Discovery of an intermediate phase in bis-thiourea pyridinium chloride inclusion compound

Abstract

This paper reports the occurrence of a new phase for bis-thiourea pyridinium chloride inclusion compound, which is intermediate to the high- and low-temperature ones already reported. The structure of the intermediate phase was determined by the x-ray diffraction and the following sequence of space groups was obtained: Cmcm-->(237 K)Cmc2(1)-->(206 K)Pbca. The triplication of the lattice a parameter in the intermediate phase was observed. Detailed investigations of its phase transitions were performed by differential scanning calorimetry, x-ray diffraction, and dielectric spectroscopy methods. The phase transition at T(1)=237 K is continuous while at T(2)=206 K is discontinuous. Both phase transitions have been classified as of order-disorder type. The high- and low-temperature phases are nonpolar in contrast to the intermediate phase, for which we could not found ferroelectric ordering.

New ferroelectric: Bis-thiourea pyridinium bromide inclusion compound

“…28 The three channels existing in the tripled unit cell split into two types. For one of them, equivalent by symmetry, the mirror symmetry perpendicular to a axis on which pyridinium cations are located is kept, involving an ordering of the N atoms of the cations in this plane.…”

“…39 The search for ferroelectricity in the third compound belonging to this family bis-thiourea pyridinium chloride inclusion compound T 2 (PyH)Cl, of the polar intermediate phase (Cmc2 1 ) has failed. 28 However, this result should not be treated as final as the experiment was performed on polycrystalline samples. The question that we would like to consider here is whether another compound from this group, namely bis-thiourea pyridinium bromide inclusion compound T 2 (PyH)Br, shows ferroelectric properties, which seems possible as the space group of its low-temperature phase (P2 1 cn) is the same as that of bis-thiourea pyridinium iodide inclusion compound T 2 (PyH)I known to show such properties.…”

“…So far four such systems are known with the chloride, bromide, iodide, and nitrate anions, hereafter denoted as inclusion compounds of bis-thiourea pyridinium chloride (T 2 (PyH)Cl), bis-thiourea pyridinium bromide (T 2 (PyH)Br), bis-thiourea pyridinium iodide (T 2 (PyH)I), and bis-thiourea pyridinium nitrate (T 2 (PyH)NO 3 ), respectively. [26][27][28] In the first three systems the anion is spherical, whereas in the fourth-it is planar. These compounds have been intensively studied as to their structural, 27,29 dynamic, 30À38 and possibly ferroelectric properties.…”

A study of out-of-plane cation dynamics in a bis-thiourea pyridinium chloride inclusion compound

“…All four inclusion complexes undergo two phase transitions and the crystallographic structures of all phases are known. 16,[18][19][20] The dynamics of the guest molecules differs from their bulk dynamics because the host lattice modifies the intermolecular interactions. Therefore the investigation of the dynamics of the guest molecules in these systems provides fundamental information on the nature of the intermolecular interactions which is a key element to understand the physico-chemical properties of these inclusion compounds.…”

“…16 Recently, detailed investigations of phase transitions in T 2 (PyH)Cl (T 1 = 237 K and T 2 = 206 K) were performed by differential scanning calorimetry, X-ray diffraction and dielectric spectroscopy methods. 20 In this paper we characterise the out-of-plane reorientation of the pyridinium cation in bis-thiourea pyridinium chloride using 1 H NMR, dielectric spectroscopy and quasielastic neutron scattering (QENS) methods. The combination of these three techniques is a powerful strategy for probing dynamic properties of organic compounds, as together they allow molecular motions to be studied across a wide range of temperatures.…”