Superior ferroelectricity and nonlinear optical response in a hybrid germanium iodide hexagonal perovskite

Abundant chemical diversity and structural tunability make organic–inorganic hybrid perovskites (OIHPs) a rich ore for ferroelectrics. However, compared with their inorganic counterparts such as BaTiO 3 , their ferroelectric key properties, including large spontaneous polarization ( P s ), low coercive field ( E c ), and strong second harmonic generation (SHG) response, have long been great challenges, which hinder their commercial applications. Here, a quasi-one-dimensional OIHP DMAGeI 3 (DMA = Dimethylamine) is reported, with notable ferroelectric attributes at room temperature: a large P s of 24.14 μC/cm 2 (on a par with BaTiO 3 ), a low E c below 2.2 kV/cm, and the strongest SHG intensity in OIHP family (about 12 times of KH 2 PO 4 (KDP)). Revealed by the first-principles calculations, its large P s originates from the synergistic effects of the stereochemically active 4 s 2 lone pair of Ge 2+ and the ordering of organic cations, and its low kinetic energy barrier of small DMA cations results in a low E c . Our work brings the comprehensive ferroelectric performances of OIHPs to a comparable level with commercial inorganic ferroelectric perovskites.