Crystal and Electronic Structures of Molecular Conductor β'-Et2Me2P[Pd(dmit)2]2(dmit = C3S5) under High Pressure

The two-dimensional molecular conductor β'-Et 2 Me 2 P[Pd(dmit) 2 ] 2 is a Mott insulator under ambient pressure and exhibits a metallic state accompanied by the superconductivity with applying pressure. And, in the higher pressure region, another non-metallic behavior appears. X-ray single-crystal structure analyses under high pressures were performed at room and low temperatures to reveal the mechanism of the pressure effect. Tight-binding band calculations based on the resultant structural data indicate that the insulator-to-metal transition is a Mott transition due to an increase of the band width of the HOMO-based band and the HOMO–LUMO band overlap mechanism proposed in the previous report does not apply to this case. The high-pressure non-metallic state is relevant to the structural transition with a symmetry breaking.