TEMPERATURE VARIABLE LOW-FREQUENCY RAMAN SPECTRA AND ORIENTATION DISORDER IN $NH_{4}IO_{3}$ CRYSTAL

1. D. F. Baisa, A. I. Barabash, E.A. Shadcin, Fisika Tverdogo Tela (in Russian), 27, 2357 (1985). 2. A.I. Barabash, J. Mol. Struct., 210, 517 (1992).Author Institution: National Academy of Sciences of Ukraine, Prosp. Nauki, 46, 252650 Kiev, Ukraine.The ammonium iodate $NH_{4}IO_{3}$ crystal is $known^{1}$ as ferroelectric with displacing type phase transition $(T_{c} = 367K)$, Polarized temperature variable lowfrequency Raman scattering $(W < 200 cm^{-1})$ studies in the phase transition region are presented. Measurements of peak positions of ``external'' rotational and translation optical modes and their half-widths allow to detect the orientation disorder and help to determine its character as well as a mechanism of the phase transition to the ordered phase. It appeared to be continuous and of the mixed displacing and order-disorder character. Quantitative analysis of the spectra according to lattice dynamics theories and using $^{127}I$ NQR $analysis^{2}$ have been performed. Previous literature data are reviewed and the apparent discrepancies concerning the transition processes are elucidated on the basis of the present results