Table 1.

Polymorphs of Gallium Oxide.

Structure		Comments	References
α-Ga2O3		This structure is another common structure, apart from beta gallium oxide. It has a similar corundum structure to Al2O3. Making fine crystals is a difficult job. The α-Ga2O3 structure can be maintained only at around 550 °C and above that a phase transformation to β-Ga2O3 takes place.	[18,51,53]
β-Ga2O3		It is the most stable of all in ambient condition and has major interest from researchers and, as already mentioned, it has a monoclinic structure with parameters a = 12.19 A, b = 3.016 A, c = 5.80 A β = 103.70	[54,55,56]
γ-Ga2O3		The preparation of this polymorph is simple as it just requires the oxidation of gallium in amino alcohol, like ethanolamine.	[57,58]
ε-Ga2O3		This structure of gallium oxide can be metastable at higher pressure conditions. Also, upon heating it can transform to alpha and beta phases. Furthermore, it exhibits ferroelectric property.	[59,60]
δ-Ga2O3		This structure was initially thought to be a phase which was similar to ε-Ga2O3. The structure was presumed to be a nano-crystal form of ε-Ga2O3. However, later it was confirmed that it is not a nanostructure or another phase and is a different cubic structure.	[61,62]