# Understanding Hydration Transitions of CaBr2

## TL;DR

This study identifies the hydration and dehydration steps of CaBr2, revealing a stable monohydrate phase and structural similarities to CaCl2 hydrates.

## Contribution

The study clarifies the hydration pathway of CaBr2, resolving debates about intermediate hydrate steps and their crystal structures.

## Key findings

- Only a stable monohydrate phase exists between anhydrate and dihydrate in CaBr2.
- The dihydrate of CaBr2 has the same crystal structure as CaCl2 dihydrate, differing only in size.
- Hydration steps of CaBr2 are consistent in both bulk and confined mesoporous silica gels.

## Abstract

Due to climate change and the energy transition, energy
storage
applications are being studied and developed. One energy storage application
is a heat storage battery, which needs materials that can store and
release heat with high energy storage capacity. One such material
is a salt hydrate. The hydration pathways of salt hydrates can have
different numbers of steps. There are salts with single-hydrate steps
like for CuCl2 (0–2) and LiBr (0–1) and multihydrate
steps like for MgCl2 (0–2–4–6) and
SrCl2 (0–1–2–6). Additionally, there
are also salts with complex hydration–dehydration pathways
like for CaCl2 (0–1/3–2–1–0).
Little is known about the hydrate steps of CaBr2. The crystal
structures of the CaBr2 nona-, hexa-, and anhydrate are
known, but there are no intermediate steps and conditions for these
transitions. The hexahydrate and anhydrate have the same structure
as CaCl2 except for the unit cell size due to the different
anions. Additionally, the equilibria were determined for the hexa-,
tetra-, and dihydrate transitions. However, the intermediate steps
are debated. The hydrates 3, 1.5, 1, and 0.5 were all proposed but
are disputed and not verified. Therefore, the hydration and dehydration
pathways of CaBr2 from the anhydrate to the dihydrate and
back were examined in this study for both the bulk salt and the confinement
of mesoporous silica gels. The kinetic phase transition onsets and
equilibrium lines were measured for the bulk salt. Powder X-ray diffractograms
were used to ensure that the same structures were formed every time
during hydration and dehydration. Single-crystal analysis was used
to determine the crystal structures of the hydrates. These experiments
showed only a stable monohydrate phase between the anhydrate and dihydrate
during hydration and dehydration. Furthermore, the dihydrate has the
same crystal structure as the dihydrate of CaCl2 except
for the size, while the monohydrate differs from the CaCl2 monohydrate. Additionally, the composites’ kinetic onsets
and powder diffractograms were measured, which showed that CaBr2 performs the same hydrate steps in confinement as in bulk
form.

The hydration and dehydration steps of
the salt hydrate
CaBr2 were determined with their formation and stable conditions
together with the crystal structures of the mono- and dihydrate.