High pressure investigations of melamine

We have performed mid- and far-infrared (IR), Raman, and angular dispersive X-ray diffraction studies on melamine at high pressure up to 36 GPa. We have confirmed the presence of three phase transitions; the first between 1 and 2 GPa, the second between 7 and 9 GPa, and the third near 16 GPa. We observed a softening of the N–H symmetric and antisymmetric vibrations with pressure, suggesting that intermolecular hydrogen bonding increases as the intermolecular distance decreases similarly to what was observed in triamino-trinitrobenzene. The molecular decompression data from core intramolecular peaks of mid-IR and Raman indicate that melamine did not chemically decompose up to the highest investigated pressures but the sample suffered some irreversible amorphization. We have further clarified the lack of observation of any phase transitions in prior Raman and IR studies by examining the pressure dependence of other uninvestigated modes of vibration.