstract

An electrolysis of bis(ethylenedithio)tetrathiafulvalene (ET) and a Mn-cluster in a 1,1,2-trichloroethane solution containing 10% (vol/vol) of ethanol yields black lustrous single crystals, β″-(ET)3(MnCl4)(1,1,2-C2H3Cl3) based on the X-ray structural study. The crystal structure can be characterized as alternating two-dimensional donor sheets and insulating sheets made of isolated [MnCl4]2− ions and the 1,1,2-C2H3Cl3 molecules. Every pair of the neighbouring donor molecules has a large displacement along both short and long molecular axes. A tight-binding band calculation suggests that this arrangement should lead to a weak but isotropic intermolecular interaction in the donor sheets, and this in turn should lead to a marginally metallic or semiconducting electronic structure. Although the polarized reflectance spectra and the temperature-dependent spin susceptibility derived from the EPR spectra on the single crystal indicate metallic nature, the electrical behaviour under atmospheric pressure is semiconductive with room temperature conductivity of 35 S cm−1 and apparent activation energy of 0.023 eV. It exhibits a resistive hump with hysteresis at around 60 K, which could be associated with a structural transition demonstrated by a series of low temperature X-ray oscillation photographs. The magnetic susceptibility at 4.5–300 K does not exhibit any anomaly and is well reproduced by the Curie–Weiss model with Curie constant C/emu K mol−1 = 3.89 and Weiss temperature θ/K = −0.10. This magnetic behaviour can be quantitatively understood as the sum of Pauli paramagnetism of the π-electrons and the contribution from the local spins on the Mn(II) ions with d5-configuration (S = 5//2). High pressure easily suppresses the increase in resistivity at low temperature, and the electrical behaviour is particularly sensitive to the first 3–5 kbar.