Phase relations in pseudobinary Ba 2 M Cu 3 O 6.5 + x ( M = Y,Gd)
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I. INTRODUCTION The Ba2YCu3O6 5 + x obtained by sintering Ba, Y, Cu oxide mixtures at about 900 °C exhibits a well-ordered Ba2YCu3O6 structural framework but it can vary in oxygen content of additional O4 atoms of orthorhombic Cu-O-Cu chains.1 The large thermal parameter1 and the present experiments suggest a high mobility of O4 atoms. Therefore the quaternary compound can be treated as a nonstoichiometric pseudobinary oxide MeOx similar to binary oxides, e.g., FeOx.2 Here Me = Ba2YCu3O6 5 was chosen as the reference for metal Me with formally monovalent, divalent, and trivalent Cu at x = — 1.5, 0, and 1.5, respectively. The oxygen content x of FeO x decreases with increasing temperature T at constant oxygen partial pressure p(O2) or with decreasing p(O2) at constant T? The variation of oxygen content is similar to hydrogen uptake of metals, which was studied in great detail.3 Usually loading and unloading of metals in several cycles improves the homogeneity of the sample. In the present investigation the pseudobinary Ba 2 YCu 3 O 65 + x framework was synthesized from carefully ground Ba, Y, Cu oxide mixtures by annealing for 24 h in 1 bar O 2 at 900 °C and loading while temperature decreased at 5° min" 1 . Afterward the powder was ground again, pressed to 4 mm pellets at 1 ton cm" 2 , unloaded by heating to 900 °C, and subjected to repeated loading at identical conditions after 24 h annealing at 900 °C to get a homogeneous sample with sufficient channels and pores between ~2-20 fim grains for rapid gas-phase reaction (Fig. 1). The samples were used as reference for further treatment at different oxygen partial pressure. The absolute oxygen content x = 0.300 was determined with a 0.1 fig balance at room temperature after 24 h reduction at 900 °C to Ba4Y2O7, Cu metal mixture in hydrogen gas.4 Slightly decreased oxygen partial pressures were obtained by mixtures of pure (5N) N 2 and O2 gases at a constant flow rate of 6 ml s" 1 . The oxygen loss was determined by the 0.1 //g balance after the cooling process. The oxygen content of the starting material with x = 0.300, which was unloaded and loaded again at p(O2) = 1 bar, increased slightly to x = 0.301. The temperature dependence of the oxygen 762
J. Mater. Res. 2 (6), Nov/Dec 1987
http://journals.cambridge.org
content at constant p(O2) was determined by thermogravimetric analysis (TGA). In addition, differential thermal analysis (DTA) and strain gauge experiments were performed similar to experiments in metal hydrogen systems3 in order to determine the phase transitions and phase boundaries. The resistivity of small pieces of pellets was measured by the four-terminal technique.
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FIG. 1. Scanning electron micrograph of Ba2YCu3O65 + x with x = - 0.248 (above) and x = 0.300 (below).
0003-6951 /87/060762-03S01.75
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© 1987 Materials Research Society
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Hauck, Bickmann, and Zucht: Pseudobinary Ba2yWCu3065+,
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