Evidence of the Coexistence of Two Structural Phases in a Single DUT-8(Ni) Microcrystal Using Raman Imaging

Metal–organic frameworks (MOFs) are a class of soft porous crystals that possess extensive capabilities for regulating and inducing morphological transitions between different crystalline phases under external influences. According to the contemporary perspective, transitions between metastable structural phases occur cooperatively throughout the material, thereby preserving its ideal crystalline structure. A phase of the metal–organic framework DUT-8(Ni), which is poorly investigated and is a transient metastable phase between phases with completely open and closed pores, was studied by using Raman spectroscopy. In this Letter, we present experimental evidence for the coexistence of two structural phases with different pore sizes within a single microcrystal using the hyperspectral Raman mapping technique. The focused light of the laser beam triggered the structural phase change of the microcrystals. The treatment spot was tiny compared to that of the transition region. The long-term stability of the microcrystal phase after the transition is demonstrated. Changes in the reflectance spectra, which characterize the crystal’s color, also confirm the observed changes. The coexistence of different phases within one crystal, on the one hand, changes the existing understanding of the phase transition mechanism between open and closed pore phases. And on the other hand, it is a very illustrative example of Raman mapping capabilities as not only the isolated Raman spectrum matters but also the whole data set obtained from the microcrystal surface.