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Abstract
The Raman spectra of GPI and DGPI crystals were investigated in the 400–3200 cm−1 internal vibrations range and in the temperature range 175–250 K and 295–355 K, respectively. Para-ferroelectric phase transition at 224 K for GPI and 322 K for DGPI was examined. The results obtained from Raman investigations confirm the order-disorder nature of a ferroelectric phase transition of second order type. One type of glycinium cations is observed in the paraelectric phase but two types in ferroelectric phase. The dynamical disorder of protons is coupled to the motions of glycinium cations.