[NH2CHNH2]3Sb2I9: a lead-free and low-toxicity organic–inorganic hybrid ferroelectric based on antimony(iii) as a potential semiconducting absorber

Abstract

We report the physicochemical characteristics of a novel Sb-based formamidinium ferroelectric material, (NH₂CHNH₂)₃Sb₂I₉ (FAIA). FAIA crystallizes at room temperature in the hexagonal P6₃mc space group. The crystal structure of FAIA is isomorphic to that of its bismuth analog, [NH₂CHNH₂]₃Bi₂I₉ (FAIB), which is characterized by a zero dimensional (0D) anionic network (discrete bioctahedral units). From the DSC results three phase transitions (PT) have been found for FAIA: at 187.4/189 K (phase I ↔ phase II), 177.6/177.9 K (II ↔ III) and 148.2/152.3 K (III ↔ IV). The compound exhibits ferroelectricity in the room temperature phase I and in the successive temperature phases (II and III). The spontaneous polarization (P_S = 0.5 × 10⁻² μC cm⁻²) reversibility has been confirmed by the presence of the ferroelectric hysteresis loops and pyroelectric measurements. The origin of ferroelectricity is discussed in terms of the cationic dynamics. This material may be considered as a semiconductor (σ ∼ 10⁻⁶ S m⁻¹, 300 K).