THE STRUCTURE OF ION DEFICIENT SODIUM BETA-''-ALUMINA

We have grown Na beta''-alumina crystals while allowing a fast evapora tion rate of the melt from an open crucible. Preferential evaporation of Na2O leads to beta''-phase crystals stabilized by the magnesium con tent but with a lower sodium concentration than that previously report ed for the beta''-phase. Oxygen vacancies are present in the conductio n planes in order to maintain charge balance. The lower sodium content , possibly coupled with a non-equilibrium magnesium configuration, lea ds to a conduction plane structure different from the a root 3 X a roo t 3 superlattice found by other workers. We have performed single-crys tal X-ray diffraction to determine the structure of this ion deficient Na beta''-alumina. The results help to explain the properties of our divalent crystals reported previously and give further insight into th e mechanisms controlling conduction ion ordering in beta'' aluminas.