Structure, phase transitions and dielectric properties of a new inclusion compound of bis-thiourea pyridinium nitrate salt

Abstract

Inclusion compounds / Phase transitions / Bis-thiourea pyridinium nitrate salt / Single crystal structure analysis / X-ray diffraction Abstract. A structure of a new inclusion compound of bis-thiourea pyridinium nitrate salt C 5 H 6 N þ NO 3 À 2[CH 4 N 2 S] was determined by single-crystal X-ray diffraction method at 293 K, 250 K and 150 K. In high temperature phase the compound crystallizes in orthorhombic system, the two low-temperature phases are monoclinic. The sequence of phases is: Pbnm ) P2 1 /c ) P2 1 . In the orthorhombic form (I) the a axis is doubled in respect to the monoclinic forms and pyridinium and nitrate ions are partially disordered. Both low-temperature monoclinic phases (II) and (III) are ordered but twinned by pseudomerohedry. In all three phases the host lattice is built of ribbons of hydrogen-bonded thiourea molecules and nitrate anions forming channels in which the guests pyridinium cations are located. Each ribbon consists of two crystallographically independent thiourea molecules and nitrate ions. Four ribbons form a channel parallel to the c axis with approximately square cross-section. In the channel there is a stack of pyridinium cations, which planes form an inclination angle with the channel axis. The angle varies from about 90 in disordered phase I to 60 in well ordered phase (III).The dielectric spectroscopy measurements were performed on the polycrystalline sample with HP-4291A impedance analyser in the frequency range 1 MHz-1 GHz. The dielectric measurements reveal that two phase transitions occur at T 1 ¼ 287 K and at T 2 ¼ 217 K. The phase transition at T 2 ¼ 287 K is of the first order with the thermal hysteresis of 10 K at cooling and heating. The phase transition at 217 K is of the second order and the dielectric anomaly suggests the ferroelectric-paraelectric phase transition. The dielectric relaxation observed below T 2 are caused by the pyridinium cation reorientation. The dielectric relaxation time changes with the temperature according to Arrhenius low and the activation energy of the relaxation process is about 14 kJ/mol.

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

“…24 The systems T 2 ͑PyH͒NO 3 and T 2 ͑PyH͒I have been discovered to show ferroelectric properties. 20,22 The ferroelectric ordering in T 2 ͑PyH͒I has been observed in the lowtemperature ͑LT͒ and INT phases, while only in the LT phase in T 2 ͑PyH͒NO 3 . Therefore, a natural consequence is to check if the other two compounds from this group ͓T 2 ͑PyH͒Cl and T 2 ͑PyH͒Br͔ also show ferroelectric properties and try to understand the origin of the differences and similarities of the orderings in the LT and INT phases of these compounds for which the high-temperature ͑HT͒ states are isomorphs.…”

“…These complexes are ionic-molecular systems held by van der Waals and Coulomb interactions as well as hydrogen bonds. Hitherto, four such systems with pyridinium chloride, bromide, iodide, and nitrate have been described ͓hereafter denoted as T 2 ͑PyH͒Cl, T 2 ͑PyH͒Br, T 2 ͑PyH͒I, and T 2 ͑PyH͒NO 3 , respectively͔ [17][18][19][20][21][22][23][24][25] and very recently the complex with thiazolium bromide was discovered. 26 The thiourea molecules connected by hydrogen bonds form ribbons, and four such ribbons make a channel of a rectangular ͑or close to rectangular͒ shape containing pyridinium cations.…”

“…Recently, crystallographic investigations resolved two phase transitions ͑three distinct phases͒ for T 2 ͑PyH͒NO 3 : at 216 and 281 K. 20 The dynamics of this system has been studied by 1 H and 2 H NMR, 19,21 dielectric spectroscopy, 20 and neutron spectroscopy. 23 Also T 2 ͑PyH͒I has been found by differential scanning calorimetry ͑DSC͒ to undergo two phase transitions, not one as postulated in Ref.…”

New ferroelectric: Bis-thiourea pyridinium bromide inclusion compound

“…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.…”

“…[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. 27,29,39 Research of ferroelectric properties in the family of thiourea pyridinium salts inclusion compounds with pyridinium salts are important because, to the best of our knowledge, other thiourea inclusion complexes with organic molecules as guest are not ferroelectric materials.…”