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

Abstract

Cation size effect in the anion radical salts of Pd(dmit) 2 with the tetrahedral cation Me 4Z + (Z=N, P, As, and Sb) has been examined by resistivity measurements under pressure, X-ray 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.