Raman spectra and phase transition in betaine potassium iodide dihydrate

Polarized Raman spectra of betaine potassium iodide dihydrate ([(CH3)(3)N+C H2 COO-](2)KI . 2H(2)O) have been studied in the temperature range 40-290 K . The behaviour of betaine external modes and of water internal vibrations provides evidence for a first order phase transition of order-disorder type at 100 K. This phase transition is related to the freezing of the successi ve reorientations of water molecules. The temperature dependence of the hal fwidth of the water stretching band centred at 3489 cm(-1) (40 K) obeys an Arrhenius-type law corresponding to different activation energies below and above the critical temperature. The comparison of Raman and dielectric dat a allows us to identify two types of reorientation of water molecules.