Phase transition of pyridinium tetrachloroiodate(III), PyHICl4, studied by a single crystal X-ray analysis and dielectric and heat capacity measurements

Change of a local environment of a polar pyridinium ion, which is associated with the phase transition of crystalline pyridinium tetrachloroiodate(III) at T-c = 217 K, was investigated by a single crystal X-ray analysis and dielectric and heat capacity measurements. The site symmetry 2/m of the ion at T > T-c indicates an orientational disorder in the high-temperature phase (HTP). The energy difference Delta E between the stable and meta-stable orientations of the pyridinium ion at the 2/m site was estimated to be Delta E/R <= 560 K at 280 K in the HTP. Below the T-c, an antiferroelectric ordering of the ions was revealed. (c) 2006 Elsevier B.V. All rights reserved.