A combined HRND and HREM study of degradation collapse in NH4+/H3O+ β″-alumina
Abstract High resolution neutron diffraction ( HRND ) from single crystals, and high resolution electron microscopy ( HREM ) are combined to study the deammoniation of NH 4 + /H 3 O + β″-alumina on heating above 200°C. HRND shows the emergence of extra reflections which cannot be indexed in terms of the original β″-alumina cell, but rather in terms of a ∼20.6A (double spinel block) c -axis. The intensities of the extra reflections are seen to change as deammoniation proceeds. This can, in turn, be related to HREM observations of the development of distinct regions of conduction plane loss with the consequent pair-wise degradation collapse of adjacent spinel blocks. Regions can also be seen where the conduction planes appear to swell out to produce disc-like pockets, presumably containing an accumulation of evolved NH 3 gas; regions of “knock-on” damage caused by protons in the conduction planes are also observed.