Crystallization of high temperature superconductors from nonstoichiometric melts

Abstract

Crystallization of high-temperature superconductors YBa 2Cu 3O 6.5+ x and (La,Sr) 2CuO 4 - y from homogeneous nonstoisiometric melts has been studied. The phase diagram of the CuO-BaO system was constructed. The real phase diagram of the CuO-BaO system in air having the complete liquidus corresponds to the superposition of two phase diagrams — those of the CuO-BaO and the CuO 0.5-BaO systems. An important part in the process of the formation of superconducting crystals is played by Cu + cations. The formation of peroxide oxygen pairs O 2 2- bound to Ba is briefly considered. These pairs provide the formation of O 2 2- → O-O pairs whose transport could be implemented by charge-density waves associated with a varying copper valence. Platelet-like YBa 2Cu 3O 6.5 + x crystals 8 × 8 × 0.1 mm 3 in size and prismatic La 2 - x Sr x CuO 4 - y crystals 6 × 4 × 10 mm 3 in size were synthesi zed by the method of spontaneous crystallization from homogenized nonstoichiometric melts enriched with copper oxides. La 2 - x Sr x CuO 4 - y single cry stals 8×8×12 mm 3 in size were grown on a seed from homogenized nonstoichiometric melts by the Czochralski technique. The measurements on annealed YBa 2Cu 3O 6.5+ x single crystals yielded: T c = 93 K, Δ T ≈ 0.2 K.