Contributions of bulk and surface energies in stabilizing metastable polymorphs: A comparative study of group 3 sesquioxides La2O3, Ga2O3 , and In2O3

Abstract

Sesquioxides are an important class of compounds which are used in various applications such as optical glasses, high-power electronics, and gate dielectrics. Using density functional theory, we investigated bixbyite ${\mathrm{La}}_{2}{\mathrm{O}}_{3}, {\mathrm{In}}_{2}{\mathrm{O}}_{3}$, and ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$; hexagonal ${\mathrm{La}}_{2}{\mathrm{O}}_{3}$; corundum ${\mathrm{In}}_{2}{\mathrm{O}}_{3}$; and monoclinic ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$. We compared the predicted structural properties for each material using the local density approximation (LDA) and the generalized-gradient approximation [GGA PBE (Perdew-Burke-Ernzerhof)]. We found that LDA predicts the correct ground state structure for all three sesquioxides; however, GGA PBE predicts the incorrect ground state structure for ${\mathrm{La}}_{2}{\mathrm{O}}_{3}$. To gain better insight on why this happens, we calculated the phonon properties and the free energy for all structures to determine phase transition temperatures. Additionally, we determined the transition pressure for ${\mathrm{In}}_{2}{\mathrm{O}}_{3}$. We also studied several surface terminations for each compound and determined the lowest-energy surface for each structure as well as the interfacial energy for ${\mathrm{La}}_{2}{\mathrm{O}}_{3}$.