Rationalisation of oxygen non-stoichiometry determination in cuprates

Chemical titrations are widely used to determine the oxygen content of cuprate superconductors; however, discrepancies between different methods and disagreements with thermal methods are often encountered. It seems that differences in titrimetric determinations principally reflect the mechanism of dissolution. The most important factor is whether the excess carriers are lost to evolved oxygen gas or whether they go into solution as oxidised species, such as peroxide., O-2(2.). Some titrimetric methods are unable to either detect evolved oxygen or to successfully compete for holes with oxygen evolution on dissolution. Such methods fail to detect the oxygen excess in compounds such as YBa2Cu3O7-x, although they do detect part of the oxygen excess in phases such as Bi2Sr2CaCu2O8+delta. It is proposed that the origins of this speciation of oxygen excess reflect structural factors. On dissolution excess carriers recombine with the 'parent' interstitial oxygens, hence the resulting redox behaviour depends on the nature of the interstitial oxygen site.