Defects in an orthorhombic MoAlB MAB phase thin film grown at moderate synthesis temperature

Abstract

Here, we report on atomic scale characterization of various defects in a MoAlB (MAB) phase thin film grown by DC sputtering at a synthesis temperature of 700 °C. Aberration-corrected scanning transmission electron microscopy reveals the formation of an intergrown metastable Mo3Al2B4 phase accompanied by thermally stable 90° twist boundaries, coexisting within the pristine MoAlB matrix. The concurrent formation of these structural defects in the MoAlB matrix can be rationalized based on minute differences in formation enthalpies as shown via density functional theory calculations. The specific structural nature of both the twist boundary and compositional defect (Mo3Al2B4) in a MoAlB matrix is hitherto unreported in literature. Apart from these defects, faceted grain boundaries are observed. In the vicinity of amorphous AlOx regions, Al is deintercalated and a 2D MoB MBene phase is formed as reported before. Besides these amorphous AlOx regions, a few nanometer-sized 3D MoB clusters are found. The advancement of aberration-corrected scanning transmission electron microscopy significantly improves characterization from 1D to 3D defects which is important for thin film materials design for the moderate synthesis temperature range. The reported defects might play an important role in the formation of 2D MoB MBenes.