Research Article: Formamidinium iodide: crystal structure and phase transitions

Date Published: April 01, 2017

Publisher: International Union of Crystallography

Author(s): Andrey A. Petrov, Eugene A. Goodilin, Alexey B. Tarasov, Vladimir A. Lazarenko, Pavel V. Dorovatovskii, Victor N. Khrustalev.


Crystal structure, thermal behaviour and phase transitions of formamidinium iodide were studied by DTG, DSC, powder diffraction and X-ray crystallography.

Partial Text

Compounds with the general formula ABX3 [where A denotes an organic cation e.g. methylammonium (MA, CH3NH3+) or formamidinium [FA = CH(NH2)2, CH3NH3]; B = Pb, Sn; X = I, Br, Cl] belong to a class of hybrid organic–inorganic perovskites and perform as outstanding light harvesters. These compounds gave birth to a new field of photovoltaics – perovskite solar cells – when Kojima and co-authors used (MA)PbI3 as a light sensitizer for the first time in dye-sensitized solar cells (DSSCs) in 2009 and showed 3.8% efficiency (Kojima et al., 2009 ▸). Since then, a revolutionary breakthrough has occured in this area and the highest efficiency now has reached 22.1%.

At a temperature of 100 K, compound I crystallizes in the monoclinic space group P21/c. The formamidinium cation adopts a planar symmetrical structure [r.m.s. deviation is 0.002 Å, and the C—N bond lengths are 1.301 (7) and 1.309 (8) Å; Fig. 1 ▸]. The iodide anion does not lie within the cation plane, but deviates from it by 0.643 (10) Å. The cation and anion in I form a tight ionic pair by the strong N1—H1A⋯I1 hydrogen bond (Table 1 ▸ and Fig. 1 ▸).

In the crystal of I, the tight ionic pairs form hydrogen-bonded zigzag-like chains propagating toward [20] by the strong inter­molecular N2—H2A⋯I1i hydrogen bonds (Table 1 ▸ and Fig. 4 ▸). The hydrogen-bonded chains are further packed in stacks along [100] (Fig. 4 ▸) [symmetry code: (i) x − 1, −y + , z + ].

Polycrystalline powder of I was purchased from Dyesol and used without further purification. Single crystals suitable for X-ray structural study were obtained by recrystallization from an anhydrous ethanol solution by slow cooling.

Crystal data, data collection and structure refinement details are summarized in Table 2 ▸. X-ray diffraction study of I was carried out on the ‘Belok’ beamline of the National Research Center ‘Kurchatov Institute’ (Moscow, Russian Federation) using a Rayonix SX165 CCD detector. Reflection intensities measured were corrected for absorption using the Scala (Evans, 2006 ▸) program.