2,4,6-Trimethylpyridinium perchlorate: Polar properties and correlations with molecular structure of organic–inorganic hybrid crystal

Abstract

[ ( CH 3 ) 3 C 5 H 2 NH ] [ ClO 4 ] has been synthesized and characterized by X-ray (at 344, 245, 180 and 115 K), calorimetric, dilatometric, dielectric and pyroelectric measurements. At room temperature the crystal structure is polar, space group Pmn 2 1 . It consists of discrete disordered [ ClO 4 ] - anions and ordered trimethylpyridinium cations giving the 3D network of hydrogen bonds. The compound reveals a rich polymorphism in the solid state. It undergoes four solid–solid phase transitions: from phases I to II at 356/327 K (heating/cooling), II → III at 346/326, III → IV at 226 K and IV → V at 182/170 K. [ ( CH 3 ) 3 C 5 H 2 NH ] [ ClO 4 ] reveals a strong pyroelectric response over a wide temperature region (phases III, IV and V) with the spontaneous polarization changes ( Δ P s ) of the order of 1.5 – 8 × 10 - 3 C / m 2 . The spontaneous polarization is irreversible over all the polar phases, however, the magnitude of the Δ P s in the vicinity of the phase transitions is characteristic of compounds with the ferroelectric order. The molecular mechanism of the successive phases transitions in the studied crystal is proposed.