A-site ordered quadruple perovskite oxides AA₃'B₄O₁₂

The A-site ordered perovskite oxides with chemical formula AA₃'B₄O₁₂ display many intriguing physical properties due to the introduction of transition metals at both A' and B sites. Here, research on the recently discovered intermetallic charge transfer occurring between A'-site Cu and B-site Fe ions in LaCu₃Fe₄O₁₂ and its analogues is reviewed, along with work on the magnetoelectric multiferroicity observed in LaMn₃Cr₄O₁₂ with cubic perovskite structure. The Cu-Fe intermetallic charge transfer (LaCuLaCu₃³⁺Fe₄³⁺O₁₂→LaCu₃²⁺Fe₄^3.75+O₁₂) leads to a first-order isostructural phase transition accompanied by drastic variations in magnetism and electrical transport properties. The LaMn₃Cr₄O₁₂ is a novel spindriven multiferroic system with strong magnetoelectric coupling effects. The compound is the first example of cubic perovskite multiferroics to be found. It opens up a new arena for studying unexpected multiferroic mechanisms.