Pressure‐induced phase transitions in L‐leucine crystal

Raman spectra of a crystal of L ‐leucine, an essential amino acid, were obtained for pressures between 0 and 6 GPa. The results show anomalies at three pressure values, one between 0 and 0.46 GPa, another between 0.8 and 1.46 GPa, and a third at P ∼ 3.6 GPa. The first two anomalies are characterized by the disappearance of lattice modes (which can indicate occurrence of phase transitions), the appearance of several internal modes, or the splitting of modes of high wavenumbers. The changes of internal modes are related to CH and CH₃ unit motions as well as hydrogen bonds, as can be inferred from the behavior of bands associated with CO₂⁻ moieties. The third anomaly is a discrete change of the slopes of the wavenumber versus pressure plots for most modes observed. Further, decompression to ambient pressure generates the original Raman spectrum, showing that the pressure‐induced anomalies undergone by L ‐leucine crystals are reversible. Copyright © 2008 John Wiley & Sons, Ltd.