CA-SUBSTITUTION AND LA-SUBSTITUTION IN YBA2CU3O7-DELTA, Y2BA4CU7O15-DELTA AND YBA2CU4O8

We report the preparation and characterisation of the homologous series of high temperature superconductors, Y2Ba4Cu(6+n)O14+n-delta with n = 0 (123), n = 1 (247) and n = 2 (124) as a function of Ca and La substitution through measurement of T(c), lattice parameters, selected Rietveld refinements and thermogravimetric analysis. Solubility limits of Ca and La are investigated and found to be widely different for each member. Data is presented showing that Ca substitutes on both the Y- and Ba- sites and as the Ca concentration increases the relative fraction on the Y site decreases. Systematic trends indicate that for n = 0 123 is slightly overdoped, 247 is slightly underdoped and 124 is well underdoped relative to T(c(max)) such that 0.06 La substitution, 0.04 Ca substitution and 0.15 Ca substitution is required to maximise T(c) in 123, 247 and 124, respectively. Differences in the orthorhombicity between the members of the series as a function of substitution are interpreted in terms of differences in hole doping and in oxygen stoichoimetry for each member. For instance, the observed rapid decrease in the orthorhombicity in 124 at a high La content is interpreted as evidence that oxygen loads into the interchain site. This destabilises the ribbons leading to the formation of tetragonal 123.