Redox state analysis for understanding the high- Tc superconductivity in highly unstable Ba 2Ca 2Cu 3O 8− δ phase with Tc≈124 K and its derivative phase with Tc≈78 K

Abstract

By means of wet-chemical analysis the redox state was established for a highly unstable Ba 2Ca 2Cu 3O 8− δ superconductive phase ( T c≈124 K) obtained by high-pressure synthesis under oxidizing conditions and also for its derivative phase with T c≈78 K formed immediately when exposing the as-synthesized sample to humid air or H 2O/N 2 gas. The former phase is of the 0 (Ba)2 (Ca)23 structure having a relatively thin blocking block consisting of only two BaO layers in between two superconductive CuO 2–Ca–CuO 2–Ca–CuO 2 layer blocks. On the other hand, the latter phase possesses a structure of H- m (Ba)2 (Ca)23 ( m ≈ 5 ) with an expanded blocking block due to accommodation of water in the structure. Wet-chemical analysis clearly manifested much higher oxidation state for the 0 (Ba)2 (Ca)23 phase than for its water-containing derivative. This suggests that the driving force for the phase transformation is the strong tendency of the 0 (Ba)2 (Ca)23 phase to be reduced. The possible origin of the unusually high overall oxidation state of the 0 (Ba)2 (Ca)23 phase is discussed on the basis of peroxide-type mixed-valent oxygen in its (BaO) 2 blocking block.