Structural characterization, magnetic behavior and high-resolution EELS study of new perovskites Sr2Ru2−xCoxO6−δ (0.5?x?1.5)

New oxides of general formula Sr₂Ru_2−xCo_xO_6−δ (0.5?x?1.5) have been synthesized as polycrystalline materials and characterized structurally by X-ray diffraction. For 0.5?x<0.67 the orthorhombic, Pnma, perovskite structure of the end member, SrRuO₃, is found. At x=0.67 a phase separation into an Ru-rich Pnma phase and a Co-rich I2/c phase occurs. The I2/c form is also found for x=1.0 but another orthorhombic phase, Imma, obtains for x=1.33 and 1.5. Reductive weight losses indicate negligible oxygen non-stoichiometry, i.e., δ∼0, for all compositions even those rich in Co. High-resolution electron energy loss spectroscopy (EELS) indicates that cobalt is high-spin Co³⁺ or high-spin Co⁴⁺ for all x. Appropriate combinations of Ru⁴⁺, Ru⁵⁺, HS Co³⁺ and HS Co⁴⁺ are proposed for each x which are consistent with the observed Ru(Co)-O distances. Significant amounts of Co⁴⁺ must be present for large x values to explain the short observed distances. Broad maxima in the d.c. susceptibilities are found between 78 and 97 K with increasing x, along with zero-field-cooled (ZFC) and field-cooled (FC) divergences suggesting glassy magnetic freezing. A feature near 155 K for all samples indicates a residual amount of ferromagnetic SrRuO₃ not detected by X-ray diffraction.