Investigation of cation ordering in triclinic sodium birnessite via 23Na MAS NMR spectroscopy
Birnessite is a widespread, naturally occurring layered manganese oxide that exerts significant influence over the geochemical cycling of environmentally relevant cations due to its high-interlayer adsorption capacity. Triclinic sodium birnessite was used as a synthetic analog to gain a better understanding of the nature of cation adsorption in this important phyllomanganate. Drawing from previous work correlating observed 23Na NMR shifts in manganese oxides with local environment and Mn oxidation state, the 23Na NMR spectra of metastable buserite and two birnessite samples, NaBi-H-I and NaBi-II, were analyzed to determine the nature of the bound interlayer sodium ions in these materials. The small 23Na chemical shift of buserite shows that its interlayer sodium is fully hydrated. X-ray diffraction indicates that NaBi-H-I is a disordered birnessite while NaBi-II is highly crystalline. High-field (14.1 T) fast MAS NMR spectra of NaBi-H-I and NaBi-II supports these observations, resolving multiple sodium environments for NaBi-H-I and only two sodium environments for NaBi-II. The observed hyperfine shifts were less than expected for sodium environments with manganate layers composed of 2/3 Mn4+ and 1/3 Mn3+ ions, and the 23Na line shapes indicated that the Na+ ions are in distorted environments. Both these factors suggest that the sodium ions are offset in the interlayers toward a single oxide layer and located near Mn3+-rich environments within the layer.