High-Pressure Synthesis of a Novel Quadruple Perovskite Oxide AgCu₃Cr₄O₁₂

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A novel complex transition metal oxide AgCu₃Cr₄O₁₂ has been obtained using high-pressure and high-temperature conditions of 12 GPa and 1223 K. The crystal structure is refined to be a cubic AA′₃B₄O₁₂-type quadruple perovskite structure based on the Rietveld refinement of the synchrotron X-ray powder diffraction data. The density-functional theory calculation obtains a metallic band structure. The valence state is estimated to be Ag^∼1.3+Cu^∼2.2+₃Cr⁴⁺₄O₁₂ by bond valence sum and X-ray absorption spectroscopy analyses. The valence state on ACu₃Cr₄O₁₂ series (A = Ag, Ca, La, Ce) sequentially transforms from Ag^∼1.3+Cu^∼2.2+₃Cr⁴⁺₄O₁₂, Ca²⁺Cu²⁺₃Cr⁴⁺₄O₁₂, La³⁺Cu^(2+δ)+₃Cr^{(3.75−0.75δ)+}₄O₁₂, to Ce⁴⁺Cu²⁺₃Cr^3.5+₄O₁₂, where the electrons are doped into the A′-site (Cu^∼2.2+ → Cu²⁺), followed by predominant doping into the B-site (Cr⁴⁺ → Cr^{(3.75−0.75δ)+} → Cr^3.5+). Their electron doping sequence is distinguished from those reported in other quadruple perovskite oxides, proposing characteristic features of the ACu₃Cr₄O₁₂ family.