Lead-free hybrid ferroelectric material based on formamidine: [NH2CHNH2]3Bi2I9

Abstract

The extraordinary thermal stability of [NH₂CHNH₂]₃Bi₂I₉ (FA₃Bi₂I₉) crystals, up to around 600 K, has been enhanced by using the evaporating method. DSC measurements have revealed reversible structural phase transitions: at 203 K (Phase I → Phase II), 173 K (II → III), 134.7 K (III → IV), 131.4 K (IV → V), and 120 K (V → VI). The crystal structures of FA₃Bi₂I₉ adopt the polar space groups P6₃mc at 220 K (Phase I) and Cmc2₁ at 190 K (Phase II). The pyroelectric current (I_pyr) measured in the temperature range covering all the phases of FA₃Bi₂I₉ confirmed their polar nature. The reversibility of the spontaneous polarization in each phase has been confirmed by the observed hysteresis loops. All the phase transitions are dielectrically active. The dielectric response (ε*(ω,T) close to 134.7 and 131.4 K) is characteristic of ferroelectrics with a critical slowing-down process. Optical bandgaps estimated from the UV-vis measurements and calculated using the DFT method are equal to 1.85 and 1.89 eV, respectively. On the basis of the structural, dielectric, and spectroscopic results, the molecular mechanisms of the phase transitions have been proposed.