AZn<sub>2</sub>BO<sub>3</sub>X<sub>2</sub>(A = K, Rb, NH<sub>4</sub>; 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, AZn<sub>2</sub>BO<sub>3</sub>X<sub>2</sub> (A = K, Rb, NH<sub>4</sub>; X = Cl, Br), are developed by the tetrahedron substitution of BeO<sub>3</sub>F for ZnO<sub>3</sub>X from KBe<sub>2</sub>BO<sub>3</sub>F<sub>2</sub> (KBBF). They preserve the structural merits of KBBF, consisting of the infinite planar [Zn<sub>2</sub>BO<sub>3</sub>X<sub>2</sub>]<sub>∞</sub> 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 [BO<sub>3</sub>] triangles and distorted ZnO<sub>3</sub>Cl/Br tetrahedra. The theoretical calculations and experimental results show that AZn<sub>2</sub>BO<sub>3</sub>X<sub>2</sub> exhibits a less-developed layer habit compared with KBBF. Especially, because of the existence of relatively strong hydrogen bond between NH<sub>4</sub><sup>+</sup> groups and [Zn<sub>2</sub>BO<sub>3</sub>Cl<sub>2</sub>]<sub>∞</sub> layers, NH<sub>4</sub>Zn<sub>2</sub>BO<sub>3</sub>Cl<sub>2</sub> crystal exhibits the best growth behavior along the <i>c</i> axis. These results show that they may have prospects as a kind of UV nonlinear optical material.