Theoretical stability and materials synthesis of a chemically ordered MAX phase, Mo₂ScAlC₂, and its two-dimensional derivate Mo₂ScC₂ MXene

We present theoretical prediction and experimental evidence of a new MAX phase alloy, Mo₂ScAlC₂, with out-of-plane chemical order. Evaluation of phase stability was performed by ab initio calculations based on Density Functional Theory, suggesting that chemical order in the alloy promotes a stable phase, with a formation enthalpy of −24 meV/atom, as opposed to the predicted unstable Mo₃AlC₂ and Sc₃AlC₂. Bulk synthesis of Mo₂ScAlC₂ is achieved by mixing elemental powders of Mo, Sc, Al and graphite which are heated to 1700 °C. High resolution transmission electron microscopy reveals a chemically ordered structure consistent with theoretical predictions with one Sc layer sandwiched between two Mo−C layers. The two-dimensional derivative, the MXene, is produced by selective etching of the Al-layers in hydrofluoric acid, resulting in the corresponding chemically ordered Mo₂ScC₂, i.e. the first Sc-containing MXene. The here presented results expands the attainable range of MXene compositions and widens the prospects for property tuning.