CRYSTAL STRUCTURE OF NA2O.MGO.5AL2O3 WITH REFERENCE TO NA2O.5AL2O3 AND OTHER ISOTYPAL COMPOUNDS

A complete X-ray crystallographic investigation was carried out on a single crystal of a ternary oxide with the nominal, idealized formula Na2O·MgO·5Al2O3, using all reflections obtainable with Cu Kα radiation, recorded by an integrating Weissenberg camera. This is the first single-crystal work on this structural type, whose prototype can be considered to be the binary sodium polyaluminate originally reported as Na2O·5Al2O3. The structure is very similar to β-alumina. It is rhombohedral, with space group R3m. The triply primitive hexagonal cell has a0 = 5.614 Å and c0 = 33.85 Å, i.e., about the same a axis as β-alumina but a c axis 1.5 times as long. Where β-alumina has two 11-Å spinel blocks related by a twofold screw axis parallel to c0, the compound described here has three such blocks, related by a threefold screw axis. As in β-alumina, the blocks are spaced well apart by Al-O-Al columns. The alkali ions reside in the incompletely filled basal plane containing the column oxygen and can diffuse quite freely in this plane.