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

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Abstract

Change of a local environment of a polar pyridinium ion, which is associated with the phase transition of crystalline pyridinium tetrachloroiodate(III) at Tc = 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 > Tc indicates an orientational disorder in the high-temperature phase (HTP). The energy difference ΔE between the stable and meta-stable orientations of the pyridinium ion at the 2/m site was estimated to be ΔE/R  560 K at 280 K in the HTP. Below the Tc, an antiferroelectric ordering of the ions was revealed.

Introduction

When reorientational motion of pyridinium ion in the crystal is discussed it might be assumed that pyridinium ion has a pseudohexad C6 axis [1], [2], [3]. However, the order–disorder process of pyridinium ion in the solid state is not so simple enough to be explained by the 6-fold disorder in the orientation. In the low-symmetry crystalline phase, site symmetry of pyridinium ion should be taken into account for the potential wells in order to explain reorientational motion of the pyridinium ions [4], [5]. Besides, our calorimetric study of pyridinium tetrabromoaurate(III) revealed the temperature dependence of the energy difference, which characterizes the nonequivalence of the potential wells, through a cooperative effect of pyridinium orientation [6].

For the better understanding of the reorientational motion and the ordering process of pyridinium ion in the crystalline state, it is worth to extend study to the other pyridinium salts. It is known from 35Cl NQR and DTA/DSC measurements that pyridinium tetrachloroiodate(III), PyHICl4, undergoes a phase transition at Tc = 217 K [7], [8]. The single NQR line observed in high-temperature phase (HTP) splits into a doublet in low-temperature phase (LTP). In our previous study [8] the following points were recognized. (i) The temperature dependence of the NQR frequencies and a very broad thermal anomaly observed in DTA suggest a second-order nature of the phase transition. (ii) The temperature dependence of the NQR spin–lattice relaxation time associated with a critical relaxation around Tc suggests the phase transition is of an order–disorder type with pyridinium ions. (iii) The large splitting of the NQR frequency in LTP suggests a strong deformation of ICl4- anion as well, which is coupled with the order–disorder of pyridinium ions.

In the present study, we have determined crystal structure of PyHICl4 both in HTP and LTP, and temperature dependencies of heat capacity,Cp, and dielectric constant, ε′, attempting to get more detailed information about the ordering process of the pyridinium ion in PyHICl4.

Section snippets

Preparation and identification

PyHICl4 was prepared according to the method described in the literature [9] and identified by elemental analysis at Center for Organic Elemental Microanalysis, Kyoto University. Anal. Calcd. for PyHICl4: C, 17.2; H, 1.7; N, 4.0; I, 36.4; Cl, 40.7%. Found: C, 17.2; H, 1.7; N, 4.1; I, 36.2; Cl, 40.4%.

Crystal structure determination

Single crystal X-ray measurements of HTP as well as LTP were carried out at 233 and 113 K, respectively, using a SMART 1000/CCD diffractometer (Bruker) with graphite-monochromated Mo Kα radiation (2θ <

Crystal structures of HTP and LTP

The positional parameters of the atoms in HTP and LTP, and the geometric parameters in both the phases are listed in Table 2, Table 3, Table 4, respectively. The crystal structures of the HTP and LTP are shown in Fig. 1, Fig. 2, respectively. In Fig. 2, the convenient axes a′ (=a), b′ (=−b), c′ (=−a  c) were chosen as to be easy to compare the structural change through the transition. The site symmetry of the pyridinium ion is 2/m in the HTP and there must exist an orientational disorder. In the

Supplementary material

Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre, CCDC Nos. 298927 and 298928 for HTP and LTP, respectively. Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44 1223 336 033; e-mail: [email protected]).

Acknowledgements

We thank Ms. Ayako Mogi, Mr. Jun Watanabe, and Ms. Chie Morikawa for sample preparation.

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