Understanding the Effect of Oxygen on M ₅ AX ₄ 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: Ti_2.5Ta_2.5AlC₄, Ti_2.675Nb_2.325AlC₄, 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.