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Acta Crystallogr E Crystallogr Commun. 2017 Mar 24;73(Pt 4):569-572. doi: 10.1107/S205698901700425X. eCollection 2017 Apr 1.

Formamidinium iodide: crystal structure and phase transitions.

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Department of Materials Science, Lomonosov Moscow State University, Lenin Hills, 119991 Moscow, Russian Federation.
Department of Chemistry, Lomonosov Moscow State University, Lenin Hills, 119991 Moscow, Russian Federation.
National Research Centre 'Kurchatov Institute', 1 Acad. Kurchatov Sq., Moscow 123182, Russian Federation.
Inorganic Chemistry Department, Peoples' Friendship University of Russia (RUDN University), 6 Miklukho-Maklay St., Moscow 117198, Russian Federation.
X-Ray Structural Centre, A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 28 Vavilov St., B-334, Moscow 119991, Russian Federation.


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.


crystal structure; formamidinium iodide; phase transitions; powder diffraction; synchrotron radiation

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