Synthesis of B 2 YCu 4 O 8 by anionic oxidation-reduction
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Synthesis of Ba2YCu4O8 by anionic oxidation-reduction K. Kourtakis, M. Robbins, P. K. Gallagher, and T. Tiefel AT&T Bell Laboratories, 600 Mountain Avenue, Murray Hill, New Jersey 07974-2070 (Received 9 May 1989; accepted 7 August 1989) The superconducting phase Ba2YCu408 has been synthesized (as a bulk powder) in flowing oxygen. This was achieved by using the anionic oxidation/reduction method (or the SCD method), in which an aqueous solution containing oxidizing (NO^ salts) and reducing components (e.g., acetate salts) is spray dried into an intimate mixture. At 300 °C, an internal exothermic oxidation/reduction reaction completely converts the spray-dried powder into the corresponding mixed oxides in one reaction step. Because the resultant mixed oxide powder is homogeneous on a fine scale, the final oxide phase can subsequently be produced at lower temperatures and shorter reaction times. Using this method, —93-94% phase pure Ba 2YCu408 (as determined by x-ray diffraction) has been synthesized in flowing oxygen. Pellets of Ba2YCu4Og fabricated from the mixed oxides were prepared and partially sintered by firing at 800 °C in flowing oxygen for 50 h. Resistivity and magnetic (AC) susceptibility measurements show onset Tc's of 82—84 and 72-73 K, respectively.
The superconducting phase Ba2YCu4O8 (124) has been of interest because it contains double C u - 0 chains along the "b" axis, as opposed to single Cu-O chains present in Ba2YCu307 (123). Initially observed in thin films,' 2 it was first prepared as a bulk phase under high oxygen pressure.3 Recently, Cava et a/.4 reported bulk synthesis at one atmosphere oxygen pressure of the 124 phase as a majority phase and —10% secondary phases (as determined by x-ray diffraction). Magnetic (DC) measurements on powders had shown Tc onsets of 77 K. Their procedure included the use of nitrate and/or nitrate/oxide starting materials, slow heating schedules, intermediate grindings, and the addition of either Na2CO3 or K2CO3 before firing. These carbonates, which are traditionally used as fluxes, were thought to act as catalysts because the reaction temperature is below their melting point. Our matter of interest was to attempt the synthesis of Ba2YCu408 powder (at one atmosphere oxygen pressure) using chemical methods, in order to eliminate the need to add secondary reagents such as fluxes. In this note we report the bulk synthesis of Ba2YCu4O8 powder in flowing oxygen by anionic oxidation/reduction. This procedure, which has been previously described5 for the synthesis of Ba2YCu3O7, involves spray drying an aqueous solution containing an oxidizing and reducing component into an intimate mixture. At 850 °C3 in one atmosphere oxygen. Tc onsets are in reasonably good agreement with those reported in thin film studies (81 K). hi summary, using the anionic oxidation-reduction chemical method, we have been able to synthesize Ba2YCu408 powder in flowing oxygen without the addition of fluxes, the use of specialized heating cycles, or high oxygen pressures (these procedures were previ
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