Superstructure in Sr 2YCu 2FeO 6+ δ

Abstract

Sr 2YCu 2FeO 6+ δ exhibits superconductivity around 50 K, when it is properly annealed in N 2 atmosphere and subsequently in O 2 atmosphere under ambient and high pressure. From electron diffraction study streaks were observed on the N 2-annealed samples, which suggests existing some kind of atomic ordering. We have optimized the annealing condition for the study of the atomic ordering, and found a new phase with superstructure in the N 2-annealed sample. This indicates that the atomic ordering is strongly correlated with the N 2-annealing. To understand the mechanism of the atomic ordering in this system, we have performed in-situ neutron powder diffraction measurement under the optimized annealing process. The in-situ study revealed that the N 2-annealing at 600 °C promoted the atomic ordering of Cu and Fe along the c-axis and that the ordering did not change at all during the following annealing process. We have also discovered beginning of the transformation occurred from tetragonal structure to orthorhombic superstructure at 750 °C in N 2 and the superstructure remained until temperature returned to room temperature. These results indicate that the transformation to the superstructure is irreversible. In addition, the following O 2-annealing changed the crystal system to tetragonal with keeping the atomic ordering of Cu and Fe. This annealing supplies oxygen and changes the coordination around Fe, which might be related with the emergence of superconductivity.