A new low-dimensional metal, Cs[Pd(S₂C₂(CN)₂)₂]·0.5 H₂O

The discovery of a low-temperature superconducting state in organic compounds of the type (TMTSF)₂CIO₄ (T_c = 1.2 K) and (BEDT-TTF)₂AuI₂ (T_C = 4 K) (where TMTSF is tetramethyltetraselenafulvalene, BEDT-TTF is bis(ethylenedithiolo)tetrathiafalvalene and T_c is the superconducting transition temperature) has stimulated the search for new materials that may show higher values of T_c (refs 1-3). The general problem encountered in molecular charge-transfer salts of this type, which have conduction bands formed by intermolecular overlap of π-electron systems, is that conduction is usually quasi-one-dimensional, with good conduction along the stacking direction. Metals with this one-dimensional character are unstable, and undergo a Peierls transition⁴ to a semiconducting state at low temperatures. The relatively few exceptions (mentioned above), which remain metallic down to low temperatures, are considered to do so because they show stronger interstack interactions. We report here a new material with inherently two-dimensional interactions between the molecular π-electron systems and which we are able to stabilize as a metal down to low temperatures (1.4 K) under hydrostatic pressure (12 kbar).