Formation of Stoichiometric CsF$_n$ Compounds

TL;DR

This study predicts the formation of stable CsF$_n$ compounds with multiple oxidation states under high pressure, revealing versatile chemistry and potential fluorine storage applications.

Contribution

It provides a comprehensive theoretical prediction of stable CsF$_n$ compounds with multiple oxidation states at various pressures, expanding understanding of alkali halide chemistry.

Findings

CsF$_n$ compounds are stable at ambient pressure.

High pressure activates 5$p$ electrons in Cs, enabling higher oxidation states.

CsF$_2$ and CsF$_4$ are stable under pressure despite not favoring Cs$^{2+}$ and Cs$^{4+}$.

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$_n$ ($n$ $\geq$ 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.