A 70 K superconductor: The oxycarbonate Tl 0.5Pb 0.5Sr 4Cu 2(CO 3)O 7

Abstract

A new thallium copper oxycarbonate Tl 0.5Pb 0.5Sr 4Cu 2(CO 3)O 7 has been isolated. This new phase crystallizes in the tetragonal system with a=3.824 Å, c=16.516 Å. Its study by X-ray diffraction and high-resolution electron microscopy allows a structural model to be proposed which consists of the intergrowth of double rock-salt layers [(TlO) (SrO)] ∞, with single perovskite layers [SrCuO 3] ∞ and layers of carbonate groups according to the general formula [(SrO)(Tl 0.5Pb 0.5O)] RS[(SrCuO 3) 2] P [SrCO 3]. In fact this structure is a derivative of a hypothetical structure “Tl 0.5Pb 0.5Sr 4Cu 3O 10” closely related to the 1223 structure Tl 0.5Pb 0.5Sr 2Ca 2Cu 3O 9. The susceptibility measurements with an AC susceptometer demonstrate that this phase is a superconductor characterized by a sharp transition with a T c of 60 K when synthesized. Moreover, the comparison with other oxycarbonates shows that this phase exhibits a much higher superconducting volume fraction which reaches 70% at 4.2 K. Annealings performed in an argon-hydrogen flow (10%H 2) at 290°C allows the T c to be increased up to 70 K, keeping the same superconducting volume fraction; this optimization of T c is explained in terms of hole carrier density governed by an oxygen non-stoichiometry according to the formula Tl 0.5Pb 0.5Sr 4Cu 2(CO 3)O 7-δ. The possible synthesis of a series of superconductors with the generic formula (Tl,Pb) n Sr 4 n Cu 3 n-1 (CO 3)O 10 n-3 is discussed.