Charge ordering mechanism in silver fluoride AgF$_2$

TL;DR

This study uses Density Functional Theory to explore charge localization mechanisms in AgF$_2$, revealing conditions under which it becomes metallic and unstable to charge density wave formation, highlighting complex interactions between lattice, charge, and spin.

Contribution

It uncovers the competition between Mott-Hubbard and intervalence charge transfer mechanisms in AgF$_2$, and explains the structural transformation related to charge ordering.

Findings

AgF$_2$ becomes metallic at reduced temperature and electron correlations.

AgF$_2$ exhibits charge density wave instability related to Fermi surface nesting.

Transformation from KBrF$_4$-type structure to ground state phase is explained.

Abstract

Using Density Functional Theory, competition between Mott-Hubbard and intervalence charge transfer mechanism of electron localization is revealed in AgF$_2$, an important silver analogue of oxocuprates. We show that at reduced temperature and electron correlations AgF$_2$ becomes metallic and dynamically unstable in respect to CDW phonon modes that promote charge ordering. The CDW instability is closely related to Kohn anomaly and Fermi surface nesting. The long advocated KBrF$_4$ type CDW Ag$^{1+/3+}$F$_2$ structure and its facile transformation to the ground state Ag$^{2+}$F$_2$ phase is explained. Our results point out to intimate interplay between lattice, charge and spin degrees of freedom in seemingly simplistic binary metal fluoride.