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.