Pressure-Induced Metal-to-Insulator Phase Transitions in the Organic Conductor (2,5 DM-DCNQI) 2 Cu

The electrical resistivity along the high-conductivity axis has been measured at pressures up to 9 kbar in the novel organic conductor (2,5 DM-DCNQI) 2 Cu. Deviations from the usual linear behaviour of the conductivity under pressure at room temperature have been observed. Moreover, at lower temperatures metal-to-insulator phase transitions have been induced with values of transition temperatures depending on the applied pressure. The obtained results are discussed and compared with corresponding data for other organic compounds. A tentative interpretation of this novel and unusual phase diagram is proposed in terms of a mixed valence state for copper ions.