STOICHIOMETRY AND KINETICS OF FORMATION OF BI2SR2CACU2O-DELTA SOLID-SOLUTIONS

Single-phase samples of the '90 K' BiSCCO superconductor can be prepared, by careful attention to the experimental conditions, for the compositions: (a) stoichiometric Bi2Sr2CaCU2O(delta); (b) Bi2+xSr2-xCaCu2O(delta), O < x less-than-or-equal-to 0.2; (c) Bi2Sr2-yCa1+yCU2O(delta), 0 < y less-than-or-equal-to 0.5; (d) Bi2+xSr2-x-yCa1+yCu2O(delta), all of which retain the same (Bi + Sr + Ca):Cu ratio of 5:2 as the ideal '2212' stoichiometry. The ranges of x and y values for the solid solutions are given in the form of a pseudoternary phase diagram. Lattice parameters were indexed on a tetragonal cell; a was independent of composition; c decreased with both x and y. Formation rates of the 90 K phase vary greatly with composition and generally increase with both x and y. The stoichiometric composition requires typically 5 days at 860-degrees-C for complete reaction, whereas the composition x = 0, y = 0.5 requires only 2 days at 820-degrees-C.