Single crystals, where M = Al or Ga, were prepared by using the flux technique. Both of the compounds crystallized in the space group, P3, with a = b = 1.0468 and c = 0.6599nm for the Al compound and a = b = 1.0560 and c = 0.6660nm for the Ga compound. It was found that the ionic conductivities of these solids were less than 10-7S/cm at room temperature. This was attributed to the highly ordered distribution of Na ions over the available crystallographic sites. There was a total of fifteen Na atoms in the unit cells of these compounds. However, the conductivity increased with increasing temperature and attained a value of about 0.001S/cm at 600K. In order to achieve this level, the Na sub-lattice of the compound had to undergo a rearrangement of the unit cell.

D.M.Poojary, A.Clearfield, V.A.Timofeeva, S.E.Sigaryov: Solid State Ionics, 1994, 73[1-2], 75-80