NEUTRON-DIFFRACTION AND TEM STUDIES OF THE CRYSTAL-STRUCTURE AND DEFECTS OF ND4NI3O8

Powder neutron diffraction and electron microscopy studies confirmed t he crystal structure of Nd4Ni3O8 (space group I4/mmm; a = b = 3.9168(2 ) Angstrom, c = 25.322(2) Angstrom, Z = 4; reliability factors R-p = 0 .125, R-wp = 0.108, chi(2) = 2.81, R-Bragg = 0.043, R-F = 0.037) previ ously determined by powder X-ray diffraction [Ph. Lacorre, J. Solid St ate Chem. 97, 495 (1992)]. It consists in the intergrowth between trip le layers of nickel in square planar coordination with Nd in cubic coo rdination (infinite layer type) and fluorite type layers. Nd4Ni3O8 is thus the first known compound with a triple T' type structure (relativ e to the single T' type structure of Nd2CuO4). A study of the coherent diffraction lengths and defects in the structure has been carried out . Two kinds of defects have been detected by HREM, for which a model i s proposed. (C) 1998 Academic Press.