Layered cuprates

Layered copper-oxide superconductors exhibit the highest critical transition temperatures of any materials. Yet all of the known double perovskites A′A″B′B″O₆ containing copper have a random or rock salt distribution of the B cations with the exception of the unique layered arrangement found in La₂CuSnO₆. Only the layered arrangement contains the CuO₂^2- planes which are necessary for high-temperature superconductivity. The occurrence of layered or two dimensional structures increases markedly when vacancies are introduced on the oxygen sublattice, as evidenced in Ln₂AE_mCu₂Ti_mO_5+3m (Ln = lanthanide, Y: AE = Ba, Ca: 2≤m≤4). Similarities among oxygen-deficient structures, especially those with two-dimensional solid-state features, are discussed. Combined conductivity and thermopower analysis are presented to elucidate their unique internal chemistry, defect structure, and conduction parameters. In particular, data for La_2-xSr_xCuSnO₆ are presented and related to the crystal chemistry of the copper-oxygen layer. These data are compared with La₂Ba₂Cu₂Sn₂O₁₁ and La₂Ba₂Cu₂Ti₂O₁₁ to illustrate the significance of oxygen vacancies on the properties of the copper oxygen planes. New layered cuprates are discussed including the mixed A-site stoichiometries Ln′Ln″AE_mCu₂Ti_mO_5+3m (Ln = lanthanide, Y: AE = Ba, Ca: 2≤m≤4) which contain the smaller lanthanide (Ln″) ordered between the closely spaced, facing sheets of Cu-O square pyramids.