SUPERSTRUCTURE OF THE DEFECT PEROVSKITE TYPE COMPOUNDS La_1/3NbO₃. Y. Tabira, K. Tanabe, K. Suda and N. Ishizawa. Research Laboratory of Engineering Materials, Tokyo Institute of Technology, Nagatsuta, Midori, Yokohama 226, Japan

Superstructure analysis of the title compound(hereafter call LNO) were carried out by the single-crystal X-ray diffraction making use of SR, with the aid of the electron diffraction. The LNO as well as La_1/3TaO₃(call LTO) are promising host crystals for the Li⁺ ion intercalation. Both compounds crystallizes in double-layered perovskite structure with perfectly vacant sites of La³⁺ in the z=1/2 plane and by two third occupied sites of La³⁺ in the z=0 plane at room temperature. Although reduction of crystal system to which the LNO belongs, from tetragonal(space group P4/mmm) to orthorhombic and a superstructure doubling the a and b dimensions of the fundamental tetragonal unit cell, have been previously reported, no structure refinements have been performed to our knowledge. X-ray diffraction experiment were made using the horizontal-type four-circle diffractometer, utilizing SR from a vertical wiggler at the beam line 14A of the PF(KEK) in Tsukuba, Japan. The geometry of the reciprocal lattice of the LNO insists on being orthorhombic unlike the LTO is tetragonal, and the refined cell parameters are a=0.7817(1), b=0.7836(1), c=0.7910(1) nm. Three dimensional diffracted intensity data including the superstructure reflections were collected in the independent reciprocal space within sin [[theta]]/[[lambda]][[second]]1.0 with [[lambda]]=0 070nm. The electron diffraction pattern of the hkO plane of the LNO contained weak but sharp superstructure reflections and obeyed an extinction rule for the C-faced lattice. Of five possible space groups, Cmmm, C2mm, Cm2m, Cmm2 and C222, Cm2m seems to be the| most probable one since in the refinements it gave the lowest conventional R factor to be 0.048 for all 1150 reflections. The R factor is 0.044 for 636 fundamental, and 0.089 for 514 superstructure reflections whose intensities are much weaker than the fundamental ones. Refined structure has an alternate shift along the b axis of La³⁺ in the z=0 plane and suggests the statistical fluctuation of Nb⁵⁺ from the averaged position, which might interact with the local distribution of La³⁺ ions in the structure.