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-(MO1+/-delta')(m)-AO- or -CuO2-B-(O-2-B)(s-1)-CuO2-AO-(MO1+/-delta')(m)-AO- along the elongated c axis. 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+/-delta) (M-mr(n - 1)n; category A) or M(m)A(2k)B(s)Cu(1+k)O(m+4k+2s+/-delta) (M-m(2k)s(1 + k); category B). Stabilization of such structures especially in the case of high values of the n/s parameter, 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 KClO4, KClO3 and Ag,Oz 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+delta and Ba2Cu3O5+delta 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.