We demonstrate that an organic Mott insulator, 0 -(BEDT-TTF) 2 ICl 2 , is metallized by application of extremely high pressure by means of the cubic anvil press. When the metallic state is stabilized, superconductivity with the highest transition-temperature (T c ) among the organic systems appears. The lower limit of the pressure at which the superconducting transition manifests itself is approximately 7.0 GPa and T c reaches a maximum value, 14.2 K (onset), at 8.2 GPa. The pressure-temperature phase diagram is constructed and discussed in terms of the comparison with those of other organic systems.

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Additional quench experiments were also carried out using a multianvil apparatus in order to further explore the phase relations and to study chemical compositions of the coexisting phases at various pressures and temperatures.

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Physical properties of an organic conductor,-(BEDT-TTF) 2 IBrCl, were characterized under ambient and extremely high pressures. Semiconducting behavior with activation energy, E a =0.11eV, and antiferromagnetic transition with T N =19.5K were uncovered in the present work. In addition, we have found that this salt showed a sign of superconductivity at 8.0GPa in common with the case of the-(BEDT-TTF) 2 ICl 2 , which is the highest-T c superconductor among organics.

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An organic Mott insulator,-(BEDT-TTF) 2 ICl 2 , is metallized by application of extremely high pressure up to 9.0GPa. When the metallic state is stabilized, superconductivity with the highest transition-temperature (T c) among organic systems appears. The maximum T c that we observed was 14.2K(onset) at 8.2GPa. A magnetic-field effect on this superconductivity and normal-state properties are discussed in terms of comparison with other organic superconductors.