High-pressure studies of melamine

Abstract

We performed the first high-pressure study of melamine using X-ray Raman spectroscopy (XRS) up to ∼17 GPa in a diamond anvil cell at ambient temperature. We utilized the 16 ID-D undulator beamline at the Advanced Photon Source for the X-ray source. The observed diminishment of the 1s→π* peak as well as other changes in the XRS spectra with pressure suggest the possibility of intra- and inter-molecular bonding changes due to two phase changes in the investigated pressure range or hybridization changes of atomic orbitals in the material with pressure. We also performed a complementary X-ray powder diffraction study of neat melamine up to 24 GPa observing at least two phase transitions with pressure. Pressure cycling indicated that the phase transitions were reversible. Density-functional theory calculations performed on the system at ambient and low pressure show a high level of agreement with the experiments.