Stability of the Tl-1223 phases

Abstract

The thermodynamic stability of TlBa2Ca2Cu3O9–y (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(Tl2O)], the sample temperature, and the sample composition. P(Tl2O) determines which of several superconducting phases form in the unsubstituted Tl–Ba–Ca–Cu–O system. TlBa2Ca2Cu3O9–y is stable only within a narrow window of P(Tl2O). Partially replacing Tl with Pb and Ba with Sr substantially increases the stability of the 1223 phase. The composition (TlxPb0.5) (Sr1.6Ba0.4)Ca2Cu3O9–y yields only the 1223 phase under two-zone conditions when P(Tl2O) 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(Tl2O).