HREM ON DEFECTS IN SR2ND1.5CE0.5NBCU2O10-DELTA

Defects occuring in Sr2Nd1.5Ce0.5NbCu2O10-delta have been studied with high resolution electron microscopy and electron diffraction. The structure of Sr2Nd1.5Ce0.5NbCu2O10-delta consists of a block of layers CuO2-SrO-NbO2-SrO-CuO2 having a perovskite-like structure sandwiched with a (Nd, Ce)-O2-(Nd, Ce) block which has a fluorite-like stacking. Defects occur in which the number of (Nd, Ce) layers varies from 1 to 5, e.g. Sr2(NdCe)nNbCu2O6+2n-delta. The n = 1 material was found to extend over a relatively larger area. n = 3, n = 4 and n = 5 phases occur only as single planar stacking faults. All n = 1, 2, 3, 4, 5 structures have an a(p) square-root 2, a(p) square-root 2, c superstructure, due to rotation of the NbO6 octahedra about the c-axis. Quite frequently intergrowths occur with an approximate composition of Sr(Nb, Cu)O3, which adopts the perovskite structure with a c-axis of 0.42 nm and with an ab plane which fits to the ab plane of Sr2Nd1.5Ce0.5NbCu2O10-delta. The thickness of the intergrown Sr(Nb, Cu)O3 is limited to 5 nm. The (Nb, Cu)O6 octahedra in Sr(Nb, Cu)O3 are also tilted about the c-axis but the amount of tilt is less than in Sr2Nd1.5Ce0.5NbCu2O10-delta.