Structures and energetics of Ga₂O₃ polymorphs

First-principles calculations are made for five Ga₂O₃ polymorphs. The structure of ε-Ga₂O₃ with the space group Pna 2₁ (No.33, orthorhombic), which is sometimes called κ-Ga₂O₃ in the literature, is consistent with experimental reports. The structure of γ-Ga₂O₃ is optimized within 14 inequivalent configurations of defective spinel structures. Phonon dispersion curves of four polymorphs are obtained. The volume expansivity, bulk modulus, and specific heat at constant volume are computed as a function of temperature within the quasi-harmonic approximation. The Helmholtz free energies of the polymorphs are thus compared. The expansivity shows a relationship of β<ε<α<δ, while β<ε<δ<α for the bulk modulus. The formation free energies have the tendency β<ε<α<δ<γ at low temperatures. With the increase of temperature, the difference in free energy between the β-phase and the ε-phase becomes smaller. Eventually the ε phase becomes more stable at above 1600K.