High oxygen pressure synthesis of new copper oxide superconductors

The unique coordination of the copper ions in +1, +2, and +3 oxidation states, which are stable in a range of oxygen partial pressure. 10^–6<P(O₂)< 10³ atm, makes possible the formation of a wide variety of distinct structures. By controlling the oxygen pressure during the synthesis and annealing, the distribution of metal and oxygen ions can be modified on an atomic scale to optimize the structural and electronic properties. We present several examples of compounds for which the critical structural elements for superconductivity, i.e., the perfectly ordered CuO₂ planes, have been obtained and doped with holes by means of high oxygen pressure,P(O₂)>1 atm. These materials, La₂CuO₄₊ , La_2–x Ca _x CuO₄, and YSr_2–x La _x Cu₃O₇₊ , offer a unique opportunity to study the relationship between superconductivity and structural, magnetic, and chemical properties.