15min:

ALEXANDER BARABASH, T. GAVRILKO, G. PUCHKOVSKAYA AND A. ROSHCHIN, Institute of Physics, National Academy of Sciences of Ukraine, 46, Prosp. Nauki, 252022 Kiev, Ukraine; J. BARAN, Institute of Low Temperature and Structure Research of the Polish Academy of Sciences, ul. Okolna, 2, 50 - 950 Wroclaw, Poland.

The lattice dynamics of the ammonium iodate crystal \amon has been investigated by means of IR, Raman and NQR methods near the phase transition temperature 367 K (T_c). It is known, that \amon crystal grows from water solution and has orthorhombic syngony ( C _2 v ⁹, z = 4). In contrast with other hydrogen-bonded iodate crystals, which undergo the order-disorder phase transitions, the phase transition in \amon crystal has clear marks of the displacive type phase transition. First, ¹²⁷I NQR measurements allow to assign the low frequency bands observed in IR and Raman spectra of \amon crystal to the lattice vibrational associated with librational and translational vibrations of the NH₄⁺ and IO₃^- ions ( < 120 \cm) and with internal vibrations of N-H and I-O ions ( < 850 \cm), whereas high frequency bands were associated with NH...O hydrogen bond. Second, the potential barriers for such vibrations were estimated from the spectroscopic data. Further, it was shown, that the temperature dependence of the asymmetry parameter of the ¹²⁷I in the pretransition temperature range ((\T - 50)~\div~\T) can not be described by a simple Arrheniuse's law. In the present paper we attempted to explain this deviation by the decreasing of potential barriers for reorientational vibrations of NH₄⁺ and IO₃^- ions due to the temperature expansion of the crystal volume and the change in the hydrogen bonds network in the pretransition temperature range.