Research on low-dimensional materials has increased drastically in the last decade, with the discovery of two-dimensional transition metal carbides and nitrides (MXenes) produced by atom-selective chemical etching of laminated parent $M_{n+1}A{X}_n$ (MAX) phases. Here, we apply density functional theory and subsequent materials synthesis and analysis to explore the phase stability and Mo/Sc intermixing on the M site in the chemically ordered quaternary $i-\mathrm{MAX}$ phase (${\mathrm{Mo}}_x{\mathrm{Sc}}_{1-x}{)}_2\mathrm{AlC}$. Transmission electron microscopy confirms the theoretical predictions of preferential in-plane ordering of Mo and Sc, with the highest crystal quality obtained for the ideal Mo:Sc ratio of 2:1 (predicted as the most stable), as well as a retained $i-\mathrm{MAX}$ structure even for an increased relative Sc content, with Sc partially occupying Mo sites. The results are supported by refined neutron diffraction data, which show space group $C2/c$ (no. 15), and a C occupancy of 1. Subsequent chemical etching produces MXene for $x=0.66$, while for $x=0.33$ and 0.5 no MXene is observed. These results demonstrate that a precise control of the $i-\mathrm{MAX}$ composition is crucial for derivation of MXene, with a MXene quality optimized for a Mo:Sc ratio of 2:1 with minimal intermixing between Mo and Sc.

• Received 22 July 2019

DOI:https://doi.org/10.1103/PhysRevMaterials.3.113607