High Pressure Synthesis of NdCuO_3−δ Perovskites (0 ≤ δ ≤ 0.5)

Oxygen-deficient perovskites NdCuO3−δ (0 ≤ δ ≤ 0.5) were prepared for the first time at high pressures in a multianvil apparatus and characterized by X-ray and electron diffraction and by electron microprobe. Several new oxygen vacancy-order phases could be stabilized depending upon δ and synthetic conditions. The crystal structures of NdCuO3−δ are related to those previously observed in the LaCuO3−δ system, but are more highly distorted due to the smaller Nd cation size, leading to different symmetries, A-site coordination numbers and unit cell dimensions. Electron diffraction shows that NdCuO2.5 crystallizes in an orthorhombic √2ap × 2√2ap × ap subcell with a 6√2ap × 4√2ap × 2ap supercell, where ap is the simple cubic perovskite lattice parameter. NdCuO2.6 crystallizes in a monoclinic √5ap × √5ap × 2ap cell. In addition, two other phases with √10ap × √10ap × 2ap and √5ap × 2√5ap × 2ap have been observed for δ = 0.3–0.4, each exhibiting superstructures related to NdCuO2.6. Oxygen vacancy ordering models are proposed to interpret these results. An orthorhombic GdFe03-type phase with δ = 0–0.07, the first example of this structure-type in the rare earth cuprates, was found at the highest oxygen pressures investigated. The major structure-types, corresponding to δ = 0.07, 0.4, and 0.5, were refined by the Rietveld method. It was not possible to synthesize other LnCuO3−δ phases for rare earths smaller than Nd at pressures up to 150 kbar. Superconductivity was not observed in any of the new perovskites down to 4.2 K. © 1995, American Chemical Society. All rights reserved.