LIQUIDUS DIAGRAM OF THE BA-Y-CU-O SYSTEM IN THE VICINITY OF THE BA2YCU3O6+X PHASE FIELD

This paper describes the melting equilibria in the vicinity of the hig h T-c phase Ba2YCu3O6+x, including evidence for two Ba-Y-Cu-O immiscib le liquids. Melting equilibria have been investigated in purified air using a combination of differential thermal analysis (DTA), thermograv imetric analysis (TGA), powder x-ray diffraction (XRD), MgO wick entra pment of liquid for analysis, scanning electron microscopy (SEM) coupl ed with energy dispersive x-ray analysis (EDS), and hydrogen reduction for determination of copper oxidation state. For relatively barium-ri ch compositions, it was necessary to prepare the starting materials un der controlled atmosphere conditions using BaO. A liquidus diagram was derived from quantitative data for the melts involved in various melt ing reactions. In general the 1/2(Y2O3) contents of the melts particip ating in these equilibria were low (mole fraction <4 %). The primary p hase field of Ba2YCu3O6+x occurs at a mole fraction of <2.0 % 1/2Y(2)O (3) and lies very close along the BaO-CuOx edge, extending from a mole fraction of approximate to 43 % CuO to a mole fraction of approximate to 76 % CuO. It is divided by a liquid miscibility gap and extends on either side about this gap. The topological sequence of melting react ions associated with the liquidus is presented as a function of temper ature. Implications for the growth of Ba2YCu3O6+x crystals are discuss ed.