The ionic distribution in trivalent Gd 3+ β″-alumina

Abstract

The crystal structure of Gd 3+ β″-alumina (idealized formula :Gd 5/9Mg 2/3Al 101/3O 17) has been studied at 22°C by single-crystal X-ray diffraction. The essential result is that the Gd 3+ ions occupy only mid-oxygen (9d) sites in the conduction slab. The column-oxygen 05 in the conduction slab is also clearly disordered over six 18f-sites, each displaced by 0.50A˚from the central 3b-position in the direction of the Gd 3+ ions. The effective crystallographic formula/cell layer is Gd 0.58Mg 0.74Al 10.26O 17, assuming charge compensation through Mg 2+ substitution of Al 3+ (at the Al2-site). Indirect evidence also supports the notion of ordering of Al and Mg in alternate Al2-sites at the centre of the spinel block. The final R W(F 2) value following the use of fourth-order cumulants to describe the Gd 3+ distribution, and the inclusion of a small amount of Gd 3+ scattering power at one of the 6c end-sites was 0.0606.