We have grown Na beta"-alumina crystals while allowing a fast evaporation rate of the melt from an open crucible. Preferential evaporation of Na2O leads to beta"-phase crystals stabilized by the magnesium content but with a lower sodium concentration than that previously reported for the beta"-phase. Oxygen vacancies are present in the conduction planes in order to maintain charge balance. The lower sodium content, possibly coupled with a non-equilibrium magnesium configuration, leads to a conduction plane structure different from the a root 3 X a root 3 superlattice found by other workers. We have performed single-crystal 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 the mechanisms controlling conduction ion ordering in beta" aluminas.