Essential role of anions in the charge ordering transition of α-(BEDT-TTF)2I3

alpha-(BEDT-TTF)(2)I-3 exhibits ametal to insulator transition around 135 K that has been ascribed to charge ordering in the donor layers containing three different donors (A-A dimers, B and C). First-principles density-functional theory (DFT) calculations provide a description of the electronic structure of this system in agreement with the presently available experimental information above and below the transition. A new mechanism of the charge ordering transition in which the anions play a major role is proposed. It is shown that lowering the temperature induces a differentiation of the two I-3(-) zigzag chains which leads to an increase/decrease of the donor-anion hydrogen bonding with the two initially identical A donors. This induces a redistribution of holes mostly affecting the A donors (thus becoming inequivalent A, A'), giving rise to the charge ordering. The interrelation between the strength of the hydrogen bonding with the anions and the concentration of holes in the HOMOs involves a subtle polarization mechanism among the sigma and pi electron densities of the donors. The redistribution of holes leads to the opening of a band gap via the modulation of the site potential of donors A/A' and the increased band mixing because of the symmetry lowering.