Charge Transfer and Structural Anomaly in Stoichiometric Layered Perovskite Sr2Co0.5Ir0.5O4

A layered Sr2Co0.5Ir0.5O4 (S.G. I4/mmm) perovskite of the K2NiF4 structure type was synthesized as a single-phase sample. Neutron powder diffraction measurements revealed full site occupancy of oxygen atoms. The Co3+/Ir5+ valence states were evaluated by the combination of X-ray absorption and X-ray photoemission spectroscopy as well as by the magnetization measurements, hence confirming the difference to the parent compounds Sr2CoO4 and Sr2IrO4 with similar structures. The Co-K edge EXAFS studies indicated a large average Co-O distances of 1.967 A suggesting a high spin state of Co3+ ion in Sr2Co0.5Ir0.5O4. No long-range magnetic ordering down to 4 K was detected by neutron powder diffraction, probably hindered by the random distribution of Co and Ir cations in the Sr2Co0.5Ir0.5O4 structure. High-pressure synchrotron powder diffraction studies showed stability of the Sr2Co0.5Ir0.5O4 structure at least up to 27 GPa at room temperature. A maximum in the c/a ratio at 10 GPa together with an anomaly in the pressure dependence of the unit cell volume could be a sign of a change in the Co ions spin state under pressure.