FT-IR AND NQR STUDIES OF PHASE TRANSTION IN $\alpha-KIO_{3}$ CRYSTAL

1. A. Petrosyan, A. Busch, V. Chechkin, V. Volkov, Yu. Venevtsev, Ferroelectrics, 21, 525 (1978) 2. A. Barabash, D. Baisa, E. Shadchin, Khimilcheskaya Fizika (in Russian), 5, 1506 (1986).Author Institution: National Academy of Sciences of Ukaraine, 46 Nauki Prosp., 252650 Kiev, Ukraine; Polish Academy of Sciences, 50-950 Wroclaw 2, P.O. Box 937, PolandFerroelectric $\alpha-kIO_{3} \cdot HIO_{3} (T_{c} = 223k)$ at T = 300k belongs to monoclinic $syngony^{1}$ (space group $p21/c, z = 4$) and consists of two sets of dimers $H(Io3)2^{-}$, connected by strong $(R_{O\ldots O} = 2, 5A)$ hydrogen bonds. In order to clarify the structure of low-temperature ferroelectric phase and the dynamics of phase transition the temperature variable FT - IR study $(4000-400 cm^{-1})$ of polycrystalline $\alpha-KIO_{3} \cdot HIO_{3}$ have been performed from T = 300 to 13k. Pressure dependable NQR measurements (0 - 5kbar) have been performed $earlier^{2}$ at T=300 and 77K. On cooling to low temperatures the ABC structure of OH stretching vibrations $(1600 - 2800cm^{-1})$ slightly varies: the middle frequency band (B) shows the asymmetric contour and moves slightly to lower frequencies; the intensity of it decreases, whereas those of bands A, and particularly C, increase. Another striking feature in these spectra is a very strong absoraption culminating between 600 and $1200cm^{-1}$ which shows a doublet structure on cooling. Such spectral distribution is typical for symmetrical strong H-bonds between two equivalent molecules related by a center of inversion or a two-fold axis. These facts correlate well with structural $data^{1}$ and confirm the NQR conclusions about distortion of anion sublattice along with the crystal space group lowering to $P2_{1}$ at the phase transition temperature