Cation intermixing and ordering phenomenon in MO layer of MSr2YCu2Oz   (M1212) compounds with M = Fe, Co, Al, and Ga: A neutron powder diffraction   study
  Various MSr2YCu2Oz (M1212 with M = Fe, Co, Al and Ga) compounds have been synthesized through a solid state reaction route. All these compounds are derived from the basic CuBa2YCu2O7 structure. Rietveld structural refinement of the room-temperature neutron diffraction data for Fe-1212 reveals that nearly half of Fe remains at the M site, while the other half goes to the Cu site in the CuO2 plane. For the M = Co, Al and Ga compounds, the basic unit cell gets doubled due to two distinct M sites in the M-O layer and the space group Ima2, without any cation intermixing is found more appropriate. For the M = Fe sample, magnetic ordering of Fe spins is seen from magnetization data below 70 K. In the case of the M = Co compound, a shallow down-turn in magnetic susceptibility is seen below 150 K, indicating low dimensional magnetic characteristics of the CoO layer. The compounds with M = Al and Ga show paramagnetic behaviour in their magnetization data down to 5 K. The oxygen content, established for each compound based on oxygen occupancies, refined from the neutron diffraction data comes close to 7.0 for each. 
V.P.S. Awana, E. Takayama-Muromachi, S.K. Malik, W.B. Yelon, M.   Karppinen, and H. Yamauchi