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

Abstract

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 MoC 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.