We have investigated the resistivity of an organic Mott insulator, β′-(BEDT-TTF)2AuCl2, under extremely high pressures of up to 9.9 GPa, using the cubic anvil press, motivated by the fact that a related material, β′-(BEDT-TTF)2ICl2, shows the highest transition-temperature superconductivity among organics at 8.2 GPa. Through characterization by resistivity and susceptibility measurements at ambient pressure, we have confirmed that these two salts have common physical properties. Nevertheless, β′-(BEDT-TTF)2AuCl2 does not show superconductivity but remains semiconducting up to 9.9 GPa. We discuss the origin of these contrasting behaviors and suggest the cause to be the difference in anion size between the two salts.
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