NUMERICAL DETERMINATION OF THE LOCAL ORDERING OF PBMG1/3NB2/3O3 (PMN) FROM HIGH-RESOLUTION ELECTRON-MICROSCOPY IMAGES

High Resolution Electron Microscopy (HREM) images of PMN (PbMg1/3Nb2/3O3) taken along a (110) axis of the cubic (perovskite) structure show that ordered regions of the {111} type exist which correspond to AB′1/2B″1/2 O3 composition. Wavy fringes in the {111} plane directions reveal some imperfect ordering. A numerical treatment of the HREM images was performed in order to determine both position and size of perfectly ordered regions. After a test experiment, this method shows that (i) ordered regions consist of clusters about 2 nm in diameter regularly spread inside the crystal, (ii) the distance between the centers of neighboring clusters is about 2.5 nm. The regular arrangement of these clusters is due to their composition which differs from the mean composition. The composition of disordered regions is close to PbMg1/4Nb3/4O3. In this model some local electrical charge equilibrium could be obtained, at least partly, by oxygen and metal-atom vacancies. © 1994 Academic Press, Inc.