High-pressure Raman spectra of racemate dl-alanine crystals

Raman spectroscopy investigations of dl-alanine crystal under high pressures have been carried out up to 18.0 GPa. For instance, around 1.0 GPa and between 1.7 and 2.3 GPa changes in the Raman profile were observed and associated to conformational changes of the molecules in the unit cell or to a phase transition accompanied to slight conformational change of the molecule through CH and CH3 groups. Moreover, between 6.0 and 7.3 GPa, the appearance of a new low energy lattice modes and to the splitting of a band assigned to the stretching vibration of the CCH3 moiety were related to a second phase transition. Finally, changes in lattice modes, red shift of the band associated to CCH3 stretching and increasing of line-width of the band associated to the wagging of CO2, between 11.6 and 13.2 GPa, are ascribed to a third phase transition. On release of pressure the original phase was obtained again.