Study of the pressure-induced phase transition on molecular conductors from the X-ray diffraction measurement

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The intention of this research is to develop the high-pressure structural study for molecular conductors. The "easy handling" powder X-ray diffraction type diamond anvil cell was produced to study the high-pressure structure investigation.Molecular conductors Me_4X(CPDT-TCNQ)_2 (X=N, P, As) having the extremely one-dimensional electronic system show the metal-insulator transition at 135K (X=N), 165K (X=P) and 185K (X=As). The origin of the transition at low temperature is different with the displacement of the cation. For Me_4N, Me_4P, Me_4As compounds, the insulating states were also observed under pressure at room temperature. From the high-pressure X-ray difrraction measurement, I identified that the transitions under pressure are originated by the 2_kF CDW for Me_4N, Me_4P salts and 4k_FCDW for Me_4As salt. I succeeded to complete the universal pressure-temperature phase diagram for the series of the compounds. The present result is significant to the research for a variety of one-dimensional molecular conductors.One-dimensional π-d conductors (DBr-DCNQI)_2Cu and (DI-DCNQI)_2Cu show drastic changes in the conducting properties depending on the electronic state of Cu atoms. The detail investigation of the lattice parameter clarifies the sight difference of their structures in the high-pressure anomalous metallic phases. This structural character is the origin of the appearance of the different high-pressure metallic state for the both compounds. This result would contribute the development of the research for the DCNQI system having the unique electronic and magnetic properties under pressure.