α-(BEDT-TTF) 2I 3 exhibits a metal 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 I3- 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 σ and π 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. © 2012 American Physical Society.
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|Publication status||Published - 10 May 2012|