K(NH₄)Zn₂(PO₄)₂: a Beryllium-Free Sr₂Be₂B₂O₇ Derivative with Enhanced Interlayer Connectivity

A novel zinc phosphate derivative of Sr2Be2B2O7 (SBBO), K(NH4)Zn-2(PO4)(2) (KNZP), featuring [Zn2P2O8](infinity)(2-) double layers akin to the [Be2B2O7](infinity)(4-) layers in SBBO, was successfully synthesized via a moderate hydrothermal method. Through the substitution of BeO4 and BO3 with ZnO4 and PO4, the issue of toxicity has been effectively resolved, while the enhanced interlayer interactions facilitated by covalent and hydrogen bonding in KNZP overcome the inherent structural instability. Notably, KNZP exhibits a wide transparent window and a moderate second-harmonic generation (SHG) intensity, reaching 0.7 times that of KH2PO4 (KDP), rendering it type-I phase-matchable, indicating that it is a promising UV nonlinear optical (NLO) material.