Widely used hardly known. An insight into electric and dynamic properties of formamidinium iodide

Abstract

The simple organic crystal formamidinium iodide (FAI) appeared to be a novel semiconducting material in a wide temperature range. The electric properties of FAI and the role of formamidinium cation (FA + ) in the molecular mechanism of the solid-to-solid phase transitions (at 345 K (III / II) and 388 K (II / I)) were analysed. The creation of the ferroelastic domain structure in phases III and II was proved on the basis of observation under a polarizing microscope. Moreover, the molecular arrangement of dipolar organic FA + was studied by 1 H NMR (spin-lattice relaxation time) and vibrational spectroscopy supported by density functional theory. The theoretical results show a good agreement with the experimental data. The infrared spectrum in a harmonic approximation was calculated and a comparative vibrational analysis was performed. All used techniques showed that the prototypic phase I exhibits the feature of plastic phase.

Vibrational spectra of methylammonium iodide and formamidinium iodide in a wide temperature range

“…The recorded IR and Raman spectra of both MAI and FAI at room temperature are in accordance with the already reported literature results [5,7,8,11,19]. For clarity and systematization of the assignation, parallel comparison of MAI and FAI was applied in every region.…”

“…The assignation of the out-of-plane CH vibration in FAI has been previously reported [11,19] but has not been addressed properly since it is more difficult to distinguish the band origin at room temperature rather than at the liquid N2 temperatures. From our detailed analysis of the bands given in Figure 11, several conclusions can be drawn.…”

“…Even in these papers, IR analyses are lacking, incomplete [9], or inaccurate [10]. More detailed assignation has been provided recently by Hills-Kimball et al [11] but in this case only the spectra at two temperatures have been covered (300 K and 350 K), so the phase transitions have not been studied.…”

“…Even though they seem simple and the assignation of the bands in the IR and Raman spectra should be straightforward, it was shown that the situation is complicated since both of them exist in different phases depending on the temperature. The structural characterization of the different phases has been previously reported [11][12][13][14][15], which can assist the process of assignation.…”

“…That suggests presence of two phase transitions in each compound. Based on the literature data [8,11,13,15], the space group parameters as well as the temperatures of the transitions of MAI and FAI are given in Scheme 1.…”