BaO-Nd2O3-CuOx subsolidus equilibria under carbonate-free conditions at pO(2)=100 Pa and at pO(2)=21 kPa

摘要

Subsolidus phase equilibria of the BaO-Nd2O3-CuO, system at PO2 = 100Pa (0.1% O-2 volume fraction, 810degreesC) and at PO2 = 21 kPa (21% O-2 volume fraction, 930degreesC have been investigated by applying controlled-atmosphere methods to minimize the presence of carbonate and CO2 and H2O contamination. Under carbonate-free conditions, the BaO Nd2O3-CuOx phase diagrams at PO2 = 100 Pa and at PO2 = 21 kPa are similar to one another except for differences in the extent of the solid solutions. Apart from the limiting binary phases, the ternary system consists of three solid solutions and one stoichiometric ternary compound. The first solid solution is the high T-c series, Ba2-xNd1+xCu3O6+z (0.3 greater than or equal to x greater than or equal to 0 at pO(2) = 100 Pa; 0.95 greater than or equal to x greater than or equal to 0 at pO(2) = 21 kPa). At pO(2) = 21 kPa, a compositionally dependent phase change was detected, from tetragonal (0.7 > x greater than or equal to 0) to orthorhombic (0.95 greater than or equal to x greater than or equal to 0.7). The second solid solution series, the 'brown-phase' Ba1+xNd2-xCuOz, has a narrow homogeneity region (0.10 > x greater than or equal to 0 at PO2 = 100 Pa; 0.15 > x greater than or equal to 0 at PO2 = 21 kPa). In the high BaO part of the phase diagram, a third solid solution (Ba2-xNdx)CuO3+z (x = 0 to approximate to 0.3 at PO2 = 100 Pa; x = 0 - 0.45 at O-2 = 21 kPa) was confirmed, as well as a nominally stoichiometric phase, Ba4Nd2Cu2Oz. The latter phase is an insulator, with a structure comprised of unusual CuO5 linear chains. A significant difference in tie line distribution involving the Ba2-xNdl+xCu3O6+z superconductor was found under carbonate-free conditions relative to literature studies completed in air. Instead of the BaCuO2 + x-Ba-2 + xNd(4-x),Cu2Oz tie line normally encountered in air, a Ba2-x,Nd1+xCu3O6+ (Ba,Nd)(2)CuO3+x, tie line was established. This tie line substantially expands the field of stability of the Ba2-xNd1+xCu3O6+z superconductor phase into the BaO-rich region of the phase diagram. Implications for the processing of materials based on the Ba2-xNd1+x xCuO(6+ z) superconductor are discussed. (C) 2003 Elsevier Science (USA). All rights reserved. [References: 84]