Závacká Kamila, Vetráková Ľubica, Bachler Johannes, Neděla Vilém, Loerting Thomas
Institute of Scientific Instruments of the CAS, v.v.i., Kralovopolska 147, 61264 Brno, Czech Republic.
Institute of Physical Chemistry, University of Innsbruck, A-6020 Innsbruck, Austria.
ACS Phys Chem Au. 2025 Feb 6;5(2):195-206. doi: 10.1021/acsphyschemau.4c00093. eCollection 2025 Mar 26.
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, (ii) simple cooling followed by pressurization, (iii) hyperquenching at 10 to 10 K s, 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 the eutectic temperature of 251 K. We determine the composition of these hydrates to be CsCl·5HO and CsCl·6HO 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 of the liquid (from i and ii) is incapable of crystallizing CsCl-hydrates. By contrast, the FCS produced upon 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.
在这项工作中，我们聚焦于氯化铯浓水溶液的低温行为，并发现了两种氯化铯水合物。我们采用了四种不同的方法，即：(i) 以0.5至80 K/s的速率进行简单冷却；(ii) 简单冷却后加压；(iii) 以10³至10⁴ K/s的速率进行超快速淬火；(iv) 超快速淬火后加压。根据方法的不同，观察到了不同类型的相行为，包括涉及冷冻浓缩的结晶、压力诱导非晶化、完全玻璃化以及多晶型转变。我们工作中发现的氯化铯水合物在超快速玻璃化（途径iii和iv）后加热时，会在150 K以上冷结晶，并显示出高于共晶温度251 K的熔化温度。我们确定这些水合物的组成为CsCl·5H₂O和CsCl·6H₂O，并在环境扫描电子显微镜（ESEM）、量热法和X射线衍射中找到了它们存在的证据。占主导且较不稳定的水合物是六水合物，其中五水合物为少数物种。我们还揭示了氯化铯水合物的诞生地，即完全玻璃态溶液（来自途径iii和iv）冷结晶时形成的冷冻浓缩溶液（FCS）。液体（来自途径i和ii）冷冻产生的海绵状FCS无法使氯化铯水合物结晶。相比之下，玻璃态溶液（来自途径iii和iv）冷冻产生的FCS呈现出微小、精细的特征，能够使氯化铯水合物结晶。我们的发现与目前的认知相矛盾，即碱金属氯化物仅对较小的阳离子Li和Na有对应的水合物，而对较大的阳离子K、Rb和Cs则没有，这为未来确定氯化铯水合物晶体结构铺平了道路。这里概述的形成亚稳晶体材料的途径可能更具普遍适用性，并且在自然界中也能找到，例如当玻璃态水溶液加热结晶时，在彗星或星际尘埃颗粒中。
