Instability of Thallium‐Containing Superconductor Phases in Isothermal Equilibrium with Thallium Oxide

Abstract

Knowledge of the equilibrium conditions under which the superconducting phases in the Tl‐Ca‐Ba‐Cu‐O system are stable would enable the controlled preparation of these materials. Isothermal equilibrium between samples in this quaternary system and Tl2O vapor from a condensed thallium oxide source has been studied. None of the superconductor phases coexisted in equilibrium with thallium oxide under the conditions employed. Instead, two previously unknown ternary thallium‐calcium‐barium oxides and one previously unknown thallium‐calcium oxide were observed. The equilibrium phase assemblage at temperatures between 825° and 925°C consisted of the most thallium‐rich ternary oxide plus CuO, although small amounts of the other thallium‐rich phases may be required to balance stoichiometry. The vapor pressure of Tl2O established by pure, condensed thallium oxide clearly exceeds the stability limit for the superconductor phases. If the thallium‐containing superconductors are stable phases at high temperature, they will exist over some range of lower Tl2O partial pressure.