Similar to the polar covalent bonding which occurs as smooth transition from pure covalent bonding to ionic bonding, a transition from metallic to ionic bonding is described as polar metallic bonding. Whereas the polar covalent bonding is well-known and widely represented in a plethora of chemical species covering every fine nuance of mixture of both bindung types, there are not that many examples for polar metallic bonding. There are two large groups of chemical structures in between salts and metals: Zintl phases (consisting of two metals which differ strongly in their respective electronegativities and hence showing mostly ionic character) and intermetallics of much higher metallic character.
It is our main effort to synthesise and characterise new substances in which polar metallic bonding can be found and which lie somwhere between the two main fields.
There are two principal access routes:
|The first route yields solid phases with spatially separated regions of either ionic od metallic bonding. The model systems we have developed here are the Alkali Metal Suboxometallates. They show metallic conductivity, however, they also have the intrinsic properties of the incorporated oxometallate anions (e. g. paramagnetism). The suboxometallates of cesium and rubidium are very sensitive materials, therefore all preparation has to be carried out under argon either in a glovebox or under Schlenk conditions.|
|The second route leads to Zintl phases whenever the electronegativities of the educts differ too much. If they are more finely balanced, solids form with cations from the electropositivie metal and delocalized negative charge over the noble metal sublattice. We are dealing with Hg-rich Amalgams of electropositive metals to investigate this class of intermetallic compounds in more detail. As most Hg-rich amalgams are air-sensitive and also show thermal lability they cannot be produced with standard solid state chemistry methods. We develop new synthetic methods, such as preparative isothermal electrolysis in aprotic solvents with reactive Hg cathodes.|
|Besides these more basic research topics we also focus on development of novel materials for lithium ion accumulators and supercapacitors. Following a new generic concept on the basis of classical structure-property relationship considerations, we extract from the large pool of known structures those candidates having potential for future applications and test them (and also newly developed materials) for use in batteries and capacitors. Our aim is to combine the typical properties of accumulators (long storage time) and capacitors (fast charge / discharge times) in one device.|