The Metal-Insulator Transition of the Magneli phase V_4O_7: Implications for V_2O_3

The metal-insulator transition (MIT) of the Magneli phase V_4O_7 is studied by means of electronic structure calculations using the augmented spherical wave method. The calculations are based on density functional theory and the local density approximation. Changes of the electronic structure at the MIT are discussed in relation to the structural transformations occuring simultaneously. The analysis is based on a unified point of view of the crystal structures of all Magneli phase compounds V_nO_2n-1 (3 =< n =< 9) as well as of VO_2 and V_2O_3. This allows to group the electronic bands into states behaving similar to the dioxide or the sesquioxide. In addition, the relationship between the structural and electronic properties near the MIT of these oxides can be studied on an equal footing. For V_4O_7, a strong influence of metal-metal bonding across octahedral faces is found for states both parallel and perpendicular to the hexagonal c_hex axis of V_2O_3. Furthermore, the structural changes at the MIT cause localization of those states, which mediate in-plane metal-metal bonding via octahedral edges. This band narrowing opens the way to an increased influence of electronic correlations, which are regarded as playing a key role for the MIT of V_2O_3.Comment: 7 pages, 3 figures, more information at http://www.physik.uni-augsburg.de/~eyert

Concentration Dependence of Superconductivity and Order-Disorder Transition in the Hexagonal Rubidium Tungsten Bronze RbxWO3. Interfacial and bulk properties

We revisited the problem of the stability of the superconducting state in RbxWO3 and identified the main causes of the contradictory data previously published. We have shown that the ordering of the Rb vacancies in the nonstoichiometric compounds have a major detrimental effect on the superconducting temperature Tc.The order-disorder transition is first order only near x = 0.25, where it cannot be quenched effectively and Tc is reduced below 1K. We found that the high Tc's which were sometimes deduced from resistivity measurements, and attributed to compounds with .25 < x < .30, are to be ascribed to interfacial superconductivity which generates spectacular non-linear effects. We also clarified the effect of acid etching and set more precisely the low-rubidium-content boundary of the hexagonal phase.This work makes clear that Tc would increase continuously (from 2 K to 5.5 K) as we approach this boundary (x = 0.20), if no ordering would take place - as its is approximately the case in CsxWO3. This behaviour is reminiscent of the tetragonal tungsten bronze NaxWO3 and asks the same question : what mechanism is responsible for this large increase of Tc despite the considerable associated reduction of the electron density of state ? By reviewing the other available data on these bronzes we conclude that the theoretical models which are able to answer this question are probably those where the instability of the lattice plays a major role and, particularly, the model which call upon local structural excitations (LSE), associated with the missing alkali atoms.Comment: To be published in Physical Review

Photocatalytic water disinfection on oxide semiconductors: Part 1 – basic concepts of TiO 2

Water disinfection (removal of microbial agents) using sunlight is an emerging technology, which has the capacity to address the global shortage of drinking water. Therefore, intensive investigations in many laboratories aim to develop photocatalyst for water disinfection. The research is focused on titanium dioxide (TiO2), which is the most promising candidate for high performance photocatalyst able to address the commercial requirements. The present work (Part 1) considers the effect of defect disorder on semiconducting and photocatalytic properties of TiO2 (rutile) in water disinfection using solar energy. It is shown that photocatalytic properties of TiO2 in water are closely related to the light induced reactivity of TiO2 with water leading to the formation of active species, such as OH*, H2O2 and O2- , which have the capacity to oxidise microorganisms. It is also shown that the ability of TiO2 to form the active radicals is closely associated with the presence of point defects in the TiO2 lattice and the related semiconducting properties. Therefore, photocatalytic properties of TiO2 may be modified in a controlled manner by changes in its defect disorder. Consequently, defect chemistry may be used as the framework in the development of TiO2 with controlled properties that are desired for solar water disinfection. The following work (Part 2) considers the structure of bacteria and their reactivity/photoreactivity with TiO2 in aqueous environments. Both Part 1 and 2 bring together the concepts of TiO2 photocatalysis and the concepts of microbiology in order to derive the theoretical models that are needed for the development of high performance photocatalysts for solar water disinfection