STRUCTURAL ASPECTS OF THE NA+ -]CD-2+ ION-EXCHANGE PROCESS IN NA+ BETA-ALUMINA

The crystal structure and ionic distributions in the conduction plane of the system Na1.22-yCdy/2Al11-O17.11, Na+/Cd2+ beta-alumina, have been determined by single-crystal X-ray diffraction (Mo K-alpha radiation, lambda = 0.71073 angstrom) for the four compositions y = 0.00, 0.22, 0.45 and 1.22 at 295 K. All crystals have the hexagonal space group P6(3)/mmc, Z = 2. Crystal data for two of the compositions: for y = 0.00, M(r) = 598.6, a = 5.5929 (4), c = 22.526 (4) angstrom, V = 610.22 (1) angstrom 3, D(x) = 3.26 Mg m-3, mu-o = 1.03, mu-c = 1.03 mm-1, F(000) = 586.6, final R(F) = 0.0187, wR(F2) = 0.0436 for 2306 observations; for y = 1.22, M(r) = 639.1, a = 5.5869 (4), c = 22.370 (4) angstrom, V = 604.71 (1) angstrom 3, D(x) = 3.51 Mg m-3, mu-o = 2.11, mu-c = 2.06 mm-1, F(000) = 618.1, final R(F) = 0.0289, wR(F2) = 0.0651 for 909 reflections. The Na+ ions occupy the z = 1/4 and 3/4 conduction planes at predominantly one 2(d) site [Beevers-Ross (BR) site] and one 6(h) site (A site) (P6(3)/mmc notation) in the crude ratio 3:2. The Cd2+ ions first replace Na+ ions at the A site as they take up a different 6(h) site (B site). When all A-site Na+ ions have been replaced, Cd2+ ions continue to occupy B sites as they replace the remaining BR-site Na+ ions. Somewhere between y = 0.45 and y = 1.22 there is a distinct qualitative shift in the relative Cd-site occupations. At full replacement (Cd2+ beta-alumina), the Cd electron density is extended over a large part of the conduction pathways and is best described with four different Cd2+-ion sites. Most of the Cd2+ ions occupy a 6(h) site (C site). The remainder are found in three less frequented sites: one 12(k) and two 12(j) sites (D, E and F, respectively). The positive charge excess in the conduction plane is stabilized by Roth-Reidinger defects, each comprising two interstitial Al atoms, one on either side of the conduction plane (at z approximately 0.175 and approximately 0.325), in combination with an extra O atom at a 6(h) mid-oxygen site. This defect exists on average once in every nine unit-cell layers (4.5 unit cells). The implications of the crystallographic results and the importance of the Roth-Reidinger defect for Na+-ion mobility are discussed.