Metal Rich Transition Metal Germanides in the System Co-Ge-Ta-Zr

From the structural point of view, compounds of the early transition metals (the elements of groups 4 through 6 of the periodic table) and the iron group metals with main group elements adjacent to the Zintl line are a very interesting group of materials. A multitude of metal-rich ternary compounds have been prepared and characterized in systems of the type M-M` and M 1 -M 2 -X (M=Zr, Hf, Nb, Ta; M`=Fe,Co,Ni; X=main group element) within the last decade. These investigations resulted in the discovery of many new compounds with unusual structural features (like e.g. the mixed occupation of lattice sites with transition metal and main group element) and this led also in a better understanding of the chemical bonding in metal systems. An extension of the experimental investigations to quaternary systems of the type M 1 -M 2 -M`-X is expected to yield an further improvement of bonding concepts and a better understanding of the stabilization effects in this interesting class of materials. The aim of the current project is the high temperature preparation of new metal-rich compounds in the system Co- Ge-Ta-Zr and their characterization by means of single crystal X-ray diffraction. The structures of new compounds will be discussed in the context of bonding concepts and special stabilization effects like the acid-base concept of Lewis or the differential fractional site occupancy. Furthermore it may be helpful to apply extended Hückel band structure calculations. Order-disorder and displacive transformations that may occur at lower temperatures will be investigated by means of high temperature X-ray diffraction and will be discussed in the context of the Landau theory of symmetry breaking transitions.