Knowledge Bank

Ferroelectric $\alpha -kIO3\cdot HIO3(Tc=223k)$ at T = 300k belongs to monoclinic $syngony1$ (space group $p21/c,z=4$) and consists of two sets of dimers $H(Io3)2-$, connected by strong $(RO\dots O=2,5A)$ 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 $(4000-400cm-1)$ of polycrystalline $\alpha -KIO3\cdot HIO3$ have been performed from T = 300 to 13k. Pressure dependable NQR measurements (0 - 5kbar) have been performed $earlier2$ 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 $data1$ and confirm the NQR conclusions about distortion of anion sublattice along with the crystal space group lowering to $P21$ at the phase transition temperature.