Fits of the γ-MnO2 Structure Model to Disordered Manganese Dioxides

Abstract

This paper defines γ-MnO2 as a manganese dioxide phase with a structure based on an intergrowth of ramsdellitic and pyrolusitic domains, a model initially proposed by de Wolff [Acta Cryst. (1959), 12, 341–345]. This model consists of a stochastic stacking of four different types of layers, each of which is based on the same underlying two-dimensional periodic lattice. An analytic expression for the scattered intensity as a function of scattering vector is given; the subsequent powder averaging to obtain the intensity as a function of scattering angle has to be performed partly numerically. This formalism is then utilized to fit experimentally observed powder X-ray diffraction patterns of typical disordered manganese dioxides with the γ-MnO2 model. The model fits the chemically prepared manganese dioxide investigated here fairly well; heating it increases the pyrolusitic character. However, the structure of electrolytically prepared manganese dioxides (EMD) is not well described by this model, implying that there are other types of disorders in EMDs.