Open-framework ammonium transition metal fluorophosphates with a Kagomé lattice network: synthesis, structure and magnetic properties

Abstract

Two open-framework transition metal fluorophosphates (NH₄)M₃(PO₃F)₂(PO₂F₂)F₂ (M = Mn and Co) have been synthesized under fluoride-rich hydrothermal conditions at 493 K. The structure consists of triplets of face-sharing trans-MO₄F₂ octahedra linked by PO₃F groups to form 2D layers. These 2D layers with formula [M₃(PO₃F)₂O₂F₂]³⁻ in the ab plane are bridged by PO₂F₂ groups to generate a 3D open framework with 1D channels filled with ammonium cations. The XPS spectra suggested the existence of Mn(II) and Co(II) in the corresponding compounds. Further, the XPS F 1s spectra indicated that fluorine anions bridged both the M and P centres. The two compounds feature transition metal layers with a staircase Kagomé network. This topology promotes magnetic frustration, which exhibits a canted antiferromagnetic ground state (T_c = 8.0 K for Mn and T_c = 9.2 K for Co).