Intra- Versus Inter-dimer Charge Inhomogeneity in the Triangular Lattice Compounds of Β′-Cs[pd(dmit)2]2: A Degree of Freedom Characteristic of an Interchange of Energy Levels in the Molecular Orbitals

We have carried out the complete analyses of the C=C stretching modes in the vibrational spectra in the triangular lattice of beta'-Cs[Pd(dmit)(2)](2) in order to solve the puzzling phenomenon that the ground state is neither spin frustration nor anti-ferromagnetic state but octamerization. We found that both charge-rich and charge-poor dimers are non-centrosymmetric dimers with the inhomogeneous charges. Because the energy levels of HOMO and LUMO are interchanged due to the tight dimerization, the cooperative interaction between the inter-site Coulomb repulsions and the valence-bond formation operates within and between dimers, those which contribute to the inter-dimer and intra-dimer charge separations, respectively. Octamer is the minimal unit under both cooperative interactions. In the high-temperature phase of beta'-Cs[Pd(dmit)(2)](2), the competition between octamerization and tetramerization is observed because of the suppression of the intra-dimer cooperative interaction. The competition between two different states indicates the degree of freedom characteristic of the molecular orbital due to the tight dimerization. The cooperative interactions of the various X[Pd(dmit)(2)](2) salts are quantitatively evaluated from the C=C stretching modes.