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We report a Series of measurements of lhe structural and electronic properties of the molecular conductor Cs[Pd(dmit),], (dmit = isotrithionedithiolate). This material has a C21c structure with stacks of the acceptor groups arranged in sheets separated by the cations, with appreciable dimerization of the acceptor groups along the stack direction. It shows metallic properties at room temperature. We have characterized this material with measurements of conductivity. thermopower. magnetic susceptibility and polarized optical reflectivity, and we find that the electronic structure is quite isotropic in the plane of the acceptor sheets, On cooling, there is a metal-insulator transition at 56.5 K, and this opens an energy gap at the Fermi energy. This transition is associated with a complex periodic lattice distortion comprising an incommensurate modulation and also a commensurate distortion, which breaks the Ccentringsymmetry. Wepresent acaldation oftheelectronic band structure using an extended Huckel formalism. and show that, owing to the strong dimerization along the stack direction. the Fermi energy lies halfway in a band of highest occupied molecular orbital character. We consider also how the observed structural instability is able to produce an energy gap over the whole Fermi surface, and propose a model that requires coupling between the two types of modulation.

The relation between crystal structure and bulk magnetic properties is investigated in the molecular charge transfer salts (BEDT-TF)2MCl4 (M = Ga, Fe). (BEDT-TTF)2GaCl4 crystallizes in the triclinic system. Its crystal structure consists of pairs of BEDT-TTF molecules arranged in layers with intermolecular S···S interactions. Band structure calculations predict semimetallic behavior contrary to the semiconductivity observed even under a pressure of 6 kbar (σ(300 K, 1 bar) = 10-1 S cm-1 and E A = 0.2eV). The static (Faraday and SQUID magnetometry) and spin (EPR) susceptibilities indicate low-dimensional Heisenberg antiferromagnetic behavior with the susceptibility tending to zero as the temperature approaches zero. The data are analyzed using several low-dimensional magnetic models and are best fitted to a model consisting of two different spin dimers (Δ1 = 108 K and Δ2 = 212 K). The static magnetic susceptibility of (BEDT-TTF)2FeCl4 is modeled by a sum of Curie−Weiss (S = 5/2 for Fe(d5) and ϑ = −4 K), χtip, and single dimer (Δ = 45 K) parameters. The BEDT-TTF layers in these compounds thus behave as Mott−Hubbard-localized systems, and the interaction between the magnetic moment on the Fe with those on the organic layer is negligible.

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