Formation of spinel-type gallium oxynitrides: a density-functional study of binary and ternary phases in the system Ga–O–N

We perform first-principles calculations to investigate the structure of spinel-type gallium oxynitrides (γ-galons) as well as their formation from and relative stability to the binary phases gallium oxide (Ga2O3) and gallium nitride (GaN). Besides the ideal cubic γ-galon Ga3O3N with a content of 50 mol% GaN, we study approximants of defective γ-galons corresponding to 36 mol% and 17 mol% GaN within the Constant-Anion-Model. Enthalpies of formation indicate that at ambient pressure the ternary compounds are unfavorable relative to a phase assemblage of monoclinic β-Ga2O3 and wurtzite GaN. A comparison to spinel-type aluminium oxynitrides γ-alons is made. We find the difficulties in synthesizing γ-galons to be related to the low energy of the ground state structure of monoclinic β-Ga2O3.