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, MnIII6F12(PO3(OH))8[Na8(Kx(H3O)4-x(H2O)2)MnIV(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 [MnIIIO4F2] octahedra and [HPO4] groups, which encapsulates a clathrate-like "guest cluster" of Na8(Kx(H3O)4-x(H2O)2)MnIV(OH)6. The guest cluster is architecturally composed of a [MnIV(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.