Stability of the Tl-1223 phases
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Stability of the Tl-1223 phases T. L. Aselage, E. L. Venturini, and J. A. Voigt Sandia National Laboratories, Albuquerque, New Mexico 87185-1421
D. J. Miller Argonne National Laboratory, Argonne, Illinois 60439 (Received 5 October 1995; accepted 26 February 1996)
The thermodynamic stability of TlBa2 Ca2 Cu3 O92y (1223) and substitutionally related phases has been studied by performing extended high-temperature anneals in a two-zone furnace. This approach allows for independent control of each of the thermodynamic variables: the oxygen and thallous oxide partial pressures [P(O2 ) and P(Tl2 O)], the sample temperature, and the sample composition. P(Tl2 O) determines which of several superconducting phases form in the unsubstituted Tl–Ba –Ca–Cu–O system. TlBa2 Ca2 Cu3 O92y is stable only within a narrow window of P(Tl2 O). Partially replacing Tl with Pb and Ba with Sr substantially increases the stability of the 1223 phase. The composition (Tlx Pb0.5 ) (Sr1.6 Ba0.4 )Ca2 Cu3 O92y yields only the 1223 phase under two-zone conditions when P(Tl2 O) exceeds a lower bound. The stability of Pb- and Sr-substituted 1223 relative to the 1212 phase is related to the substitutional stoichiometry, rather than P(Tl2 O).
I. INTRODUCTION
Thallium cuprate superconductors with the 1223 structure (TlBa2 Ca2 Cu3 O92y and substitutionally related phases) have attracted considerable interest for use as bulk conductors.1–4 This interest arises from a combination of factors. First and foremost, magnetic flux lines are more effectively pinned in superconductors having a single Tl–O layer such as 1223 than in superconductors with double Tl–O layers.5 Second, the superconducting transition temperatures of 1223 phases are substantially higher than those of the TlBa2 CaCu2 O72y (1212) phases. Finally, synthesis procedures for 1223 are more advanced than for TlBa2 Ca3 Cu4 O112y (1234). Additional cations may be substituted within TlBa2 Ca2 Cu3 O92y to enhance the ease of formation of the 1223 phase and/or its superconducting properties. In particular, substitution of Sr for Ba beyond 50% was reported to promote the development of the single Tl–O layer phase, 1223, over the double Tl–O layer phase 2223.6 Nonetheless, formation of TlSr2 Ca2 Cu3 O92y is difficult under ordinary synthesis conditions.7,8 Subramanian and co-workers first showed that pure-Sr 1212 and 1223 phases could be synthesized by substituting Pb for one-half of the Tl atoms in the structure.8 Additionally, Doi and co-workers have reported an increase in flux pinning in samples with composition (Tl0.5 Pb0.5 ) (Sr1.6 Ba0.4 )Ca2 Cu3 O92y . As a result, substantial effort has been devoted to producing wires, tapes, and thick films of this latter composition. In these efforts, competing formation of 1212 versus 1223 appears to be sensitively related to process conditions. Phases with J. Mater. Res., Vol. 11, No. 7, Jul 1996
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double Tl–O layers have not been reported in bulk materials
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