On the structural integrity of the spinel block in the β"-alumina structure

The beta''-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, 24372452]. His calculation is tested rigorously against the many single-crystal structure determinations that have been made on the beta''-alumina family compounds and excellent correlation is found.