Chemical methods to identify the origin of oxidation in the thallium cuprate superconductors

Creation of mixed valency in the CuO₂ sheets of the copper oxide superconductors is a necessary condition to suppress long-range antiferromagnetic order and promote superconductivity. The oxidation/reduction of the (CuO₂)^2- sheets is generally achieved by suitable ionic substitutions and/or oxygen intercalation/disintercalation. The mechanism for such an oxidation in the thallium cuprate superconductors has, however, not yet been established. We have adopted a simple wet chemical method to obtain the Tl content and a modified iodometric procedure to determine the total oxygen content. Our wet chemical analyses combined with thermogravimetric analysis demonstrate for the first time that oxidation of the CuO₂ sheets in Tl₂Ba₂Ca_n-1Cu_nO_2n+4 is primarily due to an overlap of the Tl-6s band with the σ*_x²-y² and/or π* bands of the CuO₂ sheets. Neither Tl vacancies nor excess oxygen in the Tl₂O₂ blocks is required to render the thallium cuprates superconductive.