Powder Neutron Diffraction Analysis of Sodium Silver and Deuterium Beta Aluminas

Abstract

The main features of the beta-alumina structure are known from X-ray diffraction studies on the Na, K, Ag and Tl compounds^1,2 but some anomalies remain. In particular, three separate mechanisms for the compensation of the excess soda have been presented recently involving vacancies on aluminum sites or extra oxygen near the mid-oxygen position.^1,3 Also, the structural basis for the anomalous lattice parameters⁴ of Li and H beta-aluminas is not known. We have prepared a pure beta-phase ceramic containing a significantly higher excess of soda (1.47Na₂O:11Al₂O₃ by atomic absorption analysis for Na and Al) than found in the single crystals studied hitherto and have so far studied the crystal structures of this and the ion-exchanged Ag and D compounds by powder neutron diffraction using profile analysis. Data are presented for these compounds at low temperature and for the Ag compound at room temperature. Refinements nearing completion show that i) atoms in the mirror plane have significant shifts in position for anisotropic relative to isotropic temperature factor refinements, ii) quite good agreement is found for the Ag refinement at room temperature compared to that done with X-rays, iii) the nonstoichiometry is almost certainly compensated by extra oxygen in the mirror plane, and iv) the location of D in the mirror plane is quite different to that of the cations in the other compounds, being found between pairs of O(2) and O(4) oxygens.