Liquidus Diagram of the Ba-Y-Cu-O System in the Vicinity of the Ba₂YCu₃O_6+x Phase Field

This paper describes the melting equilibria in the vicinity of the high T_c phase Ba₂YCu₃O_6+x , including evidence for two Ba-Y-Cu-O immiscible liquids. Melting equilibria have been investigated in purified air using a combination of differential thermal analysis (DTA), thermogravimetric analysis (TGA), powder x-ray diffraction (XRD), MgO wick entrapment of liquid for analysis, scanning electron microscopy (SEM) coupled with energy dispersive x-ray analysis (EDS), and hydrogen reduction for determination of copper oxidation state. For relatively barium-rich compositions, it was necessary to prepare the starting materials under controlled atmosphere conditions using BaO. A liquidus diagram was derived from quantitative data for the melts involved in various melting reactions. In general the 1/2(Y₂O₃) contents of the melts participating in these equilibria were low (mole fraction <4 %). The primary phase field of Ba₂YCu₃O_6+x occurs at a mole fraction of <2.0 % 1/2Y₂O₃ and lies very close along the BaO-CuO _x edge, extending from a mole fraction of ≈43 % CuO to a mole fraction of ≈76 % CuO. It is divided by a liquid miscibility gap and extends on either side about this gap. The topological sequence of melting reactions associated with the liquidus is presented as a function of temperature. Implications for the growth of Ba₂YCu₃O_6+x crystals are discussed.