Phase coexistence and critical temperatures of the (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O x phase under partial pressures of oxygen
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Phase coexistence and critical temperatures of the (Bi, Pb)2 Sr2 Ca2 Cu3 Ox phase under partial pressures of oxygen between 10–3 and 0.21 bar with and without additions of silver M. D¨aumling,a) R. Maad, A. Jeremie, and R. Fl¨ukiger D´epartement de Physique de la Mati`ere Condens´ee, Universit´e de Gen`eve, 24, quai Ernest-Ansermet, CH-1211 Gen`eve 4, Switzerland (Received 29 July 1995; accepted 4 February 1997)
We have investigated the stability of the (Bi, Pb)2 Sr2 Ca2 Cu3 Ox phase for the stoichiometry (Bi : Pb : Sr : Ca : Cu 1.72 : 0.34 : 1.83 : 1.97 : 3.13), subjecting it to temperatures between 700 and 850 ±C under various oxygen partial pressures. A narrow region was found in which Bi, Pb(2223) was the only superconducting phase. This region follows closely the thermal decomposition line. X-ray pure Bi, Pb(2223) will partially decompose if treated outside of the stability region. For a given oxygen partial pressure, the Bi, Pb(2223) phase tends to coexist with the 2201 phase for temperatures above, and the 2212 phase for temperatures below this region. At even lower temperatures an additional lead-rich phase appears. Critical temperatures Tc vary little with treatment and range between 108.5 K and 110.8 K. If 10% silver is added to the starting powder, the phase coexistence regions shift. Silver does not seem to have a significant effect on the absolute values of the critical temperature. The Bi, Pb(2223) thermal decomposition temperature for a given oxygen pressure is lowered by at least 10 K by the presence of Ag.
I. INTRODUCTION
Since the discovery of the Bi2223 phase, it was found that the phase relationships and formation conditions are extremely complex. The stability limits of the already formed Bi2223 phase in a temperatureoxygen pressure phase diagram (Van’t Hoff diagram) were determined1–3 quickly. The work shows that the decomposition line of the Bi2223 phase is coincidental to the CuO –Cu2 O line when the Bi2223 phase is decomposing in a solid state reaction. As soon as a liquid appears (at higher temperatures), the two phase boundaries separate. However, as this phase cannot be prepared in a pure state other phases are always present during the measurements. Later studies4,5 focused on the phase formation and phase formation kinetics of, in particular, the lead-doped version of the phase. Very recently the influence of lead on this decomposition line was reported.6 According to these data the leadcontaining phase is stable down to lower oxygen partial pressure than the pure phase, while the lines coincide for temperatures above about 775 ±C and oxygen pressures above about 10–3 bar. In the low temperature-high oxygen pressure corner of the phase diagram (which always has to be transversed on cooldown when reacting in air, for example), other work7,8 on Pb-doped Bi2223 report the formation of precipitates and thus a partial a)
Currently at Forschungszentrum Karlsruhe, Institut fuer Technische Physik, Postfach 3640, D-76021 Karlsruhe, Germany. J.
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