Bond patterns and charge order amplitude in 1/4-filled charge-transfer solids

Metal-insulator transition accompanied by charge-ordering has been widely investigated in quasi-one-dimensional conductors, including in particular organic charge-transfer solids. Among such materials the 1/4-filled band charge-transfer solids are of strong interest, because of the commensurate nature of the charge-ordering in these systems. The period-four charge-order pattern ...1100... here is accompanied by two distinct bond distortion patterns, giving rise to bond-charge-density waves (BCDW) of types 1 and 2. Using quantum Monte Carlo methods, we determine the phase diagram within the extended Hubbard Hamiltonian that gives both types 1 and 2 BCDW in the thermodynamic limit. We further investigate the effect of electron-electron and electron-phonon interactions on the amount of charge disproportionation. Our results show that between these two bond patterns, one (BCDW2) in general coexists with a large magnitude charge order, which is highly sensitive to electron-phonon interactions, while the other (BCDW1) is characterized by weak charge order. We discuss the relevance of our work to experiments on several 1/4-filled conductors, focusing in particular on the materials (EDO-TTF)_2X and (DMEDO-TTF)_2X with large amplitude charge-order.