First-Principles Study of Magnesium-Induced Phase Stabilization in Gallium Oxide Polymorphs

In this study, we investigate the effect of Mg incorporation on the relative phase stability of the four primary Ga₂O₃ polymorphs using density functional theory (DFT) calculations, with the goal of rationalizing experimental observations suggesting that diffusion from MgAl₂O₄ substrates contributes to the relative stabilization of the γ phase. Mg incorporation is modeled up to 25% of Ga sites within supercells derived from fully relaxed unit cells of each polymorph. Our results show that, while β-Ga₂O₃ remains the thermodynamically most stable phase, the enthalpy and free energy differences between polymorphs decrease with increasing Mg content. The inherently disordered γ phase, with its high configurational entropy, becomes less energetically unfavorable under Mg substitution, suggesting that entropy-driven stabilization may facilitate its formation under high-temperature and/or nonequilibrium growth conditions, such as those previously reported. These findings provide a thermodynamic rationale for the experimental observation of the γ phase during epitaxial growth on MgAl₂O₄ spinel substrates.