Peierls Distortion Induced Giant Linear Dichroism, Second‐Harmonic Generation, and In‐Plane Ferroelectricity in NbOBr2

Abstract The Peierls distortion plays an essential role in governing the in‐plane ferroelectricity and nonlinear optical characteristics of anisotropic niobium oxide dihalides, such as NbOCl 2 and NbOI 2 . Despite its significance, experimental investigation into the structural, optical, and ferroelectric properties of NbOBr 2 has been lacking. Here, the successful fabrication of centimeter‐sized, high‐quality NbOBr 2 single crystals, enabling direct observation of Peierls distortion using aberration‐corrected scanning transmission electron microscopy, is reported. Notably, the magnitude of Peierls distortion in NbOBr 2 falls between that observed in NbOCl 2 and NbOI 2 . Furthermore, the existence of giant linear dichroism absorption and second‐harmonic generation (SHG) phenomena associated with Peierls distortion is confirmed. Exfoliated NbOBr 2 flakes exhibit single‐domain ferroelectricity, with the polarization switchable by an electric field. Temperature‐dependent SHG measurements reveal a Curie temperature of ≈502 K for ferroelectric NbOBr 2 . This work demonstrates that NbOBr 2 holds promise for polarized nonlinear optical devices, as well as for nonvolatile information processing and storage devices.