New quadruple perovskites Ln2Ba2TixSn2–xCu2O11, (0⩽x⩽2): controlling copper–oxygen distances in potential superconducting parent phases

Abstract

The compounds Nd₂Ba₂Cu₂SnTiO₁₁, Sm₂Ba₂Cu₂Sn_0.5Ti_1.5O₁₁ and Gd₂Ba₂Cu₂Ti₂O₁₁ have been synthesized by reaction of the component oxides at 1025 °C and their structures refined from powder X-ray diffraction data. The materials crystallise with a quadrupled perovskite unit cell, similar to that of La₂Ba₂Cu₂Sn₂O₁₁ containing alternating double layers of copper and titanium/tin. Decreasing lanthanide size and increasing titanium content is associated with a marked unit cell contraction and a reduction in the mismatch between the different double-layer dimensions. A reduction in the copper–oxygen distances in the CuO₂ planes from 2.007 Å in La₂Ba₂Cu₂Sn₂O₁₁ to 1.951 Å in Gd₂Ba₂Cu₂Ti₂O₁₁ produces a copper–oxygen interaction similar to that found in high-temperature superconductors.