Charge ordering mechanism in silver difluoride

Using density functional theory, a competition between the Mott-Hubbard and intervalence charge transfer mechanism of electron localization is revealed in AgF2, an important silver analog of oxocuprates. We show that at reduced temperature and electron correlations AgF2 becomes metallic and dynamically unstable with respect to soft phonon modes that promote charge ordering. The charge density wave (CDW) instability is closely related to the Kohn anomaly and Fermi surface nesting. The long advocated KBrF4-type CDW Ag1+/3+F2 structure and its facile transformation to the ground state Ag2+F2 phase is explained. Our results point to an intimate interplay between lattice, charge, and spin degrees of freedom in this seemingly simplistic binary metal fluoride.