Abstract
Creation of mixed valency in the CuO 2 sheets of the copper oxide superconductors is a necessary condition to suppress long-range antiferromagnetic order and promote superconductivity. The oxidation/reduction of the (CuO 2) 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 CuO 2 sheets in Tl 2Ba 2Ca n−1 Cu n O 2 n+4 is primarily due to an overlap of the Tl-6 s band with the σ∗ x 2−y 2 and/or π∗ bands of the CuO 2 sheets. Neither Tl vacancies nor excess oxygen in the Tl 2O 2 blocks is required to render the thallium cuprates superconductive.
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