Order, disorder and superconductivity in FeSr2YCu2O6+δ

FeSr2YCu2O6+δ (Fe-1212) samples sintered and subsequently annealed under various atmospheres were studied by electron microscopy. In the as-sintered state, the sample showed a basic tripled perovskite tetragonal structure with the unit cell of apxapxcp. The subsequent N2-annealed sample showed a superstructure with the orthorhombic unit cell of aoxboxco∼2cpx2apx2ap with the sequence of -Cu5-Fe4-Cu5-Cu5-Fe4'-Cu5- where the subscripts 4 and 5 represent tetrahedral and pyramidal coordinations, respectively. Here Fe4 and Fe4' represent different configurations of [FeO4] tetrahedron at the Cu-chain site. The alternative order of two-type zigzag [FeO4] tetrahedron chains was observed as that in CoSr2YCu2O7 (Co-1212). Final high-pressure O2 heat-treatment which successfully brought about superconductivity with a transition temperature, Tc of about 50K introduced additional oxygen at the O(1) site, and the basic tripled perovskite tetragonal structure was observed