Oral: Unraveling the transformation pathway of the β to γ phase transition in Ga₂O₃ from atomistic simulations

Defect spinel γ-Ga₂O₃, the least stable polymorph of Ga₂O₃, frequently appears as a structural defect in or on the surface of monoclinic β-Ga₂O₃, yet the mechanism behind its formation remains unresolved. In this study, we use first-principles calculations to propose a pathway for the phase transition from β to γ-Ga₂O₃ and identify two critical driving forces: tensile strain along the crystallographic a-axis and Ga deficiency. With the inclusion of configurational entropy, the γ phase becomes energetically competitive under Ga-deficient conditions, achieving a stability crossover at room temperature when vacancy concentrations exceed 3%. We propose a model for the β → γ transformation pathway involving two primary reactions. These reactions describe the migration of Ga atoms from tetrahedral lattice sites to octahedral interstitial positions, facilitated by Ga vacancies and tensile strain along the a-axis. Our calculations reveal that the transformation barriers are prohibitively large in pristine Ga₂O₃, but are significantly reduced by Ga vacancies, strain, and volumetric relaxation during the transformation. Our findings align with experimental observations of γ-Ga₂O₃ formation on damaged surfaces or under highly n-type conditions that favor Ga deficiency and strain. This work provides new insights into the defect-induced mechanisms underlying the phase transition and highlights the roles of localized strain and non-equilibrium defect concentrations in promoting γ-phase formation.