Structural characterization,phase transition and dielectric properties of 4-cyanopyridynium perchlorate monohydrate: [(4-CNC5H4NH)][ClO4]·H2O

Crystal structure of 4-cyanopyridynium perchlorate monohydrate ([(4-CNC₅H₄NH)][ClO₄]·H₂O) has been determined at 293 and 240 K as orthorhombic space group, Pnma and monoclinic space group, P2₁/c, respectively, by means of single crystal X-ray diffraction. At room temperature the perchlorate anion reveals significant disorder, which is realized by the splitting of two oxygen atoms into four sites. DSC, dilatometric and dielectric spectroscopy techniques show that the crystal undergoes phase transition at 286/288 K (on cooling/heating scans). [(4-CNC₅H₄NH)][ClO₄]·H₂O appears to be an insulator with relatively high activation energy of the order of 100 kJ/mol. The phase transition in the title crystal is believed to be related to the dynamics of the perchlorate anion.