STRUCTURAL ORDER IN BA(ZN1 3TA2/3)O-3, BA(ZN1/3NB2/3)O-3 AND BA(MG1/3TA2/3)O-3 MICROWAVE DIELECTRIC CERAMICS/

The process and nature of structural ordering and the factors that inf luence them have been investigated in the microwave dielectric perovsk ites, barium zinc tantalate (BZT), barium zinc niobate (BZN), and bari um magnesium tantalate (BMT), sintered at various temperatures. The sa mples were characterized mainly by X-ray powder diffraction and transm ission electron microscopy. The results show that short-range 1:1 B-si te order features strongly in the early stages of ordering in BZT and BZN, but it is extremely rare in BMT, for which most grains commence w ith 1:2 order. As sintering progresses, 1:1 order is replaced by 1:2 l ong-range order in BZT and by disorder in BZN. Orientational variants of the ordered domains within grains occur in similar numbers when ord er is fine-scale, but their distribution is less homogeneous in well-o rdered samples. Local inhomogeneities in the degree of order within gr ains, which will affect dielectric properties; correlate with both res idual non-stoichiometry and the presence of dislocations. Incompletely reacted starting materials which may persist to late stages of sinter ing can also strongly influence order. Anomalously large ordered domai ns at grain boundaries are attributed to grain-boundary migration acco mpanied by enhanced diffusion. The results indicate that with starting materials that are well-mixed and homogeneous at the nanoscale, tailo ring of physical properties should be possible by controlling the type and degree of order through chemical composition.