Knowledge Bank

The lattice dynamics of the ammonium iodate crystal $NH4IO3$ has been investigated by means of IR, Raman and NQR methods near the phase transition temperature 367 K $(Tc)$. It is known, that $NH4IO3$ crystal grows from water solution and has orthorhombic syngony $(C2v0z=4)$. In contrast with other hydrogen-bonded iodate crystals, which undergo the order-disorder phase transitions, the phase transition in $NH1IO3$ crystal has clear marks of the displacive type phase transition. First, $\mathrm{<mrow\; data-mjx-texclass="ORD">127</mrow>}I$ NQR measurements allow to assign the low frequency bands observed in IR and Raman spectra of $NH4IO3$ crystal to the lattice vibrational associated with librational and translational vibrations of the $NH4+$ and $IO3-$ ions and with internal vibrations of N-H and I-O ions , whereas high frequency bands were associated with $NH\dots O$ 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 $\mathrm{<mrow\; data-mjx-texclass="ORD">127</mrow>}I$ in the pretransition temperature range $((Tc-50)÷Tc)$ 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.