α-l-Glutamic acid under high pressure: Phase transitions studied by Raman spectroscopy

l-Glutamic acid crystallizes at ambient pressure in two different polymorphic forms, the α- and β-forms. In this work the vibrational properties of the α-form are investigated by Raman spectroscopy under high pressure. A single crystal of α-l-glutamic acid, immersed in paraffin oil, was submitted to pressures up to 7.5 GPa using a diamond anvil cell. The analysis of the Raman spectra of the crystal shows that α-l-glutamic acid presents two phase transitions, one between 1.9 and 2.3 GPa and another one between 3.3 and 3.7 GPa, both characterized by modifications in the low-wavenumber spectral region. Additionally, an impressive increase of the band linewidth is observed for pressures between 5.6 and 6.0 GPa. Such increase suggests that disorder is induced by pressure, and these changes can be directly associated with the solidification of paraffin oil. A discussion on the reversibility and possible mechanisms governing the phase transitions is also provided.