Structural characterization of the Sr2Co0.5Nb(Ta)(0.5)O-4 and Sr3CoNb(Ta)O-7 compounds

The Sr2Co0.5Nb(Ta)(0.5)O-4 and Sr3CoNb(Ta)O-7 compounds, both with Ruddlesd en-Popper structures, have been synthesized by the ceramic method at 1150 d egrees C under atmospheric pressure. The crystallographic structure of the compounds was assigned to the tetragonal system with space group I4/mmm by X-ray diffraction(XRD) Rietveld refinement. The reduced lattice volume and lattice parameters increased as the Ta with 5d substitutes for the Nb with 4d in the compounds. The Co/Nb(Ta)O bond length has been determined by X-ra y absorption spectroscopic(EXAFS/XANES) analysis and the XRD refinement. Th e CoO6 octahedra were tetragonally distorted by elongation of Co-O bond alo ng the c-axis. The magnetic measurement shows the compounds Sr2Co0.5Nb(Ta)( 0.5)O-4 and Sr3CoNb(Ta)O-7 have paramagnetic properties and the Co ions wit h intermediate spin states between high and low spins in D-4h symmetry. All the compounds showed semiconducting behavior whose electrical conductivity increased with temperature up to 1000 K. The electrical conductivity incre ased and the activation energy for the conduction decreased as the number o f perovskite layers increased in the compounds with chemical formula A(n+1) B(n)O(3n+1).