B-SITE ORDER AND INFRARED REFLECTIVITY IN A(B'B'')O3 COMPLEX PEROVSKITE CERAMICS

Complex perovskite A(B'B'')O3 ceramics with various degrees of B-site order have been structurally and spectroscopically characterized by means of x-ray diffraction, transmission electron microscopy, and infrared reflectivity. In Ba(Fe1/2Nb1/2)O3 and Pb(Fe1/2Nb1/2)O3, no order could be detected whereas Pb(In1/2Nb1/2)O3, Pb(Mg1/3Nb2/3)O3, and Pb(Sc1/2Ta1/2)O3 (unannealed) all exhibited weak reflections associated with small ordered regions Pb(SC1/2Ta1/2)O3 was also prepared by appropriate heat treatment in the fully ordered form and exhibited strong maxima arising from the ordered superlattice in both electron and x-rav diffraction. The infrared reflectivity (30-4000 cm-1) revealed correlation between the degree of ordering and appearance of the extra mode in the 260-320 cm-1 range assigned to B'-B'' vibrations. Despite an apparently low volume fraction of ''ordered material'' in. ''partially ordered'' samples of Pb(SC1/2Ta1/2)O3, the extra mode was observable. It is suggested that infrared spectroscopy is sensitive to short-range ordering down to the nm range which is proposed to exist in between the small regions of order.