W0085

Crystal Structure of the Layered A_nb_no_3n+2 Compounds in the Srtio₃-Sr₂nb₂o₇ Quasibinary System. I. Levin, L.A. Bendersky, T.A. Vanderah and R.S. Roth, Materials Science and Engineering Laboratory, National Insitute of Standards and Technology, Gaithersburg, MD, 20899 USA

A series of structurally related phases A_nB_nO_3n+2 with A=Sr²⁺ and B=(Ti⁴⁺, Nb⁵⁺) have been prepared. The members of this series with n=4,5,6,7 and non-integer values between 4 and 5 were characterized by both conventional (CTEM) and high-resolution (HRTEM) transmission electron microscopy. The structure of A_nB_nO_3n+2 compounds is composed of the distorted perovskite-like layers cut parallel to {110} plane of the perovskite. For members of the series found at integral values, the layers are n [BO₆] octahedra thick. Compounds with non-integral value of n between 4 and 5, are ordered intergrowths of the n=4 and n=5 structures. For these structures, n indicates the average thickness of the perovskite-like layers. All structures share the same basic orthorhombic unit cell with a~2a_p, c~1.41a_p (a_p is the lattice parameter of the cubic perovskite) and the long b-dimension which increases incrementally with a value of n. Electron diffraction indicated that all these compounds are incommensurately modulated along a direction, with the vector of modulation k close but not equal to 1/2a* (*-indicates reciprocal space). In-situ heating/cooling experiments in the TEM were used to study the domain of stability of the incommensurate modulation as a function of composition. A general classification scheme which relates the possible tilt systems of [BO₆] octahedra to the symmetry of the A_nB_nO_3n+2 structures was developed. Based on this analysis the incommensurate modulation in all the structures studied was attributed to the tilting of [BO₆] octahedra about the axis normal the layer faces.