New quadruple perovskites Ln₂Ba₂Ti_xSn_2-x Cu₂O₁₁, (0≤x≤2): controlling copper-oxygen distances in potential superconducting parent phases

The compounds NdBaCuSnTiO, SmBaCuSnTi O and GdBaCu TiO 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 LaBaCuSn O 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 CuO planes from 2.007 Å in La BaCuSnO to 1.951 Å in GdBaCuTiO produces a copper-oxygen interaction similar to that found in high-temperature superconductors.