Inorganic metal phosphates have served as promising candidates for non-linear optical (NLO) materials owing to their advantages in environmental-friendly chemical composition, wide transmittance ranges and favorable single-crystal growth habits. Notable examples of phosphate-based NLO materials include KH₂PO₄ (KDP) and KTiOPO₄ (KTP), which have been commercialized and widely used in laser technology.

Many new phosphates have also been developed, however, due to the very small hyperpolarizability and anisotropy of the highly symmetric [PO₄]^3- tetrahedral unit, most of them display weak second harmonic generation(SHG) signals and small birefringence, which seriously hinders their application as NLO materials. 

In a recent study published in J. Am. Chem. Soc., the research group led by Prof. MAO Jianggao from Fujian Institute of Research on the Structure of Matter(FJIRSM) of Chinese Academy of Sciences proposed that introducing highly polarizable Hg²⁺ into phosphate is an effective route to enhance the second-order NLO susceptibility and anisotropy of phosphates, and they successfully synthesized LiHgPO₄ via mild hydrothermal reactions. 

In this study, LiHgPO₄ crystallizes in the acentric tetragonal space group P-42₁m. Each Hg atom is characterized by (2+4) coordination with a highly distorted HgO₆ octahedron which shares oxygen atoms with PO₄ tetrahedra to form a unique double-layer.  

Researchers found that LiHgPO4 exhibits excellent performances, including a short ultraviolet(UV) absorption edge (<300 nm), very strong SHG response (11 × KDP at 1064 nm) and large birefringence (0.068 at 1064 nm). They realized that LiHgPO₄ is a promising candidate for UV NLO material.  

Theoretical calculations indicate that the Hg²⁺ cations with highly distorted (2+4) octahedral geometry contribute greatly to the SHG effect and optical anisotropy of the compound.  

The study sheds new light on the idea of introducing highly distorted HgO₆ octahedron to improve the SHG response and birefringence of phosphates. This approach can be harnessed to design more practical NLO materials based on metal phosphates. 

The design strategy and optical properties of LiHgPO₄ (Image by Prof. MAO’s group) 

Contact: 

Prof. MAO Jianggao  

Fujian Institute of Research on the Structure of Matter 

Chinese Academy of Sciences 

E-mail: mjg@fjirsm.ac.cn