Cation Dependence of Crystal Structure and Band Parameters in a Series of Molecular Conductors, β'-(Cation)[Pd(dmit)2]2 (dmit = 1,3-dithiole-2-thione-4,5-dithiolate)

An isostructural series of anion radical salts, β'-(EtxMe4−xZ)[Pd(dmit)2]2 (x = 0–2, Z = P, As, Sb), with a quasi-triangular lattice comprising the dimer unit [Pd(dmit)2]2− belong to a strongly correlated electron system with geometrical frustration. Intra and interdimer transfer integrals between the frontier molecular orbitals, which characterize the strength of electron correlation and degree of frustration, can be tuned by selection of the counter cation. We have systematically analyzed the crystal structure with X-ray diffraction method and intermolecular transfer integrals using extended Hückel molecular orbital calculations based on structural data. The variation in the cation affects the unit cell in a manner equivalent to an anisotropic pressure. Increasing the covalent radius of the central atom Z and the number of ethyl groups (x) in the cation leads to slight arching of the Pd(dmit)2 molecule. This arch-shaped distortion of the Pd(dmit)2 molecule modifies the interdimer transfer integrals in formation of the regular triangular dimer lattice. On the other hand, the intradimer transfer integral, which is correlated with the effective on-site Coulomb interaction of the dimer, is weakly dependent on the type of cation.