3–5 μm mid-infrared (mid-IR) lasers have important applications in photoelectric areas, laser medicine, optical parametric oscillation pumping etc. Previous studies have paid considerable efforts on exploring the nonlinear optical (NLO) materials used in the mid-infrared fields. AgGaS₂ and ZnGeP₂ are typical commercialized IR NLO materials. However, their low laser damage thresholds (LDTs) dramatically hindered their practical application. Therefore, metal iodates have been considered as potential mid-IR NLO materials due to their excellent characters.  

In a recent study published in Angew. Chem. Int. Ed., a research group led by Prof. YE Ning from Fujian Institute of Research on the Structure of Matter (FJIRSM) of the Chinese Academy of Sciences, has developed the first ammonium-containing metal iodate fluoride, (NH₄)Bi₂(IO₃)₂F₅, by introducing F^- anions and NH₄⁺ anionic groups into bismuth-containing iodates, and the new NLO material achieved great performances. 

The researchers found that (NH₄)Bi₂(IO₃)₂F₅ showed a very strong second harmonic generation(SHG) response (9.2 ′ KH₂PO₄), suitable birefringence (Δn = 0.0690 at 589.3 nm), and high LDT(40.2′AgGaS₂). 

The bismuth-containing iodates usually possess high probabilities to achieve large SHG responses due to the potentiation of polarizable lone-pair electrons in Bi³⁺ as well as IO₃^- anionic groups. The superposition of polarization in (NH₄)Bi₂(IO₃)₂F₅ was beneficial to the dipole moment, which further enhanced the SHG response of (NH₄)Bi₂(IO₃)₂F₅.  

The researchers introduced fluoride anions into the bismuth-containing iodates in order to enlarge the bandgap and further increase the laser damage threshold. (NH₄)Bi₂(IO₃)₂F₅ exhibited a high LDT value (40.2′AgGaS₂). 

Moreover, the researchers revealed that the introduction of NH₄⁺ anionic groups can not only reinforce the bonding interaction between [Bi₂(IO₃)₂F₅]^- layers, but also reduce the growth temperature. H atoms in NH₄⁺ anionic groups can form N-H···F hydrogen bonds with F anions in [Bi₂(IO₃)₂F₅]^- layers. The growth temperature of (NH₄)Bi₂(IO₃)₂F₅ is 160 °C, which is lower than growth temperatures of most bismuth-containing iodates. 

This study provides an effective way to explore and design mid-IR NLO materials with strong SHG responses and large LDTs. 

Illustration of structural of (NH₄)Bi₂(IO₃)₂F₅ and the strong SHG response (Image by Prof. YE’s group)  

Contact: 

Prof. YE Ning  

Fujian Institute of Research on the Structure of Matter 

Chinese Academy of Sciences 

E-mail: nye@fjirsm.ac.cn