(NH)CdClF and (NH)CdBrF: The First Fluoride-Containing d 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, (NH)CdClF and (NH)CdBrF, are successfully synthesized via hydrothermal methods. These compounds crystallize in the noncentrosymmetric (NCS) space group, R32, and are composed of distorted octahedral [CdXF] (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 KHPO 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 d Cd ions, with (NH)CdBrF benefiting further from Br's greater polarizability. Furthermore, these compounds exhibit wide bandgaps exceeding 4.2 eV, making them the first d 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 (NH)CdClF and 243 nm for (NH)CdBrF, 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 d mixed halide systems with enhanced optical properties, offering promising candidates for future UV NLO technologies.