Could you explain the significance of your article to the non-specialist? 
Transition metal oxides with pyrochlore structure have attracted considerable interest owing to their remarkable physical properties. These phenomena are often directly related to the structurally determined geometrical magnetic frustration and are frequently not completely understood. In this contribution we present a new compound, Hg₂Ru₂O₇, which shows unusual behaviour: a sharp phase transition found in heat capacity, electrical conductivity, and magnetism. Hg₂Ru₂O₇ completes a family of mixed oxides which contains the chemical homologous elements Cd/Hg and Ru/Os, all crystallising with the pyrochlore structure. Although structurally and chemically closely related, these compounds show significant differences in their properties. With the characterisation of the last missing member of this group of compounds we can provide further insight into the nature of physical phenomena as well as a challenge for testing theoretical models. 

What has motivated you to conduct this work? 
Ruthenium oxides show a variety of interesting physical properties like ferro- or antiferromagnetism, metallic or insulating behaviour or even superconductivity, and are still a rich field to search for interesting new materials. With the solid state synthesis under elevated oxygen pressures we were able to obtain many oxides with elements in high, sometimes maximal oxidation states, e.g. the method applied for synthesis lets us expect to obtain Ru⁵⁺/Ru⁶⁺ from the experiments. The employed synthesis technique is also well suited for stabilising Hg²⁺ at higher temperatures. Realising that there are no known compounds in the Hg-Ru-O system it was obvious where to start the experiments. 

Where do you see this work developing in the future? 
The related thallium pyrochlore, Tl₂Ru₂O₇, shows defects concerning the thallium and the oxygen content, depending on the synthesis method, and slight changes of the composition induce strong changes of physical properties. With respect to this the new Ru⁵⁺ compound will have to be investigated in more detail. 

Are there any particular challenges facing future research in this area? 
Features involving magnetism and electronic conduction in transition metal oxides are often intimately connected and generally difficult to understand, even more for the geometrically frustrated pyrochlores. An explanation for the striking behaviour of Tl₂Ru₂O₇ was recently proposed. Similar to this a theoretical description also for the Ru⁵⁺ pyrochlores has to be found. Mercury as well as cadmium or thallium compounds are of course barely suitable for any practical application. So efforts to replace the toxic elements with harmless ones while conserving their properties are another challenge. 

Wilhelm Klein obtained his Ph.D. at the University of Bonn in 2000. His dissertation, which he performed under the guidance of Prof. Martin Jansen, dealt with the low temperature synthesis and crystal structures of ionic ozonides. He then moved to the Max-Planck-Institute for Solid State Research in Stuttgart to work on the synthesis and characterisation of new ternary silver oxides. He is currently working at the University of Hamburg.