ORDERED SUBSTITUTION OF IRON FOR COPPER IN THE TETRAGONAL PEROVSKITE LA6.4SR1.6CU8O20

Two series of iron-substituted cuprates with the tetragonal La6.4Sr1.6Cu8O20 oxygen deficient perovskite structure have been isolated: La6.4Sr1.6CU8-xFexO20 (0 less than or equal to x less than or equal to 1.20) and La6.4-xSr1.6+x Cu8-xFexO20 (0 less than or equal to x less than or equal to 2). The XRD and Mossbauer studies show that they correspond to a preferential substitution of Fe(III) for copper in the octahedral sites of the structure. In fact the ideal structure can be described as 90 degrees-oriented ''123'' copper ribbons interconnected with [FeO3](infinity) rows of FeO6 octahedra. The transport properties of these phases show a transition from a metallic to a semiconducting state. The difference in behavior between the La6.4Sr1.6Cu8-xFexO20 series and the La6.4-xSr1.6+x Cu8-xFexO20 series is interpreted on the basis of hole carrier density introduced by Cu(III) and of the trapping of carriers by Fe(III) species. The possibility of anisotropic transport properties with a metallic conductivity along c is discussed. (C) 1995 Academic Press, Inc.