A Series of Incommensurately Modulated AnBnO3n+2 Phases in the SrTiO3–Sr2Nb2O7 Quasibinary System 1

A series of structurally related phases AnBnO3n+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 ≈ 3.9 Å, c ≈ 5.6 Å, 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 Å 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.