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High‐Order Incommensurate Modulations and Incommensurate Superstructures in Transparent Sr 0.6 Ba 0.4 Nb 2 O 6 (SBN40) Ceramics
Author(s) -
Lee HanYoung,
Freer R.
Publication year - 1998
Publication title -
journal of applied crystallography
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.429
H-Index - 162
ISSN - 1600-5767
DOI - 10.1107/s0021889898003550
Subject(s) - microstructure , tetragonal crystal system , materials science , strontium barium niobate , superstructure , ceramic , transmission electron microscopy , tungsten , planar , crystallography , diffraction , macle , modulation (music) , condensed matter physics , crystal structure , optics , ferroelectricity , dielectric , crystal twinning , optoelectronics , composite material , nanotechnology , chemistry , metallurgy , physics , acoustics , thermodynamics , computer graphics (images) , computer science
Ceramics of strontium barium niobate (Sr 0.6 Ba 0.4 Nb 2 O 6 , SBN40), having the tetragonal tungsten bronze (TTB) structure, were prepared by a conventional processing route; sintering at temperatures in the range 1573–1723 K yielded products with three types of microstructure. Incommensurate superstructures were investigated by transmission electron microscopy (TEM). Modulation of the incommensurate superstructure (ICS) depended on microstructural development. The lowest‐order ICS modulation was a common feature in all the SBN40 ceramics, regardless of microstructure. reflections and unusually low δ (incommensurability parameter) were observed in SBN40 specimens having abnormally large grains and thus a certain degree of A ‐site ordering was expected. Transparent SBN40 ceramics which exhibited large homogeneous grains were characterized by a shortened c axis, intense [001] X‐ray diffraction reflections, a multiplication of (110) and (001) cell spacings and the appearance of higher‐order ICS reflections. Planar defects, having the thickness of one spacing, were observed aligned along . The development of a higher‐order structural modulation and of ICS reflections is believed to reduce the structural energy; the development of planar defects appears to stabilize the incommensurate phase.