Crystal structure and structural characteristics

The crystal structure of p-alumina may be described as an alternative stacking structure of spinel blocks and conduction planes . The spinel block is constructed by four layers of oxygen in a cubic close packed array, similar to that of spinel. Metal ions occupy a tetrahedral site, Al(2), and octahedral sites, Al(l) and Al(4). The conduction plane is a crystallographic mirror plane. In a unit cell section of the conduction plane 

The crystal structure of p -alumina is very similar to that of p-alumina however, the conduction plane is not a mirror plane. In a unit cell plane, there is one 0(5) oxygen and two BR sites (Fig. 13.1). Strictly speaking, the BR sites are displaced slightly from the plane, alternately up and down, and are coordinated tetrahedrally by three oxygens 0(2) and one 0(4), The c-parameter reflects the thickness of three spinel blocks and conduction planes. 

It is well-established that p-alumina prepared by the usual ceramic methods contains an excess of Na ions over its ideal composition, and that the formula is expressed by Nai+,jAlnOi7+,c/2 (x =5 0.3). The parameter, x/2, represents the number of interstitial oxide ions, 0 at mO sites - a middle point between two 0(5) oxygens, Fig. 13.1 - per unit cell plane each Oj coordinates to two Frenkel defects i.e. two interstitial AljS which are associated with A1 vacancies at the adjacent Al(l) site . The situation in K p-gallate is identical to that of P-alumina. Local electroneutrality is not guaranteed in the ideal structure, since there is a 

Several papers on the structures of proton-containing crystals, determined by X-ray and neutron diffraction have been reported as follows i)+ p-aluminai 2 , p-alumina 2 , p-alumina H3O+ 

IH O P -gallate . The structures of these proton-containing crystals are naturally very similar to those of the alkali-containing parent crystals. Accordingly, only limited structural characteristics of these compounds are discussed here, although small modifications always occur on ion exchange even to the centre of the spinel block. 

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