As the key devices of the ultraviolet (UV, 200 nm<λ<400 nm) all-solid-state lasers, UV nonlinear optical (NLO) crystals have attracted increasing attention in the near decades. For now, the 266 nm laser (fourth harmonic generation for Nd: YAG)is mainlygenerated by β-BaB₂O₄ (β-BBO) and CsLiB₆O₁₀(CLBO) crystals. However, the overlarge birefringence ofβ-BBO and hygroscopicity of CLBO cause many troubles in the practical laser output. The exploration of new UV NLO crystal is of urgent demand. 

　　KBBF is the unique crystal that allows efficient generation of DUV light by direct frequency doubling. The special two-dimensional (2D) layered structure of KBBF integrates many advantages such as large band gap, large SHG responses and moderate birefringence and so on, which is regarded as a typical prototype for designing new NLO materials.In recent years, beryllium-free KBBF family materials were continuously reported considering the toxicity of BeO used in the preparation of KBBF. It has been proved that the zinc atom is an excellent substitution candidate for beryllium atom.The replacement from Be to Znprovides large SHG responses and keeps moderate birefringence. 

　　The team of Prof. ShiliePAN in Xinjiang Technical Institute of Physics and Chemistry (XTIPC) of Chinese Academy of Sciences successfully obtained a beryllium-free UV NLO material with better growth habit, Zn₂BO₃(OH), that derives from the prototype of KBBF.Zn₂BO₃(OH) was explored to feature the same topological layer with KBBF by replacing [BeO₃F]^5- to [ZnO₃(OH)]^5- and excellent UV performances. It exhibits an SHG response of about 1.5 × KDP with the UV cutoff edge of 204 nm. The birefringence of Zn₂BO₃(OH) in the visible region is about 0.067, which is larger than those of commercial UV crystals LBO, CBO and CLBO. Additionally, it has excellent thermal and water-resistant stabilities. Owing to the removal of interlayer cations, Zn₂BO₃(OH) shows better growth habits than KBBF while achieving the balance between beneficial layered structure and layer tendency. 

　　Related work has been published in Adv. Sci., DOI: 10.1002/advs.201901679. Further researches are in progress.

　　The authors acknowledge the supportfrom the Key Research Project of Frontier Science of CAS, the National Key Research Project, the National Natural Science Foundation of China and the Youth Innovation Promotion Association of CAS. 

　　https://onlinelibrary.wiley.com/doi/epdf/10.1002/advs.201901679 

Keywordnonlinear optical materials, borates, KBBF-family, second harmonic generation, crystal growth habits  

Xuefei Wang, Fangfang Zhang, Le Gao, Zhihua Yang and Shilie Pan* 

Nontoxic KBBF Family Member Zn₂BO₃(OH): Balance between Beneficial Layered Structure and Layer Tendency