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We review the synthesis methods and properties of single crystal, powder, and thin film TlBaCaCuO high-temperature superconducting (Tl-HTS) materials. With transition temperatures ≥ 100 K for several compounds, Tl-HTS materials present real opportunities for applications above 77 K. Experiments using (1) single crystals: determined precise structural parameters and identified the complex Tl1+–Tl3+ equilibrium model; (2) powders: studied the complex thermodynamic phase diagram; and (3) epitaxial films: studied fundamental properties such as electron pair symmetry and the effect of controlled extrinsic defects on flux pinning strength, as well as providing the large-area surfaces required for device applications.

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Phase equilibria in the Tl-Ca-Ba-Cu-O system have been studied at sample temperatures that are near, but below, the onset of melting. Independent control of the partial pressures of oxygen P(O2) and thallous oxide P(Tl2O) was achieved through the use of a two-zone furnace. Stable ranges of P(Tl2O), P(O2), and sample composition for individual Tl superconductors, including 2122, 2223, 1122, and 1223 (the latter intergrown with about 10% 2223) have been determined. A qualitative understanding of these stable ranges, based on the relative Tl content and formal oxidation state of the cations within a structure type, has been developed. From this information, equilibrium phase diagrams for Tl superconductors with Ca:Ba:Cu ratios of 1:2:2, 2:2:3, and 3:2:4 are proposed. These diagrams provide process information for the two-zone synthesis of bulk materials and thin and thick films. The superconducting transition temperatures of the individual phases produced by near-equilibrium processing are both reproducible and relatively insensitive to changes in the process conditions.