Systematic Study on Crystal Structure and Properties of FeSr₂LnCu₂O_7+δ (Ln = La, Nd, Sm, Eu, Gd, Dy, Ho, Er, and Yb)

The crystal structures of a series of new compounds with the composition FeSr 2 LnCu 2 O 7+ δ (Ln = La, Nd, Sm, Eu, Gd, Dy, Ho, Er, and Yb) that were successfully synthesized by solid-state reaction are investigated. The Rietveld refinement of the X-ray diffraction data shows that the samples exhibit a transition from a tetragonal system with space group P 4/ mmm for light rare earths Ln = La, Nd, Sm, Eu, and Gd to an orthorhombic system with space group Pmmm for heavy rare earths Ln = Dy, Ho, Er, and Yb. The oxygen contents of the samples are chemically determined by the iodometric titration. The room-temperature Mössbauer spectroscopy of 57 Fe in FeSr 2 GdCu 2 O 7+ δ is used to study the site distribution of Fe between the 1a and 2g sites in the as-synthesized in the air samples and O 2 -annealed samples, respectively. In the as-synthesized sample, there are about 67% of Fe ions residing in the 1a site, whereas in the O 2 -annealed sample, there are about 61% of Fe ions residing on the 1a site. Magnetization measurements indicate that χ changes with temperature in a Curie-type manner. Resistance measurements indicate that these samples have a semiconductor-like behavior and their band gap is related to their structural type and the radius of rare earth ions.