(NH 4 ) 2 Cd 2 Cl 3 F 3 and (NH 4 ) 2 Cd 2 Br 3 F 3 : The First Fluoride‐Containing d 10 Metal Mixed Halides Exhibiting Superior Ultraviolet Nonlinear Optical Properties

In the search for new ultraviolet (UV) nonlinear optical (NLO) materials, two novel cadmium mixed halide compounds, (NH₄)₂Cd₂Cl₃F₃ and (NH₄)₂Cd₂Br₃F₃, are successfully synthesized via hydrothermal methods. These compounds crystallize in the noncentrosymmetric (NCS) space group, R32, and are composed of distorted octahedral [CdX₃F₃] (X═Cl or Br) units, which extend into a 3D framework. Remarkably, both compounds demonstrate strong second-harmonic generation (SHG) efficiencies-3.0 and 8.0 times that of KH₂PO₄ for the Cl- and Br-containing analogs, respectively-with phase-matching behavior observed. The SHG efficiency is attributed to the highly distorted coordination environment of the polarizable d¹⁰ Cd²⁺ ions, with (NH₄)₂Cd₂Br₃F₃ benefiting further from Br's greater polarizability. Furthermore, these compounds exhibit wide bandgaps exceeding 4.2 eV, making them the first d¹⁰ metal mixed halide systems incorporating fluoride that are suitable for UV NLO applications. With UV absorption cut-off edges as short as 203 nm for (NH₄)₂Cd₂Cl₃F₃ and 243 nm for (NH₄)₂Cd₂Br₃F₃, these materials represent a significant advancement in the development of UV-transparent NLO materials. This study introduces a novel synthetic strategy for the design of d¹⁰ mixed halide systems with enhanced optical properties, offering promising candidates for future UV NLO technologies.