STRUCTURE OF THE LUMINESCENT SYSTEM NA+/UO(2)2+BETA''-ALUMINA

The structure and ionic distribution of Na+ beta''-alumina with approximately 12% of the sodium ions exchanged for the luminescent molecular uranyl ion (UO22+) has been investigated at room temperature by single-crystal X-ray diffraction. The idealized formula for the crystal studied is (UO2)y-Na1+x-2yMgxAl11-xO17, with x = 3 and y = 0.10, M(r) = 627.8. The structure has the trigonal space group R3mBAR, a = 5.6206 (3), c = 33.703 (5) angstrom, V = 922.3 (2) angstrom3, Z = 3, D(x) = 3.39 Mg m-3, mu(obs) = 2.03, mu(calc) = 2.30 mm-1. Spinel-type blocks comprising Al3+ ions, tetrahedrally and octahedrally coordinated by O2- ions, are separated at z = 1/6, 1/2 and 5/6 by conduction planes containing Na+ and UO22+ ions and the column O atoms, O(5). These serve to hold together the spinel blocks through Al-O(5)-Al bonds. The Mg2+ ions stabilize the structure by substituting for one third of the Al3+ ions at Al(2) sites close to the centre of the spinel block (z congruent-to 0). The linear U022+ ions (length approximately 3.4 angstrom) arrange themselves in a disordered manner in the almost two-dimensional honeycomb-shaped pathways in the conduction planes, such that the molecules straddle the 6(c)-site intersections of the pathways. The uranium atoms occupy 18(h) sites, close to the Beevers-Ross (BR) 6(c) site, and the two uranyl O atoms occupy general 36(i) sites. The sodium ions occupy either BR sites or 18(h) sites, relaxed towards adjacent BR sites. Final R(F) = 0.0252, R(F2) = 0.0371, wR(F2) = 0.0407 for refinement on 2833 reflections.