Crystal structure and characterization of a novel ferroelastic ionic crystal: 1-Aminopyridinium iodide (C5H7N2)+I−

1-Aminopyridinium iodide (API) undergoes a reversible first-order phase transition (PT) II → I at 384 K. The single-crystal X-ray diffraction studies have been carried out at 110, 280 and 401 K. The compound crystallizes in the monoclinic space group P21/c (phase II) and exhibits ferroelastic properties. High temperature paraelastic phase (I) is described by the trigonal symmetry (space group R3¯m) with the 6-site cations disorder. The mechanism of PT is governed by both cationic dynamics and shifts of the I− anions. The conductivity that emerges when approaching the PT temperature is believed to be associated with the charge transfer from I− anion to the 1-aminopyridinium cation.