On the structural defects in synthetic γ-MnO2S

In the literature, the gamma-MnO2 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 epsilon-MnO2 to be put with gamma-MnO2 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 gamma-MnO2 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. (C) 2004 Elsevier Inc. All rights reserved.