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.