Predicted aluminum monoxide phases and their structural evolution under pressure

To explore the stable crystal structure and structural evolution of aluminum monoxide (AlO), we predict four novel structures and investigate their stability, mechanical, electronic and Raman properties using particle-swarm optimization (PSO) technique and density functional theory (DFT) calculations. Depending on the symmetry and bonding characteristics, these novel structures exhibit various stability and properties under pressure. The oP-AlO (space group Imm2) is the most stable structure under ambient pressure, while the h-AlO (space group R$\overline{3}$m) structure becomes the most stable above 3 GPa and remains so up to 100 GPa. The h-AlO structure stands out due to distinct bonding interactions at different Wyckoff positions of aluminum atoms, particularly the rhombus arrangement formed by Al-II atoms, which gives rise to a Dirac cone in its electronic structure that is insensitive to pressure. In contrast, the m-AlO (space group C2/m), oP-AlO and oD-AlO (space group I/mmm) structures undergo first-order phase transitions, accompanied by significant structural changes and discontinuities in Al-O bonds. The oP-AlO and oD-AlO structures, in particular, exhibit unstable transformations during these transitions. Additionally, the vibrational characteristics of predicted structures are discussed, and the significant differences facilitate future experimental identification through Raman spectroscopy.