Redox state analysis for understanding the high-T_c superconductivity in highly unstable Ba₂Ca₂Cu₃O_8-δ phase with T_c ≈ 124 K and its derivative phase with T_c ≈ 78 K

By means of wet-chemical analysis the redox state was established for a highly unstable Ba₂Ca₂Cu₃O_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 H₂O/N₂ 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 ≈ 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.