Fluoroborophosphates: a family of potential deep ultraviolet NLO materials

A series of new acentric fluoroborophosphates, namely, RbBPO4F, CsBPO4F and (NH4)(2)BPO4F2, have been obtained by combining BO3F or BO2F2 with a PO4 tetrahedron. The structures of the isomeric RbBPO4F and CsBPO4F feature a three-dimensional diamond framework, in which a BO3F (or PO4) tetrahedron corner is shared with three PO4 (or BO3F) tetrahedra, forming 1D tunnels of eight-membered rings (MRs) along the three axial directions, which are filled with alkali cations. (NH4)(2)BPO4F2 exhibits a 3D supramolecular structure, in which 1D [BPO4F2](infinity)(1) anionic chains along the a axes based on corner-sharing and alternating PO4 and BO2F2 tetrahedra are further interconnected with ammonium cations by electrostatic attractions and hydrogen bonds. RbBPO4F, CsBPO4F and (NH4)(2)BPO4F2 display second-harmonic generation (SHG) signals approximately 0.4, 0.6 and 0.8 times that of KH2PO4 (KDP) and exhibit short UV absorption edges (< 200 nm). Measurements of thermal stability and optical properties and theoretical calculations have also been performed.