Above‐Room‐Temperature Ferroelectricity and Giant Second Harmonic Generation in 1D vdW NbOI3

Abstract The realization of spontaneous ferroelectricity down to the one‐dimensional (1D) limit is both fundamentally intriguing and practically appealing for high‐density ferroelectric and nonlinear photonics. However, the 1D vdW ferroelectric materials are not discovered experimentally yet. Here, the first 1D vdW ferroelectric compound NbOI 3 with a high Curie temperature T C > 450 K and giant second harmonic generation (SHG) is reported. The 1D crystalline chain structure of the NbOI 3 is revealed by cryo‐electron microscopy, whereas the 1D ferroelectric order originated from the Nb displacement along the Nb‐O chain ( b ‐axis) is confirmed via obvious electrical and ferroelectric hysteresis loops. Impressively, NbOI 3 exhibits a giant SHG susceptibility up to 1572 pm V −1 at a fundamental wavelength of 810 nm, and a further enhanced SHG susceptibility of 5582 pm V −1 under the applied hydrostatic pressure of 2.06 GPa. Combing in situ pressure‐dependent X‐ray diffraction, Raman spectra measurements, and first‐principles calculations, it is demonstrated that the O atoms shift along the Nb─O atomic chain under compression, which can lead to the increased Baur distortion of [NbO 2 I 4 ] octahedra, and hence induces the enhancement of SHG. This work provides a 1D vdW ferroelectric system for developing novel ferroelectronic and photonic devices.