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

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.