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

The Peierls distortion plays an essential role in governing the in-plane ferroelectricity and nonlinear optical characteristics of anisotropic niobium oxide dihalides, such as NbOCl₂ and NbOI₂. Despite its significance, experimental investigation into the structural, optical, and ferroelectric properties of NbOBr₂ has been lacking. Here, the successful fabrication of centimeter-sized, high-quality NbOBr₂ single crystals, enabling direct observation of Peierls distortion using aberration-corrected scanning transmission electron microscopy, is reported. Notably, the magnitude of Peierls distortion in NbOBr₂ falls between that observed in NbOCl₂ and NbOI₂. Furthermore, the existence of giant linear dichroism absorption and second-harmonic generation (SHG) phenomena associated with Peierls distortion is confirmed. Exfoliated NbOBr₂ 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 NbOBr₂. This work demonstrates that NbOBr₂ holds promise for polarized nonlinear optical devices, as well as for nonvolatile information processing and storage devices.