A-site ordered perovskiteCaCu₃Cu₂Ir₂O_12-δ with square-planar and octahedral coordinated Cu ions

A novel CaCu₃Cu₂Ir₂O_12-δ polycrystalline sample was synthesized at 8 GPa and 1373 K. Rietveld structural analysis shows that this compound crystallizes in an AA'₃B₄O₁₂-type A-site ordered perovskite structure with space group Im-3. X-ray absorption spectra reveal a +2-charge state for both the square-planar and octahedral coordinated Cu ions, and the valence state of Ir is found to be about +5. Although the A-site Ca and the A' -site Cu²⁺ are 1:3 ordered at fixed atomic positions, the distribution of B-site Cu²⁺ and Ir⁵⁺ is disorderly. As a result, no long-range magnetic ordering is observed at temperatures down to 2 K. Electrical transport and heat capacity measurements demonstrate itinerant electronic behavior. The crystal structure is stable with pressure up to 35.7 GPa at room temperature.