Structural phase transition of θ-(BEDT-TTF)₂RbZn(SCN) ₄ under high pressure

θ-(BEDT-TTF)₂RbZn(SCN)₄ is known as a two-dimensional organic conductor exhibiting a metal-insulator phase transition associated with a charge ordering phenomena at T_MI = 200 K under ambient pressure. The transition temperature increases with applied pressure and reaches room temperature at 1.1 GPa. The structural change by pressure induced phase transition was investigated by X-ray diffraction using synchrotron radiation. A superlattice formation of 2c was found above 1.1 GPa. A single crystal structural analysis taking account of the superlattice was carried out at 1.2GPa. Displacements of BEDT-TTF molecules were found to be similar to the displacements in the low temperature phase at the ambient pressure. However, the shrinkage of the c-axis under pressure was four times as large as that under cooling at the transition point. Decreases in the intermolecular distances between BEDT-TTF molecules resulted in increases of the intersite Coulomb energies V and an increase of the electric bandwidth W. U/W (U: on-site Coulomb energy) decreased as a result of an increase of W. Such a change of U/W is obstructive to the enhancement of the charge ordered phase. This indicates that an effect of V is indispensable to the charge-ordered phase in this material.