Abstract
Tl 2 Ba 2 CaCu 2 O 8 has been synthesized as a pure phase by reaction in sealed quartz tube. The structural model of this oxide has been established from X-ray diffractograms and high resolution electron microscopy. This tetragonal structure (a=3.860 Å, c=29.388 Å, space group I4/mmm) can be described as formed of [ Ba 2Tl2Cu2 O 8]∞ layers interleaved with calcium ions. These layers are themselves described as the intergrowth of sodium chloride type [TIO] ∞ layers of corner-sharing pyramids [ BaCuO 3]∞ leading to the formulation [( BaCuO 3)2 (TIO) 2]∞. Such a perfect structure Tl2 Ba 2 CaCu 2O8, which contains only Cu(II) cannot be at the origin of the superconductivity at 110K. The superconducting properties may in fact be explained by the observation by HREM of extended defects of local composition “ TlBa 2 CaCu 2O7” for which a structural model is proposed. In a more general way the creation of thallium deficiency, according to the formulation Tl2−x Ba 2 CaCu 2 O 8 could be at the origin of superconductivity.
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