Cation size effect in molecular conductors related to the high-pressure superconductor beta-Me(4)N[Pd(dmit)(2)](2) (dmit=2-thioxo-1,3-dithiole-4,5-dithiolate)

Cation size effect in the anion radical salts of Pd(dmit)(2) with the tetrahedral cation Me(4)Z(+) (Z=N, P, As, and Sb) has been examined by resistivity measurements under pressure, Xray structure analyses and tight-binding band calculations. Although all compounds possess very similar dimerized column structure, resistivity behaviors under pressure are quite different. The cation size affects an overlapping mode between the Pd(dmit)(2) dimers. Tight-binding band calculations show that subtle differences in the overlapping mode have a profound effect on their electronic structures. Our results suggest that the stronger two-dimensional nature tends to induce the high-pressure metallic phase. This system is a unique dimer system, where the dimerization is closely linked to the dimensionality of the electronic structure. Copyright (C) 1996 Elsevier Science Ltd