On the structural integrity of the spinel block in the β"-alumina structure.
The β''-alumina structure is examined in detail and an analysis is presented of the three-dimensional integrity of the lattice. The layer structure that is responsible for the very high sodium conduction rate is the specific focus. Rigid layers that are derived from the cubic spinel structure are interleaved by more open honeycomb pathways where rapid ion diffusion takes place. The three-dimensional rigidity of the spinel block in this structure makes it possible to accurately quantify the conduction layer thickness based only on the hexagonal unit-cell dimensions, as suggested originally by Harbach [(1983), J. Mater. Sci. 18, 2437-2452]. His calculation is tested rigorously against the many single-crystal structure determinations that have been made on the β''-alumina family compounds and excellent correlation is found.
The ionic distribution in trivalent Gd3+ β″-alumina
Abstract The crystal structure of Gd 3+ β″-alumina (idealized formula :Gd 5/9 Mg 2/3 Al 101/3 O 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 0.58 Mg 0.74 Al 10.26 O 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.
Li and Mg stabilized beta aluminas
Abstract Beta alumina and beta″ alumina crystals were grown from Na2O-Al2O3 melts doped with Li2O, MgO and Li2O+MgO. Crystal structures were identified by x-ray diffraction, and chemical composition was determined by nuclear reaction analysis. The ionic conductivity in single crystals was measured form 25° to 400°C by AC impedance analysis. Beta and beta″ alumina crystals were obtained which contained substantial concentrations of Li+ and Mg2+ ions in solid solution. The room temperature conductivity is 0.014 ohm−1cm−1 in doped beta alumina and 0.12 ohm−1cm−1 in doped beta″ alumina. These conductivities are comparable to the highest values measured in beta and beta″ alumina. The concentrations of Li+ and Mg2+ give insights into the effectiveness of Mg2+ and Li+ ions as stabilizing agents.