Structure, phase transitions and molecular motions in 4-aminopyridiniumperchlorate

Abstract

The crystal structure of the 4-aminopyridinium perchlorate (4-apyH)ClO4 has beendetermined at 100 K by means of x-ray diffraction as monoclinic, with space groupP 21,with Z = 8.The crystal undergoes two structural phase transitions: one of first-order type,reversible, at 241/243 K (on cooling/heating respectively) and one ofweakly first-order type, irreversible, at 277 K (on heating). The crystaldynamics is discussed on the basis of the temperature dependence of the 1 Hnuclear magnetic resonance second moment(M2) and spin–latticerelaxation time T1.Both phase transitions are interpreted in terms of thechanges in the motional state of (4-apyH)+ cations and ClO4−anions. The dielectric dispersion studies disclose a relaxation process over thehigh-temperature phase (above 241 K) in the audio-frequency region. Thedielectric results are described by a Cole–Cole equation. The title crystal revealspyroelectric properties below 241 K. The ferroelastic domain structure of(4-apyH)ClO4 is observed over the whole temperature range studied.