High-pressure-induced multiple phase transitions of parabanic acid

In situ high-pressure Raman spectroscopy experiments have been performed to investigate the structural changes of parabanic acid (PA) up to similar to 12.0 GPa. The analysis of Raman spectroscopy reveals that PA undergoes two phase transitions. The collapse of spacing between molecular layers and the distortion of hydrogen-bonding networks almost in ac-plane are the main causes of the first phase transition above 2.1 GPa. The changes of Fermi resonance parameters also provide evidence for the first phase transitions. The second phase transition around 4.0 GPa can attribute to the further reduction of the molecular interlayer spacing and the resulting distortion of hydrogen-bonding network. First-principle calculations and Hirshfeld surfaces further confirm the analysis of the experimental results. The analysis of high-pressure phase transition contributes a better explanation for the high-pressure phase transition process of layered supramolecular materials with hydrogen-bond self-assembly. This study provides the possibility of two new polymorphs for PA under high pressure and is helpful for broadening its potential application of this material, especially in the field of supramolecular chemistry.