LI+ NA+ BETA-ALUMINA - A COMBINED SINGLE-CRYSTAL NEUTRON AND X-RAY-DIFFRACTION STUDY/

The crystal structure and ionic distribution in the conduction plane o f the partially exchanged Na+ beta-alumina system Li0.75Na0.47Al11O17. 11 has been determined from single-crystal X-ray diffraction at 30 and 298 K, in combination with a single-crystal neutron diffraction study at 295 K, At 30 K the Li+ ions occupy two sites: one in (2/3, 1/3, 1/ 4) and one similar to 1.0 Angstrom out of the z = 1/4 conduction plane . The Na+ ions are also found in two sites, both in the conduction pla ne. At 298 K the Li+ ions are found in three sites; the same two sites as at 30 K, as well as at a third site between these two. The Na+ ion s occupy the same sites as at 30 K, The combination of X-ray and neutr on diffraction is used to resolve the distribution of Na+ and Li+ ions . The column O5 atom in the Al3-O5-Al3 bridge between the spinel block s is found to be disordered. Charge compensation is achieved through a Frenkel defect of two interstitial Al atoms, one on either side of th e conduction plane, together with an extra oxygen in the conduction pl ane. One such defect occurs, on average, in every 4.5 unit cells.