1 : 1 Ca :Cu A-site Order in a Ferrimagnetic Double Double Perovskite.

Cation ordering in ABX perovskites is important to structural, physical and chemical properties. Here we report discovery of CaCuFeReO with the tetragonal AA'BB'O double double perovskite structure that was previously only reported for A'=Mn compositions. CaCuFeReO occurs in the same phase field as CaCu Fe Re O demonstrating that different A-cation ordered peroskites may be obtained in the same chemical system. CaCuFeReO has ferrimagnetic order of Fe, Re and Cu spins below T =567 K, in contrast to Mn analogues where the Mn spins order separately at much lower temperatures. The magnetoresistance of CaCuFeReO displays low-field "butterfly" hysteresis with an unusual change from negative to positive values as field increases. Many more AA'BB'O double double perovskites may be accessible for A'=Cu and other divalent transition metals at high pressure, so the presently known phases likely represent only the "tip of the iceberg" for this family.