Three-In-One: A Guanidinium-Containing Niobium Oxyfluoride with a Perovskite-like Structure Revealing Multifunctional Optical Properties

The pursuit of devising efficacious methodologies for the creation of innovative and exceptional noncentrosymmetric (NCS) compounds has emerged as an urgent undertaking. Herein, the first guanidinium-containing niobium oxyfluoride, C(NH2)3NbOF4, possessing a perovskite-like structure, was synthesized via a mild hydrothermal method. The title compound crystallizes in the polar space group P4bm (No. 100), with unit cell dimensions as follows: a = 13.2479(9) Å, c = 3.9492(4) Å, V = 693.11(12) Å3, and Z = 4. Its structure is composed of planar π-conjugated cation [C(NH2)3]+ and second-order Jahn–Teller distorted octahedra [NbO2F4]3–. Combining experimental and theoretical calculations, we demonstrate that C(NH2)3NbOF4 exhibits a second-harmonic-generation (SHG) response, confirming its NCS structure, along with an impressive birefringence of 0.207 at 546 nm. Notably, the title compound demonstrates light-blue emission, suggesting its potential as a trifunctional optical material. Therefore, this study not only expands the structural diversity of transition-metal oxyfluorides but also promotes the discovery of new perovskite-like functional materials.