H3ONb3O8 and HNb3O8 : New protonic oxides with a layer structure involving ion exchange properties

The oxide H₃ONb₃O₈, with a layer structure, was synthesized by ion exchange reaction of the niobate KNb₃O₈ with an acid solution. H₃ONb₃O₈ is a good ion exchanger : the ion exchange reaction is reversible, and NH₄Nb₃O₈ can be obtained by action of an ammonium salt solution. The study of the thermal stability of H₃ONb₃O₈ allowed us to isolate the layer oxide HNb₃O₈ whose hydration is reversible, the R-Nb₂O₅ form, and a new form of niobium pentoxide closely related to the R-form, called R′. The structural characteristics of these oxides are studied and their structural relationships are discussed.