Transfer integrals and the spatial pattern of charge ordering in theta-(BEDT-TTF)(2)RbZn(SCN)(4) at 90 K

theta-(BEDT-TTF)(2)RbZn(SCN)(4) is a two-dimensional organic conductor exhibiting a metal-insulator transition at T-MI = 200 K. This insulating charge ordered state below T-MI was investigated by a single crystal structural analysis taking account of the superlattice formation below T-MI. A large displacement (about 0.2 Angstrom) of BEDT-TTF molecules associated with the transition was found. Transfer integrals between BEDT-TTF molecules were also calculated from the structural data. The two-dimensional net of transfer integrals above T-MI was modulated into the quasi one-dimensional transfer integral chain by the superlattice formation below T-MI. The ionicities of BEDT-TTF molecules are estimated from the intramolecular bond length distribution. As a result, charge disproportionation between BEDT-TTF molecules was found, and the ionicity was 0 to +0.2 and +0.8 to +1.0 respectively. The hole-rich sites are laid on the quasi one-dimensional transfer integral chain. We conclude that the spatial pattern of charge ordering is a stripe alternating along the c-axis.