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 degrees 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. (C)2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.