1 : 1 Ca 2+ :Cu 2+ A-site Order in a Ferrimagnetic Double Double Perovskite.

Cation ordering in ABX 3 perovskites is important to structural, physical and chemical properties. Here we report discovery of CaCuFeReO 6 with the tetragonal AA'BB'O 6 double double perovskite structure that was previously only reported for A'=Mn compositions. CaCuFeReO 6 occurs in the same phase field as CaCu 3 Fe 2 Re 2 O 12 demonstrating that different A-cation ordered peroskites may be obtained in the same chemical system. CaCuFeReO 6 has ferrimagnetic order of Fe, Re and Cu spins below T C =567 K, in contrast to Mn analogues where the Mn spins order separately at much lower temperatures. The magnetoresistance of CaCuFeReO 6 displays low-field "butterfly" hysteresis with an unusual change from negative to positive values as field increases. Many more AA'BB'O 6 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.