Structures and energetics of Ga2O3 polymorphs

First-principles calculations are made for five Ga2O3 polymorphs. The structure of epsilon-Ga2O3 with the space group Pna2(1) (No. 33, orthorhombic), which is sometimes called kappa-Ga2O3 in the literature, is consistent with experimental reports. The structure of gamma-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 beta < epsilon < alpha < delta, while beta < epsilon < delta < alpha for the bulk modulus. The formation free energies have the tendency beta < epsilon < delta < gamma < at low temperatures. With the increase of temperature, the difference in free energy between the beta-phase and the epsilon-phase becomes smaller. Eventually the e phase becomes more stable at above 1600 K.