A structural transition of Fe-doped superconducting cuprates

A series of LaBa2Cu3-xFexOy (LBCO) samples with 0.0 less than or equal to x less than or equal to 0.75 have been synthesized. As the iron content increases, the ferric ion prefers to occupy Cu(2) sites instead of Cu(1), and T-c of LBCO decreases more rapidly than that of YBCO. This means that the action of Cu(2) on superconductivity is greater than that of Cu(1). Crystal structures of the series show an anomalous structural phase transition depending on Fe contents. As the Fe content increases, the structure changes from orthorhombic to tetragonal, and then the orthorhombic phase is regenerated from the tetragonal phase. By least squares fitting with the X-ray diffraction data, the structural transitions can be determined from the changes of the cell parameters a and b. The relation of the lattice parameters a, b, and c as a function of both the Fe content and the oxygen content y supports the fact that the excess oxygens may occupy the (001/2) and (01/20) sites of the crystal cell.