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

Abstract

A series of anion radical salts of [Pd(dmit) 2] with tetrahedral cations such as Me 4P + are paramagnetic Mott insulators under ambient pressure. Recently, we have found that sufficiently pure samples of the Et 2Me 2Sb 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 me 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.