Interactions of Ba2YCu3O6+y with the Gd3NbO7 buffer layer in coated conductors

A systematic study of the chemical interaction of Ba2YCu3O6+y and Gd3NbO7 was conducted under two processing conditions: purified air (21% po2), and 100 Pa po2 (0.1% po2). Phases present along the pseudo-binary join Ba2YCu3O6z and Gd3NbO7 were found to be in two five-phase volumes within the BaO-1/2 Y2O3-1/2 Gd2O3-Nb2O5-CuOy system. Three common phases that are present in all samples are (Y,Gd)2Cu2O5, Ba(Y,Gd)2CuO5 and Cu2O or CuO (depending on the processing conditions). The assemblies of phases can be categorized in three regions, with Ba2YCu3O6+y: Gd3NbO7 ratios of (I)<5.5:4.5; (II)=5.5:4.5; and (III)>5.5:4.5. The lowest melting temperature of the system was determined to be ∼938 deg. C in air, and 850 deg. C at 100 Pa po2. Structure determinations of two selected phases, Ba2(GdxY1-x)NbO6 (Fm3-barm, No. 225), and (GdxY3-x)NbO7 (C2221, No. 20 and Ccmm, No. 63), were completed using the X-ray Rietveld refinement technique. Reference X-ray powder diffraction patterns for selected phases of Ba2(GdxY1-x)NbO6 (x=0.2, 0.4, 0.6, and 0.8) and (GdxY3-x)NbO7 (x=0.6, 1.2, 1.8, 2.4 and 3) have been prepared for inclusion in the Powder Diffraction File (PDF). - Graphical abstract: Crystal structure for (GdxY3-x)NbO7 showing the partial layered feature. The alternate stacking of distorted NbO6 octahedra and (Gd,Y)O7 polyhedra are illustrated. The (Gd,Y)O8 polyhedra are omitted for clarity.