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

Abstract

The composition and the structure of ceramic EuBa2Cu3O6 + 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 n Ba m Cum + nO 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 n Ba m Cum + nO 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 Tc 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 n Ba m Cum + nO y series with matrices (Ba : Cu) 5 : 8, 3 : 5, 2 : 3, 5 : 7, and 3 : 4 exhibit superconducting properties with Tc = 82–90 K.