PCA Analysis of In Situ X‑ray Powder Diffraction and Imaging Data Shedding New Light on Solid-State Transformations: The Crystallization of Low Temperature Eutectic Mixtures

Eutectic mixtures are usually studied by differential scanning calorimetry (DSC), able to identify the transition temperatures, possible hysteresis, and investigate the energetic features of transformations. However, DSC is not able to give compositional, structural, or morphological information. A new approach is proposed exploiting powder X-ray diffraction (XRPD) and imaging to overcome the issues posed to diffraction by the presence of an amorphous liquid phase. Principal component analysis (PCA) is applied blindly to in situ XRPD data from both solid and liquid phases in an approach called differential scanning diffraction (DSD), with PCA scores being the reaction coordinate of melting or crystallization steps. PCA was used in a similar way to analyze the imaging data in what was named differential scanning imaging (DSI). Exploiting this approach, the structural and morphological changes during phase transitions can be characterized by XRPD and imaging respectively, complementarily to the energetic effects probed by DSC. Melting and crystallization points can be identified together with the hysteresis between downward and upward temperature ramps, by the structural and morphological viewpoints. A three-component mixture (NaBr, KCl, and water), with wide industrial applications, was studied to describe the behavior around the eutectic composition and examine how small mixture changes can affect the transition temperature and the freezing/melting behaviors. The phase composition at the solid state was elucidated and a new phase of NaBr was identified and its lattice parameters were obtained by XRPD. DSD and DSI resulted complementary to traditional DSC data with many potential applications in solid state chemistry and materials science.共晶混合物通常通过差示扫描量热法（differential scanning calorimetry, DSC）开展研究，该方法可识别相变温度、可能存在的滞后现象，并探究相变的能量特征。然而，差示扫描量热法无法提供混合物的成分、结构或形貌信息。本研究提出一种基于粉末X射线衍射（powder X-ray diffraction, XRPD）与成像技术的新方案，以解决非晶液相存在时对衍射分析造成的干扰问题。研究采用被命名为差示扫描衍射法（differential scanning diffraction, DSD）的策略，将主成分分析（Principal component analysis, PCA）以盲分析的方式应用于固液两相的原位XRPD数据，以主成分分析得分作为熔融或结晶过程的反应坐标。研究采用类似思路，将主成分分析用于成像数据分析，该方法被命名为差示扫描成像法（differential scanning imaging, DSI）。通过该联合方案，可分别通过XRPD与成像技术表征相变过程中的结构与形貌变化，作为差示扫描量热法所测得的能量效应的互补表征手段。从结构与形貌视角出发，可同时确定熔融与结晶点，以及升降温扫描过程中的滞后现象。本研究选取一种具备广泛工业应用价值的三组分混合物（溴化钠NaBr、氯化钾KCl与水）作为研究对象，以探究其共晶成分附近的相转变行为，并考察混合物组分的微小变化如何影响相变温度与冻结/熔融行为。研究阐明了该混合物固态下的相组成，通过XRPD鉴定出一种全新的溴化钠晶相，并获取了其晶格参数。差示扫描衍射法与差示扫描成像法可作为传统差示扫描量热法数据的互补手段，在固态化学与材料科学领域拥有诸多潜在应用场景。