Theoretical investigation of the MXene precursors MoxV4-xAlC3 (0 ≤ x ≤ 4)

By first-principles total-energy calculations, we investigated the thermodynamic stability of the MAX solid solution MoxV4-xAlC3 in the 0 <= x <= 4 range. Results evidence that lattice parameter a increases as a function of Mo content, while the c parameter reaches its maximum expansion at x = 2.5. After that, a contraction is noticed. Mo occupies V-I sites randomly until the out-of-plane ordered Mo2V2AlC3 alloy is formed. We employed the Defect Formation Energy (DFE) formalism to evaluate the thermodynamic stability of the alloys. Calculations show five stable compounds. At V-rich conditions and from Mo-rich to Mo-moderated conditions, the pristine V4AlC3 MAX is stable. In the region of V-poor conditions, from Mo-rich to Mo-moderated growth conditions, the solid solutions with x = 0.5, 1, and 1.5 and the o-MAX Mo2V2AlC3 are thermodynamically stable. The line profiles of the Electron Localization Function and Bader charge analysis show that the V-C interaction is mainly ionic, while the Mo-C is covalent. Also, the exfoliation energy to obtain a MXene layer is similar to 0.4 eV/angstrom(2). DFE also shows that MXenes exfoliated from the MAX phase with the same Mo content and atomic arrangement are thermodynamically stable. Our results get a deeper atomic scale understanding of the previously reported experimental evidence.

Automated summary

Through first-principles total-energy calculations, we investigated the thermodynamic stability of the MoxV4-xAlC3 MAX solid solution in the 0 ≤ x ≤ 4 range. Results showed that lattice parameters changed with Mo content and the formation of an ordered alloy was observed. Defect Formation Energy (DFE) analysis confirmed the thermodynamic stability of multiple solid solutions and the exfoliation of MXene layers. Our findings provide atomic-scale understanding of previously reported experimental evidence.