The Crystal Structure of “Beta Alumina” Na2O·11Al2O3

Bragg , Got t f r i ed and Wes t 3 ) have attempted the X-ray analysis of this crystal on the assumption of a formula \ N a j J · 11 \ A I Z 0 3 , agreeing with the best chemical analysis then available. They were not able to devise a structure which was completely satisfactory, but were led to suggest an ideal structure with a composition Ν α 2 0 · i l A l 2 0 3 to which "beta alumina" might tend. Since such a structure is now satisfactory from chemical analysis and from the density, a test can be made of its fit with X-ray intensities. This paper deals with the carrying out of such a test on N a 2 0 · 11 A l 2 0 3 itself, and on the isomorphous K 2 0 · H A 1 2 0 3 which has been prepared by the Norton Company. The unit cells of N a 2 0 · 11 A l 2 0 3 and K 2 0 · 11 A l 2 0 3 are hexagonal with a0 = 5.584, c0 = 22.45Ä, and a0 = 5.584, c0 = 22.67A, respectively. The space group is C6/mmc (Z)gA). The only point which is in doubt in the ideal structure suggested by Bragg , G o t t f r i e d and W e s t is the position of the Ν a (or K) atoms in the mirror planes. If the origin is taken at one of the centres of symmetry the Ν a may be at either (00 J ) or (§ 3 έ)· The present work suggests that the Ν a is actually upon the second of these two positions, so that the parameters of the structure become: