Determination of oxygen contents and cation valences in the superconducting bismuth and thallium cuprates

The oxygen contents and valences of cations in various superconducting cuprates such as Bi₂Sr₂(Ca, Y)Cu₂O_y, Tl₂Ba₂Ca_n–1Cu_nO_y, etc. which contain two kinds of multi-valence metals have been determined by novel wet chemical methods. The wet chemical methods are based on the differences in oxidation power of each cation and at the last step an electrochemical method is employed for measuring the amount of reducing agent, i.e., CuCl. For superconducting Bi-based cuprates, a modified constant-current coulometry has been developed. For the superconducting Tl-based cuprates a constant-potential electrolysis method which makes it easy to control the electrolysis potential has been applied. The results obtained for Bi₂Sr₂(Ca, Y)Cu₂O_y have shown that both Bi- and Cu-valences decrease with increasing Y, which is consistent with the results of earlier works. The results obtained for the double Tl-O layer superconductors show that the Tl valence is about +3 in all the thallium cuprates and different from the value obtained from XPS measurements of earlier works. The reason for this discrepancy is discussed.