Understanding the Effect of Oxygen on M5AX4 Structure, Stability, and Mechanical Properties.

M5X4, the newest and thickest structures in the MXene family, shows promise as mechanically robust nanomaterials. However, the essential role of oxide in their synthesis is poorly understood, which poses a challenge for discovering new M5AX4 MAX phase precursors. One possibility is that oxygen dissolves into the carbon sublattice, forming stable oxycarbide layers within the MAX phase. Herein, we investigate the layer-by-layer elemental composition of three M5AX4 compositions: Ti2.5Ta2.5AlC4, Ti2.675Nb2.325AlC4, and Mo4VAlC4. By modeling the structural stability of each composition, we investigate the possible stabilizing role of oxygen. To guide future application of M5X4 MXenes, we also calculate the electronic structure and mechanical properties of the parent M5AX4 MAX phases. This work clarifies the role of oxygen incorporation into MAX phases and its implications for the synthesis and potential applications of their MXene derivatives.