STABILITY OF THE TL-1223 PHASES

The thermodynamic stability of TlBa2Ca2Cu3O9-y (1223) and substitution ally related phases has been studied by performing extended high-tempe rature anneals in a two-zone furnace. This approach allows for indepen dent control of each of the thermodynamic variables: the oxygen and th allous oxide partial pressures [P(O-2) and P(Tl2O)], the sample temper ature, 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). Pa rtially replacing Tl with Pb and Ba with Sr substantially increases th e 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 stoic hiometry, rather than P(Tl2O).