Evolving microstructure in MnO2 using amorphisation and recrystallisation

The electrochemical properties of gamma-MnO(2) are governed by the rich and complex microstructure it accommodates. However, characterisation, at the atormstic level, of this material is difficult experimentally; rather the materials are typified by their X-ray diffraction (XRD) patterns. Here, we use an evolutionary simulation-amorphisation and recrystallisation (A&R)-to generate atomistic models for gamma-MnO(2), which include microstructural detail. These models conform to the pyrolusite polymorph, with small intergrowth domains (structurally similar to brookite-TiO(2)) and comprise micro-twinning together with general grain boundaries, stacking faults, dislocations and isolated point defects and defect clusters. Molecular graphics images, showing the atom positions for these microstructural features together with the (simulated) XRD patterns they give rise to, are presented and compare favourably with measured XRD providing valuable validation of the atomistic models. Surprisingly, the atomistic models also include anion sublattice domains conforming to both ccp and hcp, which are separated or facilitated by dislocations. We are not aware of this phenomenon having been reported in the literature for MnO(2) and therefore, our simulations offer the prediction of its existence. (c) 2006 Elsevier B.V. All rights reserved.