Structural and superconducting properties of oxygen-deficient NdBa2Cu3O7−δ

Abstract

Neutron diffraction was used to determine the structural properties of oxygen deficient ${\mathrm{NdBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ for 0.09<δ<0.74. It was found that superconductivity disappears at the orthorhombic-to-tetragonal phase transition which occurs at ${\mathrm{\delta }}_{\mathit{O}\mathit{T}}$=0.45. Structural parameters vary smoothly with δ but exhibit a change in slope at the orthorhombic-to-tetragonal transition. The structural properties exhibit the same features found in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ where ${\mathrm{\delta }}_{\mathit{O}\mathit{T}}$=0.65. It is shown that the repulsion energy of oxygen atoms in the O(1) and O(5) sites in ${\mathrm{NdBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ is smaller than in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$. This lower repulsion energy stabilizes the orthorhombic phase at lower values of δ. It is argued that the disappearance of superconductivity at the orthorhombic-to-tetragonal transition is an inherent property of the R${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ (R denotes rare earth) system.