Discrimination of two crystal forms of a DyBa2Cu3O7−δ superconductor: tetragonal and orthorhombic forms having very close lengths of the axis

Abstract

The oxygen content in DyBa₂Cu₃O_7−δ (0 ≤ δ ≤ 1) greatly affects superconductivity. When the oxygen content is high, the crystal takes an orthorhombic form and exhibits superconductivity, while it takes a tetragonal form, which is not superconductive, when the oxygen content is low. It is difficult to discriminate between these two forms using powder X-ray diffraction (XRD), especially when the oxygen content is around the transition region between orthorhombic and tetragonal forms; around the transition region, the orthorhombic a and b axes are close to each other, and the tetragonal a axis is close to the orthorhombic a axis. Such cases may occur not only in the present compound but also in other compounds. In this study, DyBa₂Cu₃O_7−δ samples were prepared near the transition conditions. The powder sample was suspended in a liquid medium and subjected to a magnetic field with or without sample rotation, and in situ XRD measurements were performed to obtain two different XRD patterns. The analysis of these patterns allowed us to identify a mixture of two crystal forms. The mixing ratio depended on the preparation conditions.