Redox state analysis for understanding the high-Tc superconductivity in highly unstable Ba2Ca2Cu3O8-δ phase with Tc≈ 124 K and its derivative phase with Tc ≈ 78 K

By means of wet-chemical analysis the redox state was established for a highly unstable Ba2Ca2Cu3O8-delta superconductive phase (T-c approximate to 124 K) obtained by high-pressure synthesis under oxidizing conditions and also for its derivative phase with T-c approximate to 78 K formed immediately when exposing the as-synthesized sample to humid air or H2O/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 CuO2-Ca-CuO2-Ca-CuO2 layer blocks. On the other hand, the latter phase possesses a structure of H-m((Ba))2((Ca))23 (m approximate to 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 O((Ba))2((Ca))23 phase to be reduced. The possible origin of the unusually high overall oxidation state of the O((Ba))2((Ca))23 phase is discussed on the basis of peroxide-type mixed-valent oxygen in its (BaO)(2) blocking block. (C) 2002 Elsevier Science B.V. All rights reserved.