Fluoroborophosphates: a family of potential deep ultraviolet NLO materials

Abstract

A series of new acentric fluoroborophosphates, namely, RbBPO₄F, CsBPO₄F and (NH₄)₂BPO₄F₂, 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)₂BPO₄F₂ exhibits a 3D supramolecular structure, in which 1D [BPO₄F₂]⁻_∞ 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)₂BPO₄F₂ 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.