Stoichiometry and property relations in the High-Tâ†c ceramic superconductors
High-Tc ceramic superconductors have been investigated in the Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O systems in order to understand the relationships between stoichiometry and superconducting properties. The structural and superconducting properties of YBa2Cu3Ox 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. Anomalies in the composition dependence of the c lattice parameter occur and may be associated with creation of vacancies in the 0(4) sites that separate the copper-oxygen planes and chains. Pelleted samples of YBa2Cu3Ox which were quenched into mercury after equilibration at high temperatures, showed no evidence of im90 K and im60 K plateaux in plots of Tc against x. Also, their Tc values were 15-25 K lower than those reported for samples prepared by low temperature reduction. For a given x, the Tc values were insensitive to quench temperature, as shown by preparing samples with the same x at different temperatures and oxygen partial pressures. Tc was also unaffected by subsequent annealing in Ar at 300-350oC. The absence of Tc plateaux and the reduced Tc values in quenched samples may be associated with the occurrence of oxygen vacancies in the (0 0 z) positions which effectively act as mobile-hole traps. 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 Bi2Sr2-xCa1+yCu2O8+, where O< y< 0.3 and O< x&60 0.3, was obtained. Oxygen nonstoichiometry in YBa2Cu3Ox and Bi2Sr2CaCu2O8+x has been investigated using a number of thermogravimetric and titrimetric techniques. For Bi2Sr2CaCu2O8+x samples prepared in air at 850oC 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, led to oxygen evolution on dissolution; the other, 0.05 O, retained its high oxidation state on dissolution. Thermogravimetric measurements also showed that the oxygen excess consists of these two components. One part, 0.06 O per formula unit, could be removed by heating in air at temperatures just below melting and easily reduced by hydrogen. The remainder, 0.13, was more difficult to remove, persisting after most of the bismuth and copper in the oxide had been reduced to the metals by hydrogen. The two components of oxygen excess are thought to reflect the nature of the band which contains 0.19 holes per formula unit with 2/3 Cu3+ and 1/3022- character. In YBa2Cu3Ox, Tc varies by about 10 K for each 0.1 change in oxygen content whereas in Bi2Sr2CaCu2O8+x, the Tc sensitivity to oxygen content is about an order of magnitude greater. A maximum in Tc, 87 K, was observed for an oxygen content, x, of 0.178 + 0.016 in Bi2Sr2CaCu2O8+x. This stoichiometry was achieved by, for instance, annealing in air at im820oC or in nitrogen at im400oC. For higher and lower x, Tc decreases. No structural changes associated with these changes in oxygen content could be detected. Variations in Tc for Bi2Sr2CaCu2O8+x appear to be associated with changes in carrier concentration, whereas in YBa2Cu3Ox, Tc depends on both carrier concentration and structural changes, which are closely linked.