On the structural defects in synthetic γ-MnO 2s

Abstract

In the literature, the γ-MnO 2 structure is considered to be that of ramsdellite (R), in which two types of defects exist. The occurrence of a slab of pyrolusite (r) is named a De Wolff defect and random faults r in R give intergrowths of ramsdellite and pyrolusite, which account well for the global features of many experimental diffraction patterns. The other type of defect results from “microtwinning”, which allows the so-called ε-MnO 2 to be put with γ-MnO 2 in the same classification. This paper discusses the previous models of defect and what could be the features of the “microtwinning”, giving for each possible model the corresponding expected features in reciprocal space. The results of a selected area electron diffraction study of rather well crystallized samples of γ-MnO 2 are presented. The splitting of particular diffraction spots and new diffuse intensity are interpreted as the first experimental evidence for “microtwinning”, and a model with orientation variants within microdomains embedded in a “normal” structure is proposed, which is rather different from the previous hypotheses involving parallel twin planes and parallel twin boundaries.