SPECTROSCOPIC INVESTIGATION OF THE LATTICE DYNAMICS IN HYDROGEN-BONDED CRYSTAL $NH_{4}IO_{3}$

Abstract

The lattice dynamics of the ammonium iodate crystal $NH_{4}IO_{3}$ has been investigated by means of IR, Raman and NQR methods near the phase transition temperature 367 K $(T_{c})$. It is known, that $NH_{4}IO_{3}$ crystal grows from water solution and has orthorhombic syngony $(C^{0}_{2v} z = 4)$. In contrast with other hydrogen-bonded iodate crystals, which undergo the order-disorder phase transitions, the phase transition in $NH_{1}IO_{3}$ crystal has clear marks of the displacive type phase transition. First, $^{127}I$ NQR measurements allow to assign the low frequency bands observed in IR and Raman spectra of $NH_{4}IO_{3}$ crystal to the lattice vibrational associated with librational and translational vibrations of the $NH^{+}_{4}$ and $IO^{-}_{3}$ ions $(\nu < 120 cm^{-1})$ and with internal vibrations of N-H and I-O ions $(v < 850 cm^{-1})$, whereas high frequency bands were associated with $NH{\ldots}O$ 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 $^{127}I$ in the pretransition temperature range $((T_{c}-50) \div T_{c})$ 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 $NH^{+}_{4}$ and $IO^{-}_{3}$ ions due to the temperature expansion of the crystal volume and the change in the hydrogen bonds network in the pretransition temperature range.