Application of high-pressure techniques: stabilization and oxidation-state control of novel superconductive and related multi-layered copper oxides

Copper oxide superconductors possess multi-layered structures with a layer sequence of -CuO₂ -(Q-CuO₂ )_{n -1} -AO-(MO_1± )_m -AO- or -CuO₂ -B-(O₂ -B)_{s -1} -CuO₂ -AO-(MO_1± )_m -AO- along the elongated caxis. Based on this layer sequence, the known copper oxide structures are categorized as members of the homologous series, M_m A_r Q_{n -1} Cu_n O_{m +r +2n ±} (M-mr (n -1)n ; category A) or M_m A_2k B_s Cu_1+k O_{m +4k +2s ±} (M-m (2k )s (1+k ); category B). Stabilization of such structures especially in the case of high values of the n /sparameter, i.e. the higher members of the homologous series, has been demonstrated to be apparently promoted under high pressures and/or strongly oxidizing conditions. Consequently, techniques for applying both high oxygen gas pressures (10-2000 atm) and ultra-high solid-medium pressures (2-8 GPa) have been advantageously utilized in synthesizing various superconductive copper oxide phases. Especially the ultra-high solid-medium pressure synthesis carried out in the so-called cubic-anvil/belt-type apparatus has proven to be extremely successful in synthesizing novel superconductive phases. In order to achieve high partial pressures of oxygen in the solid-medium environment, `external' oxygen-generating oxides such as KClO<sub>4</sub> , KClO<sub>3</sub>and Ag<sub>2</sub> O<sub>2</sub>are commonly added to the precursor mixtures. It is emphasized that in some cases it is possible to utilize `internal' oxidizing agents alone, i.e. highly oxidized precursors such as BaCuO₂₊ and Ba₂ Cu₃ O₅₊ containing metal constituents common with the desired copper oxide phase only. In the present paper, the potential and applications of high-pressure techniques in synthesizing multi-layered copper oxides and related structures are reviewed and discussed with emphasis on the important `historical' discoveries of novel phases and the present status of controlled production of high-quality samples of such phases.