Compositional defects in a MoAlB MAB phase thin film grown by high-power pulsed magnetron sputtering

Various compositional defects such as Mo₃Al₂B₄, Mo₄Al₃B₄, Mo₆Al₅B₆ and Al₃Mo, together with MoB MBene, are observed to be coexisting in a MoAlB MAB phase thin film grown at 800 °C by high-power pulsed magnetron sputtering. An overall film composition of Mo_0.29Al_0.33B_0.38 is measured by time-of-flight elastic recoil detection analysis. The concurrent formation of these compositional defects in the MoAlB matrix occurs during the synthesis without using any chemical reagent, and their coexistence with the MAB phase is thermodynamically possible, as elucidated by density functional theory simulations. These defect phases are imaged at the atomic scale by aberration-corrected scanning transmission electron microscopy. A rough estimation of defect populations of 0.073, 0.037, 0.042 and 0.039 nm^-1 for Mo₃Al₂B₄, Mo₄Al₃B₄, Mo₆Al₅B₆ and Al₃Mo compositional defects, respectively, is performed within the MoAlB matrix. The calculated energies of formation reveal that the Mo₄Al₃B₄ and Mo₆Al₅B₆ defect phases form spontaneously in the MoAlB host matrix, while the energy barrier towards the formation of the metastable Mo₃Al₂B₄ phase is approx. 20 meV per atom. The small magnitude of this barrier is easily overcome during vapor phase condensation, and the surface diffusion of adatoms during deposition leads to local compositional variations and the coexistence of the defect phases in the host matrix. Additionally, at grain boundaries, the presence of MoB MBene is observed, with an interlayer spacing between two Mo₂B₂ units increasing up to ∼50% compared to the pristine MoAlB phase.