Subsolidus phase relations in the system (Bi-Sr-Ca-Cu)-0 at 800°C were investigated via powder X-ray diffraction and electron probe microanalysis of solid-state reaction products. Two recently reported (Bi-Sr-Ca)-0 phases were detected in quaternary assemblages and had the approximate cation stoichiometries of 9115 and 825. The only truly quaternary compound detected is the superconducting phase with the cation stoichiometry 2212. Tie lines, tie triangles, and compatibility tetrahedra involving this phase and the 2201 superconducting phase were determined. Both superconducting compounds exhibit small but detectable Sr-to-Ca solubility and are Sr-deficient with respect to the "ideal'' formulas. [

The cation solubility limits of the n = 2 and n = 3 superconducting phases in the Bi2(Sr,Ca l-x)n+lC~nOy system were established along tie lines with compatible phases via electron probe microanalysis on bi-(or poly-) phasic samples prepared at 860°C. Pb additions (15 mol% of the Bi content) were used to facilitate formation of the n = 3 phase. In each case football-shaped volumes in composition space were established as the solubility limits which bordered on the nominal compositions 2212 or 2223 (Bi : Sr : Ca : Cu) with the long axis parallel to the Sr-Ca side of the quaternary (i.e., Sr-to-Ca intersolubility) but also extending toward Bi and Cu. This means that, for the most part, the superconducting phases are alkaline-earth deficient relative to the ideal 2212 and 2223 compositions. The Pb content in the 2223 phase is typically 10% of the Bi content. T, variations could be correlated with variations in Sr or (Sr + Bi) content and the length of the c-axis parameter. [

By a combination of conventional physical property measurements and high-temperature electrical property studies, the solid solution limit, transport parameters, and potential defect regimes of the Bi,,Sr,,(Ca,_.Y,)Cu,O, solid solution were established. A continuous solid solution extends to x = 0.7 or 0.8. The electrical properties indicate that the product of the hole density-of-states and mobility for semiconducting compositions is approximately an order of magnitude smaller than for the other p-type superconducting cuprates. A pronounced drop in hole concentration accompanies the tetragonal-to-orthorhombic transition at x = 0.5, whereafter superconductivity disappears. The electrical properties also indicate that a composition x 2 0.7 is the appropriate "reference" compound for the solid solution series. Upon doping this yttrium-rich insulating composition with calcium, holes are introduced. With increased calcium content (decreased yttrium content) the system exhibits several defect regimes reminiscent of the behavior in the La,-,AE,CuO, (AE = Sr or Ba) system.Oxygen defects (interstitial and vacancies) are believed to play an important role in the defect structure.

Via in situ electrical property measurements (conductivity, Seebeck coefficient) over the temperature range 500-800 °C and oxygen partial pressure range 1 0~4 -l atm, the equilibrium transport properties and stability range of YBa2Cu4O8 were determined. YBa2Cu408 behaves like the intrinsically mixed-valent compound, magnetite (Fe3O4), with small variations in electrical properties with changes in oxygen partial pressure. The decomposition boundary to YBa2Cu306+j/ (or YBa2Cu3.5O7.5±z) and CuO occurs at Iog(po2,atm) = -1.24 x 104/T(K) + 11.01(773 < T(K) ^ 1073).

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