Four materials forming a structural series of ␥-manganese pounds H-insertion has been carried out on a range of dioxides were identified by an increasing proportion of micro-Ͳ-manganese dioxide materials which display related but twinning (17-100%) and a nearly constant amount of de Wolff differing X-ray diffraction patterns and form a structural disorder ( P r ȃ 0.41) as determined from their XRD patterns. series. Chemical H-insertion was performed in a nonaqueous environment and the structure of the fully H-inserted compounds inves-THE STRUCTURE OF ␥-MANGANESE DIOXIDE tigated by XRD. In contrast to the starting materials the H-inserted compounds had similar XRD patterns which could Pannetier et al. (2, 8-11) have succeeded in explaining be indexed on the basis that H-insertion caused expansion prethe range of observed powder X-ray diffraction patterns dominantly in the b orthorhombic direction. The anisotropic with a new structural model which incorporates two types expansion changed the angle of the 021 and 061 microtwinning of crystal defects into an idealized ramsdellite host strucplanes and caused strain along them. The demicrotwinning ture. The first type of defect (see Fig. 1) is a random planar which resulted was the reason for the similarity of the XRD fault based on the model originally constructed by de Wolff patterns of the H-inserted compounds. The common endpoint (12, 13) which exploits the fact that ramsdellite and pyroluof H-insertion into the structural series was ␦-MnOOH, which site (which are two of the known crystalline polymorphs is an unmicrotwinned intergrowth structure based on groutite of manganese dioxide (14)) possess intimately related and manganite type layers reflecting the original intergrowth structures due to the similarity of their oxygen frameworks. of ramsdellite and pyrolusite type layers in the original host structures.