STRUCTURAL AND ELECTRONIC-PROPERTIES OF CS[PD(DMIT)2]2

We report a series of measurements of the structural and electronic properties of the molecular conductor Cs[Pd(dmit)2]2 (dmit = isotrithionedithiolate). This material has a C2/c 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 C centring symmetry. We present a calculation of the electronic 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.