By controlled substitution of Zn2+ for Ga3+ and Na+ on the cation sites in β″-Na-gallate, the negative charge on the spinel block, and, hence, the number of charge compensating mobile Na+ ions in the conduction plane, can be changed. The mobile Na+ concentration can be varied from 1.5 to 1.67 ions per conduction plane. The ionic conductivity is largest at these two terminal compositions, and falls to much lower values between. A super-cell domain model is proposed to explain the behavior. © 1981.