Formamidinium iodide: crystal structure and phase transitions

At a temperature of 100 K, CH5N2+·I- (I), crystallizes in the monoclinic space group P21/c. The formamidinium cation adopts a planar symmetrical structure [the r.m.s. deviation is 0.002 Å, and the C-N bond lengths are 1.301 (7) and 1.309 (8) Å]. The iodide anion does not lie within the cation plane, but deviates from it by 0.643 (10) Å. The cation and anion of I form a tight ionic pair by a strong N-H⋯I hydrogen bond. In the crystal of I, the tight ionic pairs form hydrogen-bonded zigzag-like chains propagating toward [20-1] via strong N-H⋯I hydrogen bonds. The hydrogen-bonded chains are further packed in stacks along [100]. The thermal behaviour of I was studied by different physicochemical methods (thermogravimetry, differential scanning calorimetry and powder diffraction). Differential scanning calorimetry revealed three narrow endothermic peaks at 346, 387 and 525 K, and one broad endothermic peak at ∼605 K. The first and second peaks are related to solid-solid phase transitions, while the third and fourth peaks are attributed to the melting and decomposition of I. The enthalpies of the phase transitions at 346 and 387 K are estimated as 2.60 and 2.75 kJ mol-1, respectively. The X-ray powder diffraction data collected at different temperatures indicate the existence of I as the monoclinic (100-346 K), ortho-rhom-bic (346-387 K) and cubic (387-525 K) polymorphic modifications.