WE16		15min5:46

A. I. BARABASH, T. A. GAVRILKO, G. A. PUCHKOVSKAYA, I. V. SEKIRIN, Institute of Physics, National Academy of Sciences of Ukraine, Prosp. Nauki, 46, 252650 Kiev, Ukraine; J. BARAN AND H. RATAJCZAK, Institute of Low Temperature and Structure Research, Polish Academy of Sciences, 2 Okolna Street, 50-950 Wroclaw, Poland.

The \KIOHIO \, crystals belong to the family of acidic iodate compounds which is well-known for its ferroelectric phase transitions and has four crystal modifications. Here we report the results of IR and Raman study of vibrational spectra (4000-60~cm^-1) of \alpha and -modifications in the temperature range from 300 to 13~K. The \alpha -\KIOHIO \, crystal (P2₁/b space group, z=4 at room temperature) is built of two nonequivalent sets of dimers IO₃\ppp H\ppp O₃I, joined by strong symmetric hydrogen bond (O\ppp O distance is about 2.5~Å). At 223~K~(\Tc ) the ferroelectric phase transition occurs in this crystal. The stable -modification (space group Fdd2, z=24 at room temperature) is formed from structure units of three distinct types: I₂O₅ and I₂O₅(OH), which are joined by the H-bond network. In order to understand the normal modes of vibrations in \alpha -\KIOHIO \,and -\KIOHIO, we have carried out lattice dynamics calculations using a generalized valence force field for the dimers [H(IO₃)₂]^- and other structure units. The introduction of the translation and libration coordinates related to the normal coordinate analysis leads to a precise assignment of the frequencies and shows that both stretching and bending vibrations of IO₃ groups are strongly coupled with the translational vibrations of the hydrogen atom in case of \alpha -modification. In the case of -modification those vibrations are localized on separate I-O bonds. A comparative analysis of the vibrational spectra and structure is presented. On the base of analysis of vibrational spectra and according to measurements\footnoteA.I.Barabash, J. Mol. Struct., \underline 270, 517 (1992). of pressure dependencies of the ¹²⁷I NQR spectra at 77~K it was concluded that the mechanism of ferroelectric phase transition in \alpha -modification of potassium biiodate crystal involves the proton ordering in the double-well potential at \Tc.