(NH <sub>4</sub> ) <sub>2</sub> Cd <sub>2</sub> Cl <sub>3</sub> F <sub>3</sub> and (NH <sub>4</sub> ) <sub>2</sub> Cd <sub>2</sub> Br <sub>3</sub> F <sub>3</sub> : The First Fluoride‐Containing d <sup>10</sup> Metal Mixed Halides Exhibiting Superior Ultraviolet Nonlinear Optical Properties
Seung-Hun Choi, Yang Li, Yunseung Kuk, Kang Min Ok
Abstract
Abstract In the search for new ultraviolet (UV) nonlinear optical (NLO) materials, two novel cadmium mixed halide compounds, (NH 4 ) 2 Cd 2 Cl 3 F 3 and (NH 4 ) 2 Cd 2 Br 3 F 3 , are successfully synthesized via hydrothermal methods. These compounds crystallize in the noncentrosymmetric (NCS) space group, R 32, and are composed of distorted octahedral [CdX 3 F 3 ] (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 KH 2 PO 4 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 10 Cd 2+ ions, with (NH 4 ) 2 Cd 2 Br 3 F 3 benefiting further from Br's greater polarizability. Furthermore, these compounds exhibit wide bandgaps exceeding 4.2 eV, making them the first d 10 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 4 ) 2 Cd 2 Cl 3 F 3 and 243 nm for (NH 4 ) 2 Cd 2 Br 3 F 3 , 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 10 mixed halide systems with enhanced optical properties, offering promising candidates for future UV NLO technologies.