A series of incommensurately modulated AnBnO3n+2 phases in the SrTiO3-Sr2Nb2O7 quasibinary system

A series of structurally related phases A(n)B(n)O(3n+2) with A = Sr2+ and B = (Ti4+,Nb5+) have been prepared. Members of the homologous series with n = 4, 5, 6, 7, and noninteger values between 4 and 5 were characterized by high-resolution electron microscopy/transmission electron microscopy, bulk X-ray powder diffraction, and capacitive measurements of relative permittivities and their temperature coefficients. The structures of these compounds feature distorted perovskite-like slabs sliced parallel to the cubic (110) plane. For members of the series found at integral values, n indicates the width of the slabs (in number of [BO6] octahedra). Samples at nonintegral values of n between 4 and 5 exhibited "line structures," i.e., ordered intergrowths of the n = 4 and n = 5 structures. For these samples, n indicates the average width of the perovskite-like slabs. The compounds share a basic orthorhombic unit cell with a approximate to 3.9 Angstrom, c approximate to 5.6 Angstrom, and a long b dimension that increases incrementally as the width of the slabs increases. High resolution electron microscopy indicated that all of the compounds are incommensurately modulated along the shortest 3.9 Angstrom cell dimension. These effects were not apparent in the bulk X-ray powder diffraction patterns. Experimental X-ray powder patterns indexed with the orthorhombic subcells are given for the new compounds with n = 4.33, 6, and 7, i.e., Sr4.33Ti0.33Nb4O15, Sr6Ti2Nb4O20, and Sr7Ti3Nb4O23. Preliminary capacitive measurements from 100 Hz to 5 MHz of relative permittivities and temperature coefficients indicated little frequency dependence but some unusual differences for successive members of the series, despite their crystal-chemical similarities. (C) 1998 Elsevier Science Ltd.