ABSTRACT
A thorough understanding of the thermal stability of thallium-based superconductors is required to optimize processing conditions to obtain high-Jc superconductors in a reproducible fashion. This chapter provides a brief review of several important thermodynamic features of the Thallium–Barium–Calcium–Copper Oxide system, including the formation of volatile thallium-bearing oxides, the stability of thallium-bearing compounds, and cation and anion nonstoichiometry in thallium-bearing superconductors. A number of authors have applied refinement methods to x-ray and neutron diffraction data in order to determine the cation site occupancies in single-crystal or polycrystalline specimens. Chemical analyses have also indicated a significant variation in the compositions of thallium-bearing superconductors from the ideal compositions. Despite the general trends discussed above for cation nonstoichiometry, significant differences in the thallium and calcium contents for any given type of thallium-bearing compound have been reported by different authors.
