Formation of Stoichiometric CsF${}_n$ Compounds

Abstract

Alkali halides $MX$, have been viewed as typical ionic compounds, characterized by 1:1 ratio necessary for charge balance between M${}^{+}$ and X${}^{-}$. It was proposed that group I elements like Cs can be oxidized further under high pressure. Here we perform a comprehensive study for the CsF-F system at pressures up to 100 GPa, and find extremely versatile chemistry. A series of CsF${}_n$ ($n$ $\ge$ 1) compounds are predicted to be stable already at ambient pressure. Under pressure, 5$p$ electrons in Cs atoms become active, with growing tendency to form Cs${}^{3+}$ and Cs${}^{5+}$ valence states at fluorine-rich conditions. Although Cs${}^{2+}$ and Cs${}^{4+}$ are not energetically favoured, the interplay between two mechanisms (polyfluoride anions and polyvalent Cs cations) allows CsF${}_2$ and CsF${}_4$ compounds to be stable under pressure. The estimated defluorination temperatures of CsF${}_n$ (n=2,3,5) compounds at atmospheric pressure (218 ${}^{\circ }$C, 150 ${}^{\circ }$C, -15 ${}^{\circ }$C, respectively), are attractive for fluorine storage applications.

7 pages, 6 figures

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