Properties and structures of R_2-xA_xCuO₄ phases: R = La, Pr and Nd; A = Sr, Pb and Cd

Abstract

The R₂CuO₄ phases for R = Pr and Nd were prepared and found to be semiconducting as previously reported. Crystal structures of both compounds were examined by profile refinements of neutron powder diffraction data. A series of phases of the type R_2-xSr_xCuO₄ which are isostructural with R₂CuO₄ for x up to 0.1 were prepared and found to remain semiconducting. Chemical analysis indicates the presence of Cu^III, yet these materials are not superconducting. This supports the view that R₂CuO₄ phases should be regarded as Mott insulators. A new series of phases possessing the tetragonal K₂NiF₄ structure appear in the Pr_2-xSr_xCuO₄ system for 1.0≤x ≤ 1.25. The new phases are semiconducting. We also find semiconductivity and not superconductivity for La_2-xA_xCuO₄ where A is Pb or Cd. Reasons are discussed for why Ba, Sr, Ca and Na substitutions into La₂CuO₄ lead to superconductivity whereas the Pb and Cd substitutions do not.