SPECTROSCOPIC EVALUATION OF THE STRUCTURE AND DYNAMICS OF THE HYDROGEN BONDS IN DIFFERENT CRYSTAL MODIFICATION OF POTASSIUM BIIODATE
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Date
1996
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Publisher
Ohio State University
Abstract
The $KIO_{3}$.$HIO_{3}$ 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 $\sigma$-modifications in the temperature range from 300 to 13 K. The $\alpha-KIO_{3}$ $HIO_{3}$ crystal ($P2_{1}/b$ space group, z=4 at room temperature) is built of two nonequivalent sets of dimers $IO_{3}\cdots H \cdots O_{3}I$, joined by strong symmetric hydrogen bond (O...O distance is about 2.5 \AA). At 223 K ($T_{e}$) the ferroelectric phase transition occurs in this crystal. The stable $\sigma$-modification (space group $Fdd2, z=24$ at room temperature) is formed from structure units of three distinct types:$I_{2}O_{5}(OH)$, which are joined by the H-bound network. In order to understand the normal modes of vibrations in $\alpha-KIO_{3}-HIO_{3}$.and $\sigma-KIO_{3}\cdot HIO_{3}$, we have carried out lattice dynamics calculations using a generalized valence force field for the dimers $[H(IO_{3})_{2}]^{-}$ and othe 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 $IO_{3}$ groups are strongly coupled with the translational vibrations of the hydrogen atom in case of $\sigma$-modification. In the case of $\sigma$-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 $measurements^{a}$ of pressure dependencies of the $^{127}I$ 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 $T_{c}$.
Description
$^{a}$A. I.Barabash, J. Mol. Struct., 270, 517 (1992)
Author Institution: National Academy of Sciences of Ukraine, Institute of Physics; Polish Academy of Sciences, Institute of Low Temperature and Structure Research
Author Institution: National Academy of Sciences of Ukraine, Institute of Physics; Polish Academy of Sciences, Institute of Low Temperature and Structure Research