THE RANGE IN THE COMPOSITION OF YBa2Cu3O7−x AS CRYSTALLIZED FROM A MELT
Hou, Chun-Ming 1987-
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The primary phase field of YBa2Cu3O7−x (Y123) is roughly triangular (with curvilinear boundaries). The apices of the triangle are the maximum temperature at which liquid, Y123, and Y2BaCuO5 (Y211) coexist and the ternary invariant points at which liquid, Y123, Y211, and BaCuO2 coexist, and at which liquid, Y123, Y211, and CuO coexist. The eutectic (liquid, Y123, BaCuO2, and CuO) is on the low-temperature side of the triangle. Melting experiments on appropriate compositions in a pure oxygen atmosphere are used to document not only the range in liquid compositions and their temperatures but also the range in the Y123 composition. Charges are drop-quenched from run temperature in order to preserve high-temperature assemblages to the maximum extent possible. Not all liquids quench to crystal-free glasses, and distinguishing equilibrium Y123 from quench Y123 is a challenge. Compositions of Y123 and liquid were determined by wavelength dispersive spectrometry on an electron microprobe. The thermal maximum on L(Y123,Y211) is at 1027 ◦C in pure oxygen. The most Y-rich Y123 crystals form from this liquid and at this temperature and they have Cu:Y slightly above 3. Liquids in equilibrium with Y123 and Y211 descend from that maximum to L(Y123,Y211,CuO) at a temperature of 955 ◦C. As 955 ◦C is approached, Y123 shows a decrease in both Ba and Cu. The Ba:Y of Y123 decreases to 1.88 and Cu:Y decreases to 2.92. The liquid line of descent loses Y211 at that temperature and moves along L(Y123,CuO) to the eutectic at 910 ◦C. The Cu:Y of Y123 along this coprecipitation curve keeps increasing and Y123 has its maximum Cu content at 910 ◦C, which has Cu:Y up to 3.29. To the more Ba-rich side of the eutectic, liquids coexist with Y123 and BaCuO2 and temperature increases to L(Y123,BaCuO2). The Cu:Y of Y123 along that line has a composition that remains constant except near the invariant point L(Y123,Y211,BaCuO2). The Cu:Y decreases from 3.00 at 970 ◦C to 2.96 at 990 ◦C. These are greater variations in composition of Y123 than previously determined and raise issues of how superconducting properties of Y123 vary with composition of that phase.