Functional magnetic materials metal ions

Oxides play many roles in modem electronic technology from insulators which can be used as capacitors, such as the perovskite BaTiOs, to the superconductors, of which the prototype was also a perovskite, Lao.sSro CutT A, where the value of x is a function of the temperature cycle and oxygen pressure which were used in the preparation of the material. Clearly the chemical difference between these two materials is that the capacitor production does not require oxygen partial pressure control as is the case in the superconductor. Intermediate between these extremes of electrical conduction are many semiconducting materials which are used as magnetic ferrites or fuel cell electrodes. The electrical properties of the semiconductors depend on the presence of transition metal ions which can be in two valence states, and the conduction mechanism involves the transfer of electrons or positive holes from one ion to another of the same species. The production problem associated with this behaviour arises from the fact that the relative concentration of each valence state depends on both the temperature and the oxygen partial pressure of the atmosphere. 

The coordination chemistry of oxalate (ox, C2042-) compounds provides a series of very interesting compounds from the stereochemical and magnetic points of view [197]. Most frequently the compounds form honeycomb layers in the presence of transition metal ions, in which the stereochemistry of the metal ion coordination sphere alternates between A and A. However, a three-dimensional homochiral structure is also possible. On the other hand, the negative charge of the oxalates necessitates the incorporation of cations between them, which provides the opportunity to introduce chirality and additional functionality in materials. The compound formed between homochiral manganese II oxalate and iron II tris bipyridinc (bpy) with formula [Mn oxls]2 " [Fcn(bpy)3]2+ crystallises in the space group fJ4 32. 

Figure 13.11. Room temperature gram magnetic susceptibilities, % of rare earth sol-gel materials as a function of the atomic number showing the expected trend for pure compounds A. %s correlate linearly with the susceptibilities of the pure oxides B. reflecting the stoichiometric similarity within each series of materials. Slopes are proportional to the relative abundance of the metal ions. Inset in A Polyurea-crosslinked dysprosium aerogel powder picked up by a 1-cm diameter Nd-Fe-B magnet.

---