ORDERING IN LEAD MAGNESIUM NIOBATE SOLID-SOLUTIONS

Nonstoichiometric ordering of magnesium and niobium cations in undoped and lanthanum-doped lead magnesium niobate solid solutions has been i nvestigated by means of high-resolution transmission electron microsco py and computer image simulation. High-resolution lattice images were obtained using different objective apertures for various microscope im aging conditions. Computer image simulations were performed for a wide range of sample thicknesses, defocusing conditions, local Mg/Nb ratio s, and long-range order parameters. The simulated images demonstrated that the lattice images of the ordered region are predominantly depend ent on the long-range order parameter. By a comparison of the experime ntal and simulated images, a range of values of the long-range order p arameter 0.2 less-than-or-equal-to S < 0.8 was obtained for the ordere d regions. It was also shown that the ordered structure has a (NH4)3Fe F6 structure, which consists of alternating magnesium- and niobium-pre ferred sublattices along the [111] directions.