Structural Study of Poorly Crystalline Layered Manganese Oxides Using the Atomic Pair Distribution Function Technique.

The atomic pair distribution function (PDF) is a powerful technique to analyze crystal structures of nanoparticulate, poorly crystalline and amorphous materials. The PDF technique recently has been applied to structurally characterize environmental minerals, such as ferryhydrite. In this study, we used PDF analysis to investigate structures of manganese oxides. Manganese oxides, including layered and tunneled structures, are strong oxidants and extraordinary metal sorbents in nature. Naturally-occurring Mn-oxides often possess poor crystallinity, and using XRD and EXAFS techniques, one is unable to fully interpret and differentiate their structures. For example, it is difficult to differentiate amorphous todorokite (3×3 tunnel structure) and birnessite (layered structure) by XRD and EXAFS if they are too disordered to show 7 or 10 Å d-spacing. Our PDF results conclusively confirm that d-MnO₂ and polymeric MnO₂ are layered structures, which are two poorly crystalline Mn-oxides most often used as analogues for natural amorphous Mn-oxides. The presence of Mn(III) in Mn-layers causes modification in the PDF spectra. Todorokite shows a very distinct PDF spectrum like d-MnO₂ and polymeric MnO₂, which can be used for differentiation. Aging effects on the stability of d-MnO₂ and polymeric MnO₂ were also investigated. d-MnO₂ in suspension was fairly stable at 4^oC for three years. However, polymeric MnO₂ in suspension completely transformed to cryptomelane in five years at room temperature.