Peierls Distortion Induced Giant Linear Dichroism, Second‐Harmonic Generation, and In‐Plane Ferroelectricity in NbOBr 2

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.