Niobium Oxydichloride (NbOCl₂)

NbOCl₂ is an emerging nonlinear-optical van der Waals crystal that has recently attracted intense interest for its giant Second Harmonic Generation (SHG) efficiency. It adopts a monoclinic structure with space group C2, built from one-dimensional NbO chains; within the NbO₆ octahedra, the Nb atoms undergo an off-centering (Peierls-type) distortion, resulting in broken inversion symmetry—the origin of its strong SHG and ferroelectric-like behavior. Its bandgap is around 1.9 eV. Its most striking feature is the near-complete interlayer electronic decoupling: the nonlinear optical response is almost independent of layer number, and the material serves as a candidate for on-chip quantum light sources (entangled photon pairs / spontaneous parametric down-conversion), giving it significant promise for integrated photonics and quantum optics.