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 4s(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.