HIGH-ORDER INCOMMENSURATE MODULATIONS AND INCOMMENSURATE SUPERSTRUCTURES IN TRANSPARENT SR0.6BA0.4NB2O6 (SBN40) CERAMICS

Ceramics of strontium barium niobate (Sr0.6Ba0.4Nb2O6, SBN40), having the tetragonal tungsten bronze (TTB) structure, were prepared by a con ventional processing route; sintering at temperatures in the range 157 3-1723 K yielded products with three types of microstructure. Incommen surate superstructures were investigated by transmission electron micr oscopy (TEM). Modulation of the incommensurate superstructure (ICS) de pended on microstructural development. The lowest-order ICS modulation was a common feature in all the SBN40 ceramics, regardless of microst ructure. (h+1/2,h+1/2,l) reflections and unusually low delta (incommen surability parameter) were observed in SBN40 specimens having abnormal ly large grains and thus a certain degree of A-site ordering was expec ted. Transparent SBN40 ceramics which exhibited large homogeneous grai ns were characterized by a shortened c axis, intense [001] X-ray diffr action reflections, a multiplication of (110) and (001) cell spacings and the appearance of higher-order ICS reflections. Planar defects, ha ving the thickness of one d(110) spacing, were observed aligned along {110}. The development of a higher-order structural modulation and of ICS reflections is believed to reduce the structural energy; the devel opment of planar defects appears to stabilize the incommensurate phase .