A reversible pressure-induced phase transition in β-glycine at 0.76 GPa

Single crystals of β-glycine were studied at pressures up to 7.6 GPa in a diamond anvil cell (DAC) in situ by Raman spectroscopy and polarized optical microscopy. A reversible phase transition was observed at 0.76 GPa. A boundary between the two phases could be observed in the microscope if the phase transition was slow enough. The phase transition was accompanied by cracking of the crystal. The birefringence in the high-pressure phase 2 was close to that in the starting phase 1, so that the phase transition can be supposed to preserve the monoclinic crystal system. Every vibrational band in the spectrum of the high-pressure phase can be related to a corresponding band in the starting phase, excluding lattice vibrations. The frequencies of most vibrations changed by a jump at the transition point. Low-frequency lattice modes showed linear pressure dependence when pressure increased from 0.76 to 7.6 GPa. This can be a manifestation of the softening of the libration modes of glycine at the phase transition point. At high pressures (in the range of 6.5-7.6 GPa) the shifts of the frequencies of some vibrational bands were nonlinear. This can be related to the rotations and twisting of the zwitter-ions of glycine.