The Effect of Synthesis Conditions on the Phase Composition and Structure of EuBa2Cu3O6 + δ Samples

Abstract

The composition and the structure of ceramic EuBa₂Cu₃O_{6 + δ} (Eu-123) oxide samples annealed in steps with varying processing conditions (in air or oxygen and argon atmosphere at a temperature of 940–960°С for 1–70 h with or without homogenization) were studied by the X-ray phase and chemical analysis, electron diffraction pattern analysis, elemental analysis, and high-resolution transmission electron microscopy. Regardless of the processing conditions, Eu-123 nanostructured oxide with a tetragonal or orthorhombic structure and domains 1–20 nm in size was obtained as a result of annealing. Nanostructuring of the samples, which was revealed by high-resolution electron microscopy, is attributed to their chemical nature: the presence of identical structural elements in members of the homologous Eu_nBa_mCu_{m + n}O_y series of oxides allows them to intergrow coherently and create an illusion of a single crystal. Just like any other member of the Eu_nBa_mCu_{m + n}O_y series, oxide Eu-123 is disproportionate depending on the annealing conditions to form other members of this series located on either side of the dominant oxide. Temperature T_c of the superconducting transition of each member of the series depends on the average oxidation state of copper \(\overline {Cu} \). At \(\overline {Cu} \) < 2, all members of the series have a tetragonal structure and do not exhibit superconducting properties. At \(\overline {Cu} \) = 2.28, five members of the Eu_nBa_mCu_{m + n}O_y series with matrices (Ba : Cu) 5 : 8, 3 : 5, 2 : 3, 5 : 7, and 3 : 4 exhibit
superconducting properties with T_c = 82–90 K.