Novel phase transition in Et2Me2Sb[Pd(dmit)2]2 at 70 K:: A possible mechanism based on strong dimerization of two-level molecules

A series of anion radical salts of [Pd(dmit)(2)] with tetrahedral cations such as Me4P+ are paramagnetic Mott insulators under ambient pressure. Recently, we have found that sufficiently pure samples of the Et2Me2Sb salt undergo a novel first-order phase transition at about 70 K. The spin susceptibility exhibits a sharp drop to zero as temperature decreases below this temperature, accompanied by a steep rise of resistivity. We have attributed this transition to the charge separation, 2[Pd(dmit)(2)](2)(-) -> [Pd(dmit)(2)](2) + [Pd(dmit)(2)](2)(2-). We discuss the possible mechanism of this phase transition, emphasizing the role of strong dimerization in [Pd(dmit)(2)](2). It is pointed out that the observed charge separation is favored by the HOMO-LUMO interplay in [Pd(dmit)(2)](2), which stabilizes the doubly occupied state of the dimer by forming double-bond. This mechanism is never expected for the conventional charge-ordered systems such as the ET salts with quarter-filling, in which the intermolecular Coulomb repulsion is believed to play a dominant role in charge separation.