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Description
Two polymorphic forms have been reported for solid AgF$_2$ at ambient pressure: a layered Ag$^{\rm II}$F$_2$ ($\alpha$) and a charge density wave Ag$^{\rm I}$Ag$^{\rm III}$F$_4$ one ($\beta$). The $\alpha$ phase is better known of the two. It has recently received attention due to numerous structural and electronic similarities with oxocuprate precursors of high-temperature superconductors [1]. The key common feature of AgF$_2$ and undoped oxocuprates is the presence of antiferromagnetic sheets of A$^{\rm II}$B$_2$ stoichiometry(AgF$_2$ resp. CuO$_2$). Correspondingly, structural, electronic and magnetic properties of $\alpha$ form at ambient and high pressures have been thoroughly examined in number of experimental and theoretical studies [1--4]. $\beta$ form has been observed before only once as a red-brown amorphous product of reaction of AgBF$_4$ with KAgF$_4$ in anhydrous HF [5]. It is known to undergo rapid exothermic conversion to the alpha form when temperature is raised from $-80^\circ$C to ca. 0$^\circ$C: Ag$^{\rm I}$Ag$^{\rm III}$F$_4$ $\rightarrow$ 2Ag$^{\rm II}$F$_2$ [5] and its has been confirmed in an early theoretical study, which assumed that $\beta$ form adopts that of KAgF$_4$ type structure. In the present work, we have thoroughly scrutinized the relative thermodynamic stability and lattice dynamics of the two phases, Ag$^{\rm II}$F$_2$ and Ag$^{\rm I}$ Ag$^{\rm III}$F$_4$, in a comparative theoretical and experimental study employing Raman spectroscopy and Density Functional Theory (DFT). We provide theoretical evidence for dynamical stability of both polymorphs, calculate thermodynamic potentials, perform normal mode analysis and discuss relative thermodynamic stability of the two phases.
The individual Ag$^{\rm II}$F$_2$ $\alpha$-phase layers are isoelectronic with [CuO$_2$] sheets in oxocuprates [1]. Both systems are AFM semiconductors with charge-transfer band gap. In Ag$^{\rm II}$F$_2$ ($\alpha$) we have identified Ag--F bond stretching modes of $B_{2g}$ symmetry with unusually strong response to on-site Coulombic correlation. The response of electronic structure to the $B_{2g}$ mode is characterized by modulation of intervalence charge transfer (ICT), which is accompanied by bandgap closure and subsequent reopening as the system progresses from antiferromagnetic Ag$^{\rm II}$Ag$^{\rm II}$F$_4$ to diamagnetic mixed-valence Ag$^{\rm I}$Ag$^{\rm III}$F$_4$ state. AgF$_2$ in this respect is analogous to oxocuprates family.
Refs
[1] J. Gawraczyński {\it et al.}, Proc. Natl. Acad. Sci. USA {\bf 116}, 1495 (2019).
[2] J. Gawraczyński {\it et al.}, Inorg. Chem. {\bf 56}, 14651 (2017).
[3] D. Kurzydłowski {\it et. al.}, Chem. Commun. {\bf 54}, 10252 (2018).
[4] T. Jaroń and W. Grochala, Phys Stat Sol (RRL) {\bf 2}, 71 (2008).
[5] B. Žemva {\it et al.}, Inorg. Chem. {\bf 38}, 4570 (1999).
