# In Spite of the Chemist’s Belief: Metastable Hydrates of CsCl

## TL;DR

The paper discovers two new hydrates of cesium chloride (CsCl·5H2O and CsCl·6H2O) formed under specific cooling and heating conditions, challenging previous assumptions about alkali chloride hydrates.

## Contribution

The discovery of metastable CsCl hydrates expands the known hydration behavior of alkali chlorides to include larger cations like Cs+.

## Key findings

- CsCl·5H2O and CsCl·6H2O hydrates were identified using ESEM, calorimetry, and X-ray diffraction.
- The hexahydrate (CsCl·6H2O) is more stable than the pentahydrate (CsCl·5H2O).
- The hydrates form from freeze-concentrated solutions during cold-crystallization of glassy solutions.

## Abstract

In this work, we focus on the low-temperature behavior
of concentrated
aqueous solutions of cesium chloride and discover two hydrates of
CsCl. We employ four different methods, namely, (i) simple cooling
at rates between 0.5 and 80 K s–1, (ii) simple cooling
followed by pressurization, (iii) hyperquenching at 106 to 107 K s–1, and (iv) hyperquenching
followed by pressurization. Depending on the method, different types
of phase behaviors are observed, which encompass crystallization involving
freeze-concentration, pressure-induced amorphization, full vitrification,
and polyamorphic transformation. The CsCl hydrates discovered in our
work cold-crystallize above 150 K upon heating after ultrafast vitrification
(routes iii and iv) and show melting temperatures below the eutectic temperature of 251 K. We determine the composition
of these hydrates to be CsCl·5H2O and CsCl·6H2O and find evidence for their existence in ESEM, calorimetry,
and X-ray diffraction. The dominant and less metastable hydrate is
the hexahydrate, where the pentahydrate appears as a minority species.
We also reveal the birthplace for the CsCl hydrates, namely, the freeze-concentrated
solution (FCS) formed upon cold-crystallization of the fully glassy
solution (from iii and iv). The spongy FCS produced upon cooling of the liquid (from i and ii) is incapable of crystallizing CsCl-hydrates.
By contrast, the FCS produced upon heating the glassy
solution (from iii and iv) shows tiny, fine features that are capable
of crystallizing CsCl-hydrates. Our findings contradict the current
knowledge that alkali chlorides only have hydrates for the smaller
cations Li+ and Na+, but not for the larger
cations K+, Rb+, and Cs+ and pave
the way for future determination of CsCl-hydrate crystal structures.
The pathway to metastable crystalline materials outlined here might
be more generally applicable and found in nature, e.g., in comets
or on interstellar dust grains, when glassy aqueous solutions crystallize
upon heating.