HIGH-PRESSURE SYNTHESIS OF NDCUO3-DELTA PEROVSKITES -THAN-OR-EQUAL-TO-DELTA-LESS-THAN-OR-EQUAL-TO-0.5)

Oxygen-deficient perovskites NdCuO3-delta (0 less than or equal to del ta less than or equal to 0.5) were prepared for the first time at high pressures in a multianvil apparatus and characterized by X-ray and el ectron diffraction and by electron microprobe. Several new oxygen vaca ncy-order phases could be stabilized depending upon delta and syntheti c conditions. The crystal structures of NdCuO3-delta are related to th ose previously observed in the LaCuO3-delta system, but are more highl y distorted due to the smaller Nd cation size, leading to different sy mmetries, A-site coordination numbers and unit cell dimensions. Electr on diffraction shows that NdCuO2.5; crystallizes in an orthorhombic ro ot 2a(p) x 2 root 2a(p) x ap subcell with a 6 root/2a(p) x 4 root(2)ap x 2a(p), supercell, where a(p) is the simple cubic perovskite lattice parameter. NdCuO2.6 crystallizes in a monoclinic root 5a(p), x root 5 a, x 2a(p) cell. In addition, two other phases with root 10a(p) x root 10a(p) x 2a(p) and root 5a(p) x 2 root 5a(p) x 2a(p) have been observ ed for delta = 0.3-0.4, each exhibiting superstructures related to NdC uO2.6. Oxygen vacancy ordering models are proposed to interpret these results. An orthorhombic GdFeO3-type phase with delta = 0-0.07, the fi rst example of this structure-type in the rare earth cuprates, was fou nd at the highest oxygen pressures investigated. The major structure-t ypes, corresponding to delta = 0.07, 0.4, and 0.5, were refined by the Rietveld method. It was not possible to synthesize other LnCuO(3-delt a) 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.