Understanding the lifetime of water with dynamic network analysis: the case of CsOH.H2O

TL;DR

This paper investigates atomic motions and proton exchange dynamics in CsOH·H2O, revealing a chemical reaction-driven order-disorder transition, hydrogen vacancy diffusion, and a novel Raman activity model.

Contribution

It introduces a detailed analysis of hydrogen bond interconversion and proton exchange mechanisms in CsOH·H2O, highlighting a chemical reaction-driven transition and associated Raman activity.

Findings

Hydrogen bonds continually interconvert via proton exchange.

Order-disorder transition proceeds through chemical reactions, not molecular rotation.

Rapid hydrogen vacancy diffusion leads to fast-ionic conduction.

Abstract

We describe the atomic-level motions in caesium hydroxide monohydrate (CsOH$\cdot$H$_2$O), which is a chemical compound containing layers of water and hydroxide ions. At this composition, each oxygen is involved in three hydrogen bonds which, in the hexagonal structure, form a quasi-2D honeycomb lattice. While oxygen and caesium atoms form a typical crystal lattice, the dynamics of the hydrogen atoms are more complex. Here we show that the covalent and hydrogen bonds are continually interconverting, meaning that the water and hydroxyl are interconverting by proton exchange. The order-disorder transition of the water and hydroxyl proceeds by chemical reaction rather than rotation or diffusion of the molecules. A hydrogen can rotate out of the layer, leaving a vacant site in the 2D layer. Such a hydrogen vacancy can diffuse rapidly by single molecule rotation, leading to fast-ionic conduction. The proton exchange leads to a novel type of Raman activity combining stretch and exchange processes, for which we develop a theoretical model. This would manifest in a broad single peak associated with both H$_2$O and OH stretches and a low frequency peak appearing at elevated temperature.