AZn2BO3X2(A = K, Rb, NH4; X = Cl, Br): New Members of KBBF Family Exhibiting Large SHG Response and the Enhancement of Layer Interaction by Modified Structures

A new category of five KBBF-analogy
nonlinear optical (NLO) materials,
AZn2BO3X2 (A = K, Rb, NH4; X = Cl, Br), are developed by the tetrahedron substitution of BeO3F for ZnO3X from KBe2BO3F2 (KBBF). They preserve the structural merits of KBBF, consisting
of the infinite planar [Zn2BO3X2]∞ layers. Optical measurements on this series of NLO
crystals reveal that they are phase-matchable in the visible and UV
region with powder second-harmonic generation (SHG) responses being
more than twice that of isostructural KBBF. First-principles calculations
and atom-cutting analysis were carried out to demonstrate that enhanced
SHG response originates from the cooperative effect of coparallel
[BO3] triangles and distorted ZnO3Cl/Br tetrahedra.
The theoretical calculations and experimental results show that AZn2BO3X2 exhibits a less-developed layer
habit compared with KBBF. Especially, because of the existence of
relatively strong hydrogen bond between NH4+ groups and [Zn2BO3Cl2]∞ layers, NH4Zn2BO3Cl2 crystal exhibits the best growth behavior along the c axis. These results show that they may have prospects as a kind
of UV nonlinear optical material.

一类新型的五元KBBF类比非线性光学（NLO）材料，即AZn2BO3X2（A = K, Rb, NH4; X = Cl, Br），通过将BeO3F替换KBe2BO3F2（KBBF）中的ZnO3X以四角体结构的方式进行研发。此类材料保留了KBBF的结构优势，包括无限延伸的平面[Zn2BO3X2]∞层。对这一系列NLO晶体的光学测量揭示，它们在可见光和紫外区域具有相位匹配性，粉末二次谐波产生（SHG）响应超过同构KBBF的两倍。通过第一性原理计算和原子切割分析，证实了增强的SHG响应源于平行[BO3]三角形与扭曲的ZnO3Cl/Br四面体的协同效应。理论计算与实验结果同时表明，与KBBF相比，AZn2BO3X2展现出较为不发达的层状习性。特别是，由于NH4+基团与[Zn2BO3Cl2]∞层之间存在相对强烈的氢键，NH4Zn2BO3Cl2晶体在c轴方向上表现出最佳的生长行为。这些结果表明，它们在作为紫外非线性光学材料方面具有广阔的应用前景。