Superstructure determination of the perovskite beta La0.33NbO3

New interesting results in the crystal structure of the perovskite beta La0 .33NbO3 were revealed using selected area electron diffraction, powder X-ra y diffraction techniques and Rietveld refinement method. Although the super structure of beta La0.33NbO3 could not be seen by conventional X-ray powder diffraction technique, the electron diffraction patterns revealed weak spo ts resulting in a superstructure array for the atoms of beta La0.33NbO3. Th e crystal symmetry is compatible with an orthorhombic cell, space group Cmm m. From Rietveld refinement, the resulting lattice parameters are: a = 7.82 (1) Angstrom, b = 7.83(9) Angstrom, c = 7.90(9) Angstrom and goodness of fi t R = 0.1107, Rwp = 0.15. The superstructure is built from distorted octahe dra NbO6 along the [001] axis. Results suggest that this distortion may be produced by occupation of La atoms in (2a) and (4l) sites. (C) 2000 Kluwer Academic Publishers.