Durelle Maxime, Charton Sophie, Gobeaux Frédéric, Chevallard Corinne, Belloni Luc, Testard Fabienne, Trépout Sylvain, Carriere David
CEA, DES, ISEC, DMRC, Univ. Montpellier, 30207 Marcoule, France.
Université Paris-Saclay, CNRS, CEA, NIMBE, LIONS, 91191 Gif-sur-Yvette, France.
J Phys Chem Lett. 2022 Sep 15;13(36):8502-8508. doi: 10.1021/acs.jpclett.2c01829. Epub 2022 Sep 6.
Crystallization from solution often occurs via "nonclassical" routes; that is, it involves transient, non-crystalline states like reactant-rich liquid droplets and amorphous particles. However, in mineral crystals, the well-defined thermodynamic character of liquid droplets and whether they convert─or not─into amorphous phases have remained unassessed. Here, by combining cryo-transmission electron microscopy and X-ray scattering down to a 250 ms reaction time, we unveil that crystallization of cerium oxalate involves a metastable chemical equilibrium between transient liquid droplets and solid amorphous particles: contrary to the usual expectation, reactant-rich droplets do not evolve into amorphous solids. Instead, at concentrations above 2.5 to 10 mmol L, both amorphous and reactant-rich liquid phases coexist for several tens of seconds and their molar fractions remain constant and follow the lever rule in a multicomponent phase diagram. Such a metastable chemical equilibrium between solid and liquid precursors has been so far overlooked in multistep nucleation theories and highlights the interest of rationalizing phase transformations using multicomponent phase diagrams not only when designing and recycling rare earths materials but also more generally when describing nonclassical crystallization.
溶液结晶通常通过“非经典”途径发生；也就是说，它涉及富反应物液滴和无定形颗粒等瞬态非晶态。然而，在矿物晶体中，液滴明确的热力学特性以及它们是否会转化为非晶相仍未得到评估。在这里，通过结合低温透射电子显微镜和低至250毫秒反应时间的X射线散射，我们揭示了草酸铈的结晶涉及瞬态液滴和固体无定形颗粒之间的亚稳化学平衡：与通常的预期相反，富反应物液滴不会演变成无定形固体。相反，在浓度高于2.5至10 mmol/L时，无定形相和富反应物液相共存数十秒，它们的摩尔分数保持恒定，并遵循多组分相图中的杠杆规则。到目前为止，在多步成核理论中，固体和液体前驱体之间的这种亚稳化学平衡一直被忽视，这凸显了在设计和回收稀土材料时，以及更普遍地在描述非经典结晶时，使用多组分相图来合理化相变的重要性。
