Raman study of nonhydrostatic pressure-induced phase transitions in monoclinic L-aspartic acid crystals

To explore the properties of the L-aspartic acid crystal structure under high pressure, we employed Raman spectra combined with vibrational mode assignments at pressures ranging from atmospheric pressure to 20.1 GPa. A comparison of the assignment of the vibration mode by density functional theory calculations and by isotopic shift measurements was also discussed. The spectra showed that modifications might be present mainly at pressures less than 10.0 GPa. Variations in the whole spectrum indicated that the crystal underwent three phase transitions at the pressure ranges of 2.2-3.6, 5.2-6.6, and 9.4-10 GPa. A molecular conformational variation might occur at 14.3-15.9 GPa. The behaviour of the wavenumber as a function of pressure was also presented. This work provides useful information on the hydrogen bonds within the stable monoclinic crystal structure of L-aspartic acid.