VIBRATIONAL SPECTROSCOPY AND PHASE-TRANSITIONS IN ETHYLENE DIAMMONIUM BIS DIHYDROGENMONOPHOSPHATE NH3(CH2)(2)NH3(H2PO4)(2)

The Raman and infrared of polycrystalline samples of NH3(CH2)(2)NH3(H2PO4)(2) have been recorded at different temperatures between 12 and 300 K. An assignment of the observed bands at low temperature is proposed. A temperature dependence study of the frequencies nu and half width Delta nu(1/2) of some Raman bands has revealed three phase transitions at about 132, 237 and 270 K. These transitions are believed to be governed by libration of the cation coupled with the NH3 group jumps between different potential wells and correspond to an order-disorder mechanism involving NH3 torsional motion.