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.