The unusual properties of the κ-phases ET2[N(CN)2]X (X = Cl, Br, I): a result of critical Coulomb interactions

A review is given of the astonishingly different physical properties of the above-mentioned three isostructural compounds (ET = bis(ethylenedithio)tetrathiafulvalene). While the Cl crystal is an insulator with ferrimagnetic order at low temperatures, turning into a re-entrant superconductor at a pressure of only 300 bar, the Br crystal is an inhomogeneous superconductor with coexisting magnetically ordered (and presumably insulating) domains. The I crystal is semimetallic, and its magnetic properties are not yet known. We focus on the possibility that the conduction electron system in these crystals is in the immediate vicinity of a Mott transition due to critical Coulomb interactions. Small changes in intermolecular distances, such as those caused by thermal expansion and especially by a lattice-dynamical second-order distortive transition can decide between a metallic (superconducting) and non-metallic (magnetic) ground state.