Phase Diagram Studies in the System Tl₂O₃–BaO–CaO–CuO–Ag

Abstract

oxygen atmosphere. 1223 has been found to be in equilibrium with a liquid phase that is Tl poor. 2223 and 2212 exhibit varying Tl/(Ba ؉ Ca) ratios. The three-phase field 1223 ؉ 2223 ؉ 2212 has been identified. The results of this study emphasize that multiphase samples can be prepared which consist of three superconducting phases, each exhibiting a critical temperature of 100 K or above.

(Cu,Tl)Ba2Ca3Cu4Ox compositions: I. The influence of synthesis time and temperature on the phase formation and evaporation$ndash$condensation mechanism

“…The reaction path, in which Tl-1212 was used as a precursor for the synthesis of the Tl-1223 phase also has been reported by Biondo et al [29]. Phase 1212 can be in equilibrium with 1223 phase (figure 4(c)) and this result is consistent with the data from [12,13], where the authors pay attention to phase formation using 223-precursors, but contrary to phase diagrams from [9][10][11] in which the 234 composition is investigated. In fact, in [9][10][11] for all 212, 223 or 234 precursor compositions, at any p(TlO x ) and p(O 2 ), phase 1212 was not found to be in equilibrium with 1223 phase.…”

“…Indeed our data show that the Tl-based liquid phase is consumed in reaction to form Tl-12(n − 1)n phases. On the other hand, phase diagrams given in [12,13] indicate liquid phases in equilibrium with Tl-superconducting phases. This phase can be identified in our case with the Ba(Ca,Tl)-Cu-O residual liquid phase.…”

“…Another idea is that continuous increase in the concentration of phase 4 (2θ ∼ 29.2 • ) with temperature and time is a reliable reason to believe that this phase is a crystallized liquid phase in equilibrium with 12(n − 1)n superconducting phases. Majewski et al [12,13] talk about a liquid phase that occurs in 1223 Ag-sheated tapes at Tl deficient compositions. The authors could not determine the stoichiometry of this phase because they could not see the amorphous phase by SEM and XRD.…”

“…The only phase diagrams (to the knowledge of the authors), showing the domain of stability for Tl-1234 in the Tl 2 O and O 2 partial pressure representation, at 850 • C and when synthesizing by two-furnace method in mixtures of Tl 2 O 3 and a Ba 2 Ca 3 Cu 4 O xtype precursor, are presented in [9][10][11]. Majewski et al [12,13] proposed and studied phase relations in the quaternary system Tl 2 O 3 -BaO-CaO-CuO, in the presence of Ag, at 890-900 • C, but the emphasis has been made on 1223, 2223, 1212 and 2212 phases.…”