Stoichiometry and Property Relations in the High Critical Transition Temperature Ceramic Superconductors

Available from UMI in association with The British Library. The structural and superconducting properties of YBa_2Cu_3O _{rm x} have been investigated for a wide range of stoichiometries (6 < x < 7). Results showed that large discrepancies occur between the temperature of the orthorhombic -tetragonal transition determined at high temperatures and reported in the literature from that found in samples quenched into mercury from different temperatures. This is because for intermediate x values (6.41 < x < 6.5), samples appear to undergo a non-quenchable transition from tetragonal to orthorhombic during cooling which is not associated with oxygen uptake. Pelleted samples of YBa_2Cu _3O_{rm x} which were quenched into mercury after equilibration at high temperatures, showed no evidence of ^{~}90 K and ^{~}60 K plateaux in plots of T_{rm c} against x. For a given x, the T_{ rm c} values were insensitive to quench temperature, as shown by preparing samples with the same x at different temperatures and oxygen partial pressures. The x-ray powder patterns of Bi2212 and Bi2223 phases have been identified and fully indexed using a pseudo -tetragonal subcell with a = 5.408, c = 30.83 A and an incommensurate supercell with reciprocal lattice vector, q*, given by q* = 0.211b* - c* and using an orthorhombic subcell a = 5.410, b = 5.420 and c = 37.29 A, with q* = 0.210b* - 0.78c*, respectively. An area of 2212 based solid solution formation given by the formula Bi_2Sr_{rm 2-x}Ca_{rm 1+y} Cu_2O_{8+ delta}, where 0 < y < 0.3 and 0 < x < 0.3, was obtained. Oxygen nonstoichiometry in YBa_2 Cu_3O_{ rm x} and Bi_2Sr _2CaCu_2O _{rm 8+x} has been investigated using a number of thermogravimetric and titrimetric techniques. For Bi_2Sr _2CaCu_2O _{rm 8+x} samples prepared in air at 850^circC and air quenched, the total oxygen content was found to be 8.19 O per formula unit. Titrimetric measurements showed that there were two components to the oxygen excess. One, 0.14 O, lead to oxygen evolution on dissolution; the other, 0.05 O, retained its high oxidation state on dissolution. Variations in T_{rm c} for Bi_2Sr _2CaCu_2O _{rm 8+x} appear to be associated with changes in carrier concentration, whereas in YBa _2Cu_3O_ {rm x}, T_{ rm c} depends on both carrier concentration and structural changes, which are closely linked. (Abstract shortened by UMI.).