Crystal structure and Moessbauer spectroscopy of Y2SrFeCuO6.5, a double layer perovskite intergrowth phase

The crystal structure of Y2SrFeCuO6.5 was determined from single-crystal X-ray and neutron powder diffraction studies. Mr = 488.81, orthorhombic, Ibam, a = 5.4036(8)[5.4149(1)] angstrom, b = 10.702(1)[10.7244(1)] angstrom, c = 20.250(2)[20.2799(2)] angstrom; values in square brackets are neutron data. V = 1171.0(4), Z = 8, Dx = 5.544 g cm-3, λ = 0.71069 angstrom, μ = 345.1 cm-1, R = 0.048 for 567 observed reflections. The Fe/Cu atoms occupy randomly the approximate center of oxygen pyramids. The pyramids share the apical oxygen and articulate laterally by corner sharing of oxygen to form a double pyramidal layer perpendicular to c. The pyramidal slabs are separated by double layers of Y that are in 7-fold coordination to oxygen, forming a defect fluorite unit. Moessbauer spectra indicate a unique iron environment and magnetic ordering at about 265 K. The paramagnetic phase coexists with the magnetic phase over an approximate temperature range 300-263 K, characteristic of magnetic ordering in 2-D magnetic structures