Structure and electronic properties of porous manganese oxides
Manganese oxides with varying pore sizes from ~2 to ~7 have been prepared by standard solid-state techniques and by low temperature hydrothermal methods. These materials have an open framework composed exclusively of manganese oxide, which in simple tunnel and layered structures is built up exclusively of edge and corner shared MnO6 octahedra. In more complex structures, such as that exhibited by Na0.44MnO2, this framework is built up of MnO6 octahedra and MnO5 square pyramids, where Mn3+ and Mn4+ are ordered on crystallographically distinct sites. The size of the tunnels or layer gap is dependent on the size of the cation used as a template. This thesis shows that it is possible to remove the 'template' ion from within many framework materials without destroying the structural integrity, this is due primarily to the ready conversion between the various manganese oxidation states to maintain charge balance. This makes it possible to tune the properties by incorporating varying amounts of foreign cations and/or small molecules into the vacant pore sites. These intercalation reactions result in small changes in the average manganese oxidation state, which in turn leads to differences in the thermal stability, observed magnetic and transport properties. We have also shown that it is possible to intercalate conducting polymers into the framework of some layered materials. Whilst the mechanism is not known, it can be seen that the oxidation state of the framework plays an important part in the ordering of the monomer/polymer units within the layers. Incorporation of these polymers leads to large changes in the magnitude of the observed magnetic moment as well as in the magnetic ordering. This work shows that these materials have a versatile framework, which leads to the real possibility of tuning the properties of a material to achieve desired effects leading to many possible uses for these types of materials.