Crystal structure of new formamidinium triiodide jointly refined by single-crystal XRD, Raman scattering spectroscopy and DFT assessment of hydrogen-bond network features

A novel triiodide phase of the formamidinium cation, CH5N2 +·I3 −, crystallizes in the triclinic space group P at a temperature of 100 K. The structure consists of two independent isolated triiodide ions located on inversion centers. The centrosymmetric character of I3 − was additionally confirmed by the observed pronounced peaks of symmetrical oscillations of I3 − at 115–116 cm−1 in Raman scattering spectra.

A novel triiodide phase of the formamidinium cation, CH 5 N 2 + ÁI 3 À , crystallizes in the triclinic space group P1 at a temperature of 110 K. The structure consists of two independent isolated triiodide ions located on inversion centers. The centrosymmetric character of I 3 À was additionally confirmed by the observed pronounced peaks of symmetrical oscillations of I 3 À at 115-116 cm À1 in Raman scattering spectra. An additional structural feature is that each terminal iodine atom is connected with three neighboring planar formamidinium cations by N-HÁ Á ÁI hydrogen bonding with the N-HÁ Á ÁI bond length varying from 2.81 to 3.08 Å , forming a deformed two-dimensional framework of hydrogen bonds. A Mulliken population analysis showed that the calculated charges of hydrogen atoms correlate well with hydrogen-bond lengths. The crystal studied was refined as a three-component twin with domain ratios of 0.631 (1):0.211 (1): 0.158 (1).

Chemical context
Polyiodides are a large class of compounds with organic and inorganic cations and a great diversity of anion shapes varying from simple linear I 3 À up to I 29 3complexes (Svensson & Kloo, 2003). Such a great diversity of cations and anions allows one to tune the chemical and physical properties of the target compounds. Consequently, polyiodides have attracted great interest for a wide set of applications, such as dye-sensitized solar cells (DSSC) (O'Regan & Grä tzel, 1991;Jeon et al., 2011), different electrochemical devices (Weinstein et al., 2008;Weng et al., 2017) and light-polarizing materials (Kahr et al., 2009).
Another recently proposed prospective application of polyiodides is to use liquid methylammonium and formamidinium polyiodides as a precursor for the fabrication of lightabsorbing materials for perovskite solar cells (Petrov, Belich et al., 2017). Whereas the application of formamidinium polyiodides was shown to be successful for scalable fabrication of solar cells with efficiencies over 17% (Turkevych et al., 2019), the structures of formamidinium polyiodides have not been studied so far.
In this work, we investigated the features of the new structure of the single-crystalline CH 5 N 2 + ÁI 3 À (I, FAI 3 ) phase by means of Raman scattering spectroscopy and DFT calculations. ISSN 2056-9890

Structural commentary
Dark-red transparent rhombic-shaped single crystals (Fig. 1a) were obtained by slow heating of preliminary powdered stoichiometric FAI/I 2 (FA = CH 5 N 2 + ) mixture up to 355-358 K. Such a temperature range allowed us to obtain wellshaped single crystals as a result of recrystallization from the liquid state near its melting point, which was determined to be T m = 360 K by visual thermal analysis.
FAI 3 was found to crystallize in a triclinic unit cell, space group P1. The structure (Fig. 1b) consists of two types of isolated centrosymmetric triiodide ions (D 1h point symmetry) located on centers of inversion. Therefore, only centrosymmetric I 3 À anions are present, which is rare for structures with relatively small cations such as formamidinium (Svensson & Kloo, 2003;Gabes & Gerding, 1972). For instance, in the CsI 3 crystal structure, the I 3 À anion is asymmetric (C s point symmetry) with /(I-I-I) = 178 (Runsink et al., 1972). For a similar CH 3 NH 3 I/I 2 system, the CH 3 NH 3 I 3 structure was not isolated .
The centrosymmetric character of the I 3 À anions in FAI 3 was confirmed by Raman scattering spectroscopy. The Raman spectrum recorded using a 633 nm laser (Fig. 2) contains a pronounced peak of 1 (I 3 À ) symmetrical oscillations at 116 cm À1 with an additional 235 cm À1 2 1 (I 3 À ) peak and an asymmetrical 3 (I 3 À ) component at 126 cm À1 (Svensson & Kloo, 2003). The latter might be observed because of the presence of two types of I 3 À units in the structure with different environments. In addition, no splitting of symmetric oscillations is observed in the Raman spectrum because of the very small difference between the two types of I 3 À in the structure. Raman spectrum of a transparent single crystalline plate of FAI 3 . Laser wavelength, 633 nm; laser power, 20 mW; accumulation time, 1 min. The insert demonstrates the results of spectroscopic analysis. Table 1 Hydrogen-bond geometry (Å , ) and calculated charges for the specified hydrogen atom derived according to the Mulliken scheme. Symmetry codes: (i) Àx + 1, Ày + 1, Àz + 1; (ii) Àx + 1, Ày, Àz + 1; (iii) x À 1, y À 1, z À 1; (iv) Àx, y, z À 1; (v) Àx + 2, Ày + 1, Àz + 1.  The first of the two types of I 3 À anions in FAI 3 [d(I1-I2) = 2.9165 (14) Å ] is connected with three neighboring planar formamidinium cations by N-HÁ Á ÁI hydrogen bonding with the bond length varying from 2.81 to 3.08 Å (Table 1), which is similar to the distances in formamidinium iodide  as well as in other polyiodides Said et al., 2006;van Megen & Reiss, 2013 A Mulliken population analysis showed that charges of hydrogen atoms forming a single hydrogen bond with a terminal iodine atom is +0.152 for atom H1A, which is connected with carbon, whereas it is +0.171 for atom H2A connected with nitrogen (Table 1), which correlates with the corresponding hydrogen-bond lengths [d(CH1AÁ Á ÁI4) = 3.03 Å vs d(NH2AÁÁI2) = 2.81 Å ]. For the H2B hydrogen atom, the high atomic charge (+0.189) is distributed by two hydrogen bonds. An analysis of the Bader atomic charges for the isolated cation also shows a higher charge for the H2A atom in comparison with H1A (Table 2), which correlates well with the hydrogen-bond lengths.
Besides, the FAI 3 structure can be represented as a pseudocubic close-packed structure with both iodine and formamidinium units in the close-packing layers (Fig. 3). In the discussed crystal structure, each center of mass of the formamidinium cation and each iodine have 12 neighbors in the first coordination sphere, resulting in a distorted cuboctahedra occupancy, which is typical for pseudocubic close-packing. In comparison, the formamidinium iodide structure can be described as a pseudohexagonal close-packed structure with both iodine and formamidinium units in the close-packing layers .

Synthesis and crystallization
FAI and I 2 were purchased from Dyesol (99.9% purity) and Ruskhim (99% purity) without further purification. To obtain single crystalline I, the stoichiometric FAI/I 2 mixture was previously powdered in dry air glovebox. After that, the powdered mixture was slowly heated up to 355-358 K and the Representation of FAI 3 as a deformed cubic close-packed crystal structure with both iodine and formamidinium units in the close-packing layer. Purple and violet atoms represent positions of iodine from the first and second types of I 3 À , respectively. Formamidinium cations are decreased in size for clarity. (a) A single close-packed layer and (b) representation of FAI 3 as a deformed three-layered close-packed structure (the different types of alternating close-packed layers are shown in red, orange and blue).

Table 2
Calculated Bader atomic charges for the isolated symmetric formamidinium cation. The order of the atoms in the isolated cation matches with that in the formamidinium cation in the refined crystal structure.  Computer programs: APEX3 and SAINT (Bruker, 2019), SHELXT (Sheldrick, 2015a), SHELXL2018 (Sheldrick, 2015b) and SHELXTL (Sheldrick, 2008).
obtained single crystals were used for the refinement of crystal structure.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. All H atoms were found in an electron density-difference map and refined with isotropic displacement parameters. The crystal studied was refined as a three-component twin with domain ratios of 0.631 (1)

DFT calculations
The electronic structure of the crystal FAI 3 was calculated using the DMol3 module from the Materials Studio software package (Delley, 2000(Delley, , 1990. In the applied DFT method, the PBE functional was used with the DNP 4.4 (double numerical plus polarization) basis set of atomic functions with all electron relativistic core treatment. The charges (Table 4) were derived according to Mulliken's scheme. The calculations were performed without further optimization.
Computations of Bader atomic charges were performed in the GAUSSIAN 09 program (Frisch et al., 2016) using density functional theory (PBE0) (Perdew et al., 1996) and the def-2-TZVP basis set. The geometry was optimized using the very tight optimization criteria and empirical dispersion corrections on the total energy (Grimme et al., 2010) with Becke-Johnson damping (D3) (Grimme et al., 2011).