Crystal structure and Mössbauer spectroscopy of Y₂SrFeCuO_6.5, a double layer perovskite intergrowth phase

The crystal structure of Y₂SrFeCuO_6.5 was determined from single-crystal X-ray and neutron powder diffraction studies. M_r = 488.81, orthorhombic, Ibam, a = 5.4036(8)[5.4149(1)] Å, b = 10.702(1)[10.7244(1)] Å, c = 20.250(2)[20.2799(2)] Å; values in square brackets are neutron data. V = 1171.0(4), Z = 8, D_x = 5.544 g cm^-3, λ = 0.71069 Å, 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. Mössbauer 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. The isomer shift, 0.26, and quadrupole splitting, 0.56 mm sec^-1, are consistent with Fe³⁺ in 5-fold coordination and H_int values also indicate classic high spin Fe³⁺. The average YO bond length is 2.331(6) Å and Sr is in a dodecahedral environment in which, however, two oxygen atoms at the corners of the cube are missing. The average SrO bond length is 2.793(10) Å. The structure is derived from the Ruddlesden-Popper phase Sr_n+1Ti_nO_3n+1 with n = 2.