Structural studies of pressure-induced organic superconductor β″-(DODHT)₂PF₆ having charge-ordering phase at ambient pressure

We performed the crystal structure analyses of the pressure-induced organic superconductor β″-(DODHT)₂PF₆ (DODHT denotes (1,4-dioxane-2,3-diyldithio)dihydrotetrathiafulvalene) at the hydrostatic pressures of 0.75 GPa and 1.9 GPa in order to investigate the key parameter for inducing the superconductivity. At ambient pressure, β″-(DODHT) ₂PF₆ undergoes an insulator-to-insulator transition at 255 K accompanied by charge ordering. By applying pressure, this charge-ordering transition is suppressed and superconductivity is generated at around 3 K above 1.32 GPa. Using the obtained atomic coordinates, we estimated the transfer integrals t and intermolecular nearest-neighbor Coulomb interaction energies V between conduction electrons and found that one of V/|t| ratios perpendicular to charge-ordering stripes plays an important role in suppressing the charge-ordering phase.