STRUCTURAL STUDIES OF ORDERING IN THE (PB1-XBAX)(MG1 3NB2/3)O-3 CRYSTALLINE SOLUTION SERIES/

The ordered structures of the (Pb1-xBax)(Mg1/3Nb2/3O3 crystalline solu tion series were investigated by selected area electron diffraction (S AED) and high-resolution electron microscopy (HREM). At low Ba content s (e.g., x < 0.40), the ordered structure was found to be isostructura l with Pb(Mg1/3Nb2/3)O-3, with a doubled unit cell characterized by 1/ 2{111} superlattice reflections. At higher Ba contents (e.g., x > 0.60 ), the ordered structure was characterized by 1/3{111}) superlattice r eflections. For intermediate Ba contents (e.g., x = 0.60), diffuse sca ttering along the {lll} between diffuse 1/2{111} and 1/3{111} reflecti ons was observed. The ordering is attributed to the distribution of th e B-site cations between multiple sublattices. Strong fluctuations in the B-site cation ratio between ordered and disordered regions are bel ieved not to exist; however, the possibility of weak fluctuations is c onsistent with the observed lattice images.