POLARIZED INFRARED AND RAMAN-SPECTRA OF DIGLYCINE NITRATE SINGLE-CRYSTAL

Polarized infrared (4000-ca. 350 cm-1) and Raman (4000-10 cm-1) spectr a of diglycine nitrate (DGN) single crystal were measured at room temp erature; polarized infrared spectra were also measured at several low temperatures (230, 180, 15 K). In the paraelectric phase, the glycine ion pairs were found to be joined by a symmetrical (C(i)) O...H...O hy drogen bond and the nitrate ions exhibited high symmetry (most likely D3h) as a result of effectively free rotation. The ferroelectric phase transition occurs because of inhibition of the rotation of the nitrat e ions, with consequent lowering of their symmetry to C1, as a result of increased N-H...O hydrogen bonding interaction with the +NH3 groups of the neighbouring glycine ions. The symmetry of the glycine ion pai r also falls to C1 in the ferroelectric phase, but the proton motion i n the O...H...O bond does not play a significant role in the phase tra nsition.