Formamidinium iodide: crystal structure and phase transitions

Area of Science:

• Solid-state chemistry
• Crystallography
• Materials science

Background:

• Formamidinium iodide (CH5N2+·I−) is a salt with potential applications in various fields.
• Understanding its crystalline structure and thermal properties is crucial for material design.

Purpose of the Study:

• To elucidate the crystal structure of formamidinium iodide at low temperatures.
• To investigate the thermal behavior and phase transitions of formamidinium iodide.
• To identify different polymorphic modifications under varying temperatures.

Main Methods:

• Single-crystal X-ray diffraction at 100 K.
• Physicochemical analysis including thermogravimetry (TG) and differential scanning calorimetry (DSC).
• Powder X-ray diffraction (PXRD) at variable temperatures.

Main Results:

• Formamidinium iodide crystallizes in the monoclinic space group P21/c at 100 K, featuring planar formamidinium cations and iodide anions forming tight ionic pairs.
• DSC revealed endothermic peaks corresponding to solid-solid phase transitions at 346 K and 387 K, followed by melting and decomposition around 525 K and 605 K.
• PXRD data confirmed the existence of monoclinic (100–346 K), orthorhombic (346–387 K), and cubic (387–525 K) polymorphic forms.

Conclusions:

• The study provides a detailed structural and thermal characterization of formamidinium iodide.
• Multiple polymorphic transitions and distinct crystalline phases were identified, highlighting the material's sensitivity to temperature.
• The findings are essential for understanding and controlling the solid-state properties of formamidinium iodide.