Influence of oxygen annealing on phase composition of the samples synthesized in the primary crystallization field of EuBa2Cu3O6 + δ

L.A. Klinkova, V. I. Nikolaichik, N. V. Barkovskii, V. K Fedotov, A. F. Shevchun

ABSTRACT:

Eu 0.01–0.03BaxCu1xOy (x = 0.03–0.41) samples of different cation composition were baked in the primary crystallization field of “EuBa2Cu3O6 + δ”(“123”) oxide of the BaO–CuOx (59–97 mol % CuO) matrix system at 915–1110°C (0.5–1 h) in air and additionally in oxygen atmosphere (P(O2) = 101 kPa, 450°C, 5 h). The phase composition of the samples was studied by X-ray powder diffraction, chemical analysis, electron diffraction (ED), and elemental analysis in transmission electron microscope. The samples from the discrete crystallization filed of BamCum + nOy oxides and pseudoeutectic region (70–80 mol % CuO), which were baked in air, consist of four phases: “123”, BaCuO2 (“011”), BaCu2O2 (“012”),and CuO. After oxygen annealing, the “012” phase disappears in the samples, the Ba2Cu3O5 + δ (“023”) phase is formed. Samples from the CuO–Cu2O crystallization field, which were baked in air, consist of five phases: “123”, “012”, Eu2CuO4 (“201”), CuO, and Cu2O. Phase composition of these samples remains unchanged. Oxygen annealing causes a structural transformation tetra-ortho of the “123” phase. The ortho and tetra phase “123”are characterized by a considerable deviation of cation composition from the stoichiometric ratio “123”. A deviation degree matches the interval of (Ba : Cu) 1 : 2–5 : 6 matrix oxides, which composition depends on baling temperature and CuO fraction in the starting sample. Hence, the tetragonal “123” phase formed in the Eu0.01–0.03Bax Cu1 xOy (x = 0.03–0.41) samples in the primary crystallization field of the “123” oxide baked in air is inhomogeneous in cation composition. This cation inhomogeneity is retained in particles of the “123” ortho phase with some changing of composition range due to disproportionation into copper rich and copper poor oxides of the EunBamCum + nOy series. The range of cation non stoichiometry of the “123” oxide is minimal for the sample, which composition corresponds to a boundary (940°С) of the subsolidus (pseudoeutectic) region at the boundary with melt of the BaO–CuOx system at 75 mol % CuO. No supercell reflections in addition to the basic reflections are seen in ED patterns of the orthorhombic phase Eu-123.

KEYWORDS:

 oxide EuBa2Cu3O6 + δ (Eu_123), cation non stoichiometry, primary crystallization field, oxygen annealing