Ceramic and Glass Materials

1 Alumina 5

of 1.38 Å for oxygen atoms, so the structure has less “open” volume even than close-
packing (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 [8] for some previous ref-
erences. 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 configura-
tion 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 Al 2 O 3 and water.

Table 3Structures of hydrated alumina phases

Lattice parameters (Å)/angle

Phase Formula AB c

Bayeriteβ-Al(OH) 3 Monoclinic 4.72 8.68 5.06/90°7′

Gibbsiteα-Al(OH) 3 Monoclinic 8.64 5.07 9.72/85°26′

Boehmiteα-AlOOH Orthorhombic 2.87 12.23 3.

Diasporeβ-AlOOH Orthorhombic 4.40 9.43 2.

Fig. 1The structure of alpha alumina from [31]. The structure of corundum (alpha-alumina) from

[31]. 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