School of Chemical Engineering and Technology, State Key Laboratory of Chemical Engineering, Tianjin University, Tianjin 300072, People's Republic of China.
IUCrJ. 2023 Jan 1;10(Pt 1):38-51. doi: 10.1107/S2052252522011034.
The ubiquitous presence of crystal defects provides great potential and opportunities to construct the desired structure (hence with the desired properties) and tailor the synthetic process of crystalline materials. However, little is known about their regulation role in phase transition and crystallization pathways. It was generally thought that a phase transition in solution proceeds predominantly via the solvent-mediated phase-transformation pathway due to energetically high-cost solid-state phase transitions (if any). Herein, we report an unprecedented finding that an orientational disorder defect present in the crystal structure triggers an unusual pathway of a core-shell phase transition with apparent shape-preserved evolution. In the pathway, the solid-state dehydration phase transition occurs inside the crystal prior to its competitive transformation approach mediated by solvent, forming an unconventional core-shell structure. Through a series of combined experimental and computational techniques, we revealed that the presence of crystal defects, introduced by urate tautomerism over the course of crystallization, elevates the metastability of uric acid dihydrate (UAD) crystals and triggers UAD dehydration to the uric acid anhydrate (UAA) phase in the crystal core which precedes with surface dissolution of the shell UAD crystal and recrystallization of the core phase. This unique phase transition could also be related to defect density, which appears to be influenced by the thickness of UAD crystals and crystallization driving force. The discovery of an unusual pathway of the core-shell phase transition suggests that the solid-state phase transition is not necessarily slower than the solvent-mediated phase transformation in solution and provides an alternative approach to constructing the core-shell structure. Moreover, the fundamental role of orientational disorder defects on the phase transition identified in this study demonstrates the feasibility to tailor phase transition and crystallization pathways by strategically importing crystal defects, which has broad applications in crystal engineering.
晶体缺陷的普遍存在为构建所需结构（从而获得所需性能）和定制晶体材料的合成过程提供了巨大的潜力和机会。然而，人们对它们在相变和结晶途径中的调控作用知之甚少。通常认为，由于固-固相变的能量成本高，如果存在的话，溶液中的相变主要通过溶剂介导的相转变途径进行。在此，我们报告了一个前所未有的发现，即晶体结构中存在的取向无序缺陷会引发一种不寻常的核壳相变途径，且具有明显的形状保持演化。在该途径中，固-固脱水相变发生在晶体内部，然后是溶剂介导的竞争转化途径，形成一种非常规的核壳结构。通过一系列组合的实验和计算技术，我们揭示了晶体缺陷的存在，这些缺陷是在结晶过程中尿酸互变异构体引起的，提高了尿酸二水合物（UAD）晶体的亚稳性，并触发 UAD 在壳层 UAD 晶体表面溶解和核相再结晶之前脱水到尿酸无水物（UAA）相。这种独特的相变也可能与缺陷密度有关，而缺陷密度似乎受到 UAD 晶体厚度和结晶驱动力的影响。这种核壳相变的异常途径的发现表明，固-固相变不一定比溶液中的溶剂介导的相转变慢，并提供了一种构建核壳结构的替代方法。此外，本研究中确定的取向无序缺陷在相变中的基本作用证明了通过战略性引入晶体缺陷来定制相变和结晶途径的可行性，这在晶体工程中有广泛的应用。
