Charge order with structural distortion in organic conductors: Comparison between θ-(ET)₂RbZn(SCN)₄ and α-(ET) ₂I₃

Charge ordering with structural distortion in quasi-two-dimensional organic conductors θ-(ET)₂RbZn(SCN)₄ (ET = BEDT-TTF) and α-(ET)₂I₃ is investigated theoretically. By using the Hartree-Fock approximation for an extended Hubbard model which includes both on-site and intersite Coulomb interactions together with Peierls-type electron-lattice couplings, we examine the role of lattice degrees of freedom on charge order. It is found that the experimentally observed, horizontal charge order is stabilized by lattice distortion in both compounds. In particular, the lattice effect is crucial to the realization of the charge order in θ-(ET)₂RbZn(SCN)₄, while the peculiar band structure whose symmetry is lower than that of θ-(ET)₂RbZn(SCN) ₄ in the metallic phase is also an important factor in α-(ET)₂I₃ together with the lattice distortion. For α-(ET)₂I₃, we obtain a phase transition from a charge-disproportionated metallic phase to the horizontal charge order with lattice modulations, which is consistent with the latest X-ray experimental result.