Structures of Pure and Hydrated Alumina
The structures of aluminas and hydrated aluminas are given in Tables 2 and 3. The only stable phase of Al2O3 is corundum at all temperatures and up to at least 78 GPa pressure (see earlier discussion). The corundum structure is shown in Fig. 1. It consists of oxygen ions in a slightly distorted close-packed hexagonal (rhombohedral) lattice, space group R3c. The aluminum ions occupy two-thirds of the octahedral sites in the oxygen lattice. The lattice parameters for corundum in Table 2 are for a hexagonal unit cell containing 12 Al2O3 molecules. The rhombohedral lattice parameters are a = 5.128 A and a = 55.28°. The ionic porosity Z of a solid is given by the formula
Z= 1 – V/V (3)
a v 7
in which Va is the volume of atoms in a molecule (or in the unit cell) and V is the specific volume, or the volume of the unit cell. For alumina Z = 0.21 with the radius
Lattice parameters (A)/angle
Fig. 1 The structure of alpha alumina from . The structure of corundum (alpha-alumina) from . The aluminum atoms occupy two-thirds of the octahedral interstices in a hexagonal close – packed array of oxygen atoms, which is distorted because the octahedral share faces in pairs
of 1.38 A for oxygen atoms, so the structure has less “open” volume even than closepacking (Z = 0.26) of uniform spheres.
The various metastable alumina structures are all less dense than corundum. Several other allotropic structures have been suggested, but are less well-verified than those in Table 2. All these metastable aluminas have oxygen packings that are near to close-packed cubic. Usually, eta or gamma aluminas are formed at low temperatures, and transform in the sequence gamma^delta^theta^alpha alumina with increasing temperatures. However, many other variants are possible, with gamma formed at higher temperatures and transforming directly to alpha. See  for some previous references. Factors such as particle size, heating rate, impurities, and atmosphere can influence the kinetics of transformation and the sequence of phases. Above about 1,200°C, only alpha phase (corundum) is usually present.
The structures of various hydrated aluminas are given in Table 3. One configuration suggested for these structures is chains of Al-O bonds with hydrogen bonding between chains. These hydrated aluminas decompose at low temperature (about 300°C) to Al2O3 and water.