Nowadays, a variety of commercial new nonlinear optical (NLO) crystals have been developed. Pb(II) is often used to substitute alkaline earth metal in borates to create NLO materials with excellent second harmonic generation (SHG) response due to the lone pair activity. However, large enhancement is rare by replacing the alkaline earth metal with Pb(II) in Pb-free borates and the mechanism of the SHG enhancement is still a mystery.  

A research group led by Prof. PAN Shilie and Prof. YANG Zhihua at Xinjiang Technical Institute of Physics & Chemistry of Chinese Academy of Sciences, in collaboration with American scientists from Northwestern University clarified the reason of the SHG enhancement, and they designed and synthesized a new NLO material Pb₂Ba₃(BO₃)₃Cl for the first time based on the result. The obtained Pb₂Ba₃(BO₃)₃Cl shows a phase-matching SHG response approximately 3.2 times that of KDP, and as large as six times that of isomorphic Ba₅(BO₃)₃Cl, which is the first acentric lead borate with isolated BO₃ groups.

Motivated by the large SHG response obtained from the properties of isolated boron−oxygen groups and lead−oxygen groups, researchers designed and synthesized the acentric compound Pb₂Ba₃(BO₃)₃Cl with isolated BO₃ groups. To further study the relationship between the SHG response and the electronic structure, they used the first principle calculation and discovered that the enhanced response originates from the unique edge sharing connection style between the Pb−O polyhedra and B−O groups.

This unique connection style enhances SHG responses by making the metal−oxygen bond more polarizable to external optical excitation. Therefore, researchers conjectured that among lead A-L replacement borates, those compounds with isolated boron−oxygen building units would be potential NLO materials with enhanced SHG responses and merit continued investigation.

This study, which is published in Journal of the American Chemical Society, provides a route to identify systems that would benefit from SHG-active cation substitution in isomorphic structures that exhibit weak or null SHG responses.

This work was financially supported by the National Basic Research Program of China, National Natural Science Foundation of China, the Western Light of Chinese Academy of Sciences, the Recruitment Program of Global Experts, etc.

Figure 1: Phase-matching curves of Pb₂Ba₃(BO₃)₃Cl, Ba₅(BO₃)₃Cl, and KDP (Image by XTIPC)

Figure 2: Diagram of the structures and the frontier molecular orbitals (isovalue 0.02 au/Å3) of PbBO₃ (Image by XTIPC)