EXAFS investigations of titanium doped copper tantalum oxide

The recent discovery of apparent giant dielectric response in CaCu3Ti4O12 has sparked a high interest in this perovskite-type material [1, 2]. A special characteristic of its structure is a collective rotation of BO6-octahedtra resulting in a quadratic-planar geometry of of A-sites. These can be occupied by Jahn-Teller active copper ions (otherwise typical B-cation). Depending on the valence of the B-cations the remaining of A-sites are occupied by alkaline metal, alkaline earth metal or lanthanide ions or remain vacant. Although this type of material was known for a long time and was subject of various investigations [3-6] its physics is not fully understood. Our investigations concentrated on B-site doped copper tantalum oxide which possesses a structure closely related to CaCu3Ti4O12. The replacement of Ti4+ by pentavalent ions like tantalum allowed us to synthesise A-site deficient polycrystalline phases, with Ti: Ta ratios ranging from Cu3Ti2Ta2O12 to Cu2Ta4O12. To gain single phase products it was necessary to use a small copper excess during preparation. Rietveld refinement calculations of X-ray powder diffraction data showed a linear shift of the lattice parameter a as