High pressure Raman scattering of dl-leucine crystals

We have obtained the Raman spectra of dl-leucine crystal through a diamond anvil cell for pressures between 0 and 5 GPa. The observation of several anomalies in the regions of both the lattice mode and the internal mode suggests that the crystal undergoes a phase transition between 2.4 and 3.2 GPa. This phase transition is preceded by a gradual change of the molecular conformation of leucine molecules in the unit cell. We show that, up to 5 GPa, the dl-leucine crystal is more stable than the chiral l-leucine crystal because while the former presents only one phase transition in the 2.4–3.2 GPa interval, the latter presents three different transitions, the first of which is observed at 0.46 GPa. Additionally, when pressure is released to 0.0 GPa, the original Raman spectrum is recovered, indicating that the modification at high pressure on dl-leucine crystal is reversible.