The crystal forms of Al2O3 are: α, γ, η, δ, θ, κ, χ and so on. When the external conditions change, the crystal form will change. In the Al2O3 variant, since the oxygen in α-Al2O3 (density 3.99g/cm3) is the most compactly packed, only α-Al2O3 (corundum) is stable, and other crystal forms are unstable. Transformed into α-Al2O3. In addition to corundum, the common Al2O3 crystal type should be γ-Al2O3. γ-Al2O3 has a spinel structure, but in its structure, some tetrahedral voids are not filled, so the density of γ-Al2O3 (3.65g/ cm3) is smaller than corundum. When Al(OH)3 is heated for dehydration, γ-Al2O3 is formed at about 450°C, and γ-Al2O3 is heated to a higher temperature to transform into corundum. However, the conversion speed will be faster when the change is more than 1000?.
Some other unstable crystal forms of alumina are also formed under different conditions when Al(OH)3 is heated and dehydrated. ρ-Al2O3 is amorphous, but some people think it is a transition state between amorphous and crystalline. Because ρ-Al2O3 is the only form of Al2O3 that can hydrate spontaneously at room temperature, it can be used as a cementing agent for refractory castables, so it has received attention in recent years.
