Li⁺/Na⁺ β-Alumina: A Combined Single-Crystal Neutron and X-ray Diffraction Study

The crystal structure and ionic distribution in the conduction plane of the partially exchanged Na⁺ -alumina system Li_0.75Na_0.47Al₁₁O_17.11 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 ∼1.0 Å out of the z = 1/4 conduction plane. The Na⁺ ions are also found in two sites, both in the conduction plane. 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⁺ ions occupy the same sites as at 30 K. The combination of X-ray and neutron 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 blocks is found to be disordered. Charge compensation is achieved through a Frenkel defect of two interstitial Al atoms, one on either side of the conduction plane, together with an extra oxygen in the conduction plane. One such defect occurs, on average, in every 4.5 unit cells.

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