Pressure-induced phase transition in N-H center dot center dot center dot O hydrogen-bonded crystalline malonamide

In this study, malonamide (C3H6N2O2) was compressed under up to 10.4 GPa of pressure in a diamond anvil cell at room temperature. Pressure-induced structural evolutions were monitored with in situ Raman spectroscopy. The significant changes in Raman spectra at 2.1 GPa provided evidence for a phase transition. The variations in the Raman spectra were discussed, including the disappearance of original modes, appearance of new modes, and abrupt changes in the pressure dependence of Raman modes under different pressures. Ab initio calculations were employed to account for the changes in molecular arrangements and hydrogen-bonded networks. Hirshfeld surfaces and fingerprint plots were employed for the direct comparison of the variations in packing patterns and intermolecular interactions. Based on the calculated results, variations in the NH2 stretching Raman vibrations, and degree of freedom of the molecules, the rearrangements of the hydrogen-bonded networks likely caused the phase transition of crystalline malonamide.