A SINGLE-CRYSTAL X-RAY-DIFFRACTION STUDY OF THE ION-EXCHANGE OF CD-2+ INTO AG+ BETA-ALUMINA

Conventional single-crystal X-ray diffraction (Mo K-alpha radiation, lambda = 0.71073 angstrom) at 295 K has been used to study the ionic distribution of five compositions of the mixed-cation system Ag+/Cd2+ beta-alumina, Ag1.22-yCdy/2Al11O17.11, y = 0.00, 0.10, 0.41, 0.99 and 1.11. M(r) = 702.1, 697.1, 680.7, 651.1 and 644.8, F(000) = 674.4, 670.0, 655.4, 629.0 and 623.4. The compounds are made by the controlled ion exchange of Cd2+ ions into Ag+ beta-alumina from a CdCl2 melt at 873 K. The structure comprises a hexagonal framework of spinel-type blocks of aluminium oxide separating two-dimensional conduction planes with a spacing of approximately 11 angstrom. All atoms in the spinel framework follow the centrosymmetric hexagonal space group P6(3)/mmc, Z = 2, with cell dimensions a = 5.5914 (3), 5.6032 (2), 5.5959 (2), 5.5896 (5) and 5.5900 (7) angstrom and c = 22.430 (4), 22.504 (3), 22.458 (1), 22.434 (8) and 22.396 (6) angstrom for y = 0.00, 0.10, 0.41, 0.99 and 1.11, respectively. Final wR(F(o)2) values are 0.0796, 0.0693, 0.0698, 0.0874 and 0.0853, respectively. In Ag+ beta-alumina, the Ag+ ions occupy three different 6(h) sites: one near the 2(d) site, Ag(1); one close to the 2(b) site, Ag(3); and a third site, Ag(2), between these two sites, close to Ag(1). The occupations are 41, 33 and 26% for Ag(1), Ag(2) and Ag(3), respectively. The Cd2+ ions enter a new 6(h) site, not previously occupied by Ag+ ions. At low replacement (approximately 8%), the Ag(1)-site occupation is unchanged. By the time approximately 34% of the Ag+ ions have been replaced, three Ag+ sites are replaced with almost equal amounts by Cd2+ ions. When 81 and 91% of the Ag+ ions have been replaced, Ag(1) is the only site still containing Ag+ ions. The implications of these results are discussed with reference to the general aspects of ion exchange in beta-alumina.