Synthesis and characterization of mixed-valence manganese fluorophosphate and analogues with clathrate-like structures: MnIII6F12(PO3(OH))8[Na8(Kx(H3O)4−x(H2O)2)MIV(OH)6] (MIV = Mn, Ti, Ge)

Abstract

A novel, mixed- and high-valence manganese (Mn³⁺/Mn⁴⁺) fluorophosphate, Mn^III₆F₁₂(PO₃(OH))₈[Na₈(K_x(H₃O)_4−x(H₂O)₂)Mn^IV(OH)₆] (denoted as MN), has been prepared via a water-deficient hydrothermal route with phosphoric acid as the sole solvent. This compound features a cubic three-dimensional open-framework structure built from corner-sharing [Mn^IIIO₄F₂] octahedra and [HPO₄] groups, which encapsulates a clathrate-like “guest cluster” of Na₈(K_x(H₃O)_4−x(H₂O)₂)Mn^IV(OH)₆. The guest cluster is architecturally composed of a [Mn^IV(OH)₆] octahedron in a cubic cage of Na⁺ cations, which in turn is surrounded by an octahedral arrangement of K⁺/H₂O ions, resulting in an unprecedented octahedral @ cubic @ octahedral @ cubic arrangement (OCOC). The +4 oxidation state of Mn in the guest cluster has been confirmed by the synthesis of isotypic Ti- and Ge- analogues (denoted as TI and GE) using TiO₂ and GeO₂ as the replacement for MnO₂ in the starting materials. The compounds MN, TI and GE are not stable in aqueous solution and are peeled off layer-by-layer after the absorption of water. This report provides a new route for the synthesis of mixed- and high-valence manganese phosphates that cannot be produced by conventional hydrothermal methods.