Understanding Hydration Transitions of CaBr₂

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.