Crystal chemistry of the La 2− yLnyCuO 4 ( Ln = Pr, Nd) systems

Abstract

An investigation of the value of the room temperature crystallographic tolerance factor t at which stability of the La 2CuO 4 T-tetragonal structure becomes unstable relative to the Nd 2CuO 4 T′-tetragonal structure in the systems La 2− y Ln y CuO 4, Ln = Pr or Nd, has led to the discovery of a new T″-tetragonal phase. It has a powder X-ray diffraction pattern like that of the T′ phase, but with a slightly larger c parameter than that obtained by extrapolation from the T′ phase field. Its occurrence as nearly a line phase at y ≈ 0.5 suggests cation ordering within a LnO layer; with a larger size difference between La 3+ and the Ln 3+ ion, interlayer ordering occurs to give either a two-phase sample T O + T′ or the hybrid T∗ phase.