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

[(CH3)(3)C5H2NH][ClO4] has been synthesized and characterized by X-ray (at 344, 245, 180 and 115K), calorimetric, dilatometric, dielectric and pyroelectric measurements. At room temperature the crystal structure is polar, space group Pmn2(1). It consists of discrete disordered [ClO4](-) 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 356/327K (heating/cooling), II -> III at 346/326, III -> IV at 226K and IV -> V at 182/170K. [(CH3)(3)C5H2NH][ClO4] reveals a strong pyroelectric response over a wide temperature region (phases III, IV and V) with the spontaneous polarization changes (Delta P-s) of the order of 1.5-8 x 10(-3) C/m(2). The spontaneous polarization is irreversible over all the polar phases, however, the magnitude of the Delta 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. (C) 2009 Elsevier Inc. All rights reserved.