Structural characterization of the Sr₂Co_0.5Nb(Ta)_0.5O₄ and Sr₃CoNb(Ta)O₇ compounds

The Sr₂Co_0.5Nb(Ta)_0.5O₄ and Sr₃CoNb(Ta)O₇ compounds, both with Ruddlesden-Popper structures, have been synthesized by the ceramic method at 1150 °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-ray absorption spectroscopic(EXAFS/XANES) analysis and the XRD refinement. The CoO₆ octahedra were tetragonally distoned by elongation of Co-O bond along the c-axis. The magnetic measurement shows the compounds Sr₂Co_0.5Nb(Ta)_0.5O₄ and Sr₃CoNb(Ta)O₇ have paramagnetic properties and the Co ions with 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 increased and the activation energy for the conduction decreased as the number of perovskite layers increased in the compounds with chemical formula A(n+1)B(n)O(3n+1).