A SERIES OF INCOMMENSURATELY MODULATED A(N)B(N)O(3N-SR2NB2O7 QUASI-BINARY SYSTEM(2) PHASES IN THE SRTIO3)

A series of structurally related phases A(n)B(n)O(3n+2) with A = Sr2and B = (Ti4+,Nb5+) have been prepared. Members of the homologous seri es with n = 4, 5, 6, 7, and noninteger values between 4 and 5 were cha racterized by high-resolution electron microscopy/transmission electro n microscopy, bulk X-ray powder diffraction, and capacitive measuremen ts 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 fo und 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 wi dth of the perovskite-like slabs. The compounds share a basic orthorho mbic 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 ind icated that all of the compounds are incommensurately modulated along the shortest 3.9 Angstrom cell dimension. These effects were not appar ent in the bulk X-ray powder diffraction patterns. Experimental X-ray powder patterns indexed with the orthorhombic subcells are given for t he new compounds with n = 4.33, 6, and 7, i.e., Sr4.33Ti0.33Nb4O15, Sr 6Ti2Nb4O20, and Sr7Ti3Nb4O23. Preliminary capacitive measurements from 100 Hz to 5 MHz of relative permittivities and temperature coefficien ts indicated little frequency dependence but some unusual differences for successive members of the series, despite their crystal-chemical s imilarities. (C) 1998 Elsevier Science Ltd.