Structural study of low temperature charge-separated phases of Pd(dmit)2-based molecular conductors

We have studied crystal structures of two Pd(dmit)(2)-based molecular conductors, Et2Me2Sb-[Pd(dmit)(2)](2) and Cs[Pd(dmit)(2)](2), above and below their phase transition temperatures. The Et2Me2Sb salt undergoes a first-order transition from a Mott insulator to a non-magnetic state at ca. 70 K, while the Cs salt exhibits a metal-to-insulator (M-I) transition at ca. 56 K with decreasing temperature. These salts are composed of crystallographically equivalent dimeric units [Pd(dMit)(2)](2)(-) above the transition temperatures. The low temperature structures have been revealed to have two crystallographically independent dimers arranged alternately along the b axis. The structural changes of the dimers have been analyzed by calculations of overlap integrals, to show that the degree of the dimerization changes through the phase transitions. This indicates that both of the phase transitions are characterized by the charge separation, 2dimer(-) -> dimero(0) dimer(2-). Crystal structure of a related compound, Et3MeSb-[Pd(dMit)(2)](3) is also described.