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

Abstract

A structure of a new inclusion compound of bis-thiourea pyridinium nitrate salt C₅H₆N⁺NO₃^- 2[CH₄N₂S] 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 → P2₁/c → P2₁. 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 pseudo-merohedry. 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 T₁ = 287 K and at T₂ = 217 K. The phase transition at T₂ = 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 T₂ 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.