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.