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

TL;DR: In this paper, a MAX phase alloy with out-of-plane chemical order, Mo2ScAlC2, was presented, with a formation enthalpy of −−24meV/atom.

About: This article is published in Acta Materialia.The article was published on 2017-02-15 and is currently open access. It has received 155 citations till now. The article focuses on the topics: Phase (matter) & High-resolution transmission electron microscopy.

Summary

1. Introduction

• It has been about six decades since Nowotny et al. discovered a family of laminated material called H-phases [1].
• To date, more than 70 MAX phases have been synthesized in both bulk and thin film form.
• The first Mo2C MXene was reported in 2015 [18, 19], and has since been found to have high potential for e.g. energy storage, in particular for electrode material in e.g. Li-ion batteries [20].
• Structural and compositional characterization show separation of the elements into individual atomic layers.

2. Computational details

• First-principles calculations were performed by means of density functional theory (DFT) and the projector augmented wave method [23, 24] as implemented within the Vienna abinitio simulation package (VASP) [25-27].
• The authors adopted the non-spin polarized generalized gradient approximation (GGA) as parameterized by Perdew-Burke-Ernzerhof (PBE) [28] for treating electron exchange and correlation effects.
• These are modelled using the special quasi-random structure (SQS) method [30, 31] on supercells of 4×4×1 M3AX2 unit cells, with a total of 96 M-sites, respectively.
• Evaluation of phase stability was performed by identifying the set of most competing phases at a given composition, using a linear optimization procedure [31, 32] including all competing phases in the system.
• This gives an estimate above which temperature the disordered structure is energetically favorable as compared to the ordered structure.

3. Experimental details

• These powders were mixed in an agate mortar and placed in a covered Al2O3 crucible, which was inserted in a tube furnace.
• Both MAX and MXene samples were also characterized by using the Linköping double Cs corrected FEI Titan3 60– 300 operated at 300 kV, equipped with the Super-X EDX system to perform atomic structural analysis.
• The suspension was afterwards filtered and dispersed in water ~10 times in order to remove all the remaining acid and the reaction products.
• Subsequent intercalation of the MXene sheets were realized by adding ~0.1g of the powder in ~1ml of an organic base, tetrabutylammonium hydroxide , and shaking it manually for ~5 min.

4. Results and discussion

• For Mo2ScAlC2 and Sc2MoAlC2, six different layer sequences were considered, see Anasori et al. [37] for layer stacking definitions.
• The in-plane and out-of-plane lattice parameters, a and c, determined from Rietveld refinement, are 3.03 and 18.77 Å, respectively.
• Furthermore, if the M element closest to the Al layer has larger electronegativity than Al, this results in fewer electrons available for populating antibonding Al-Al orbitals, which is energetically expensive [6].
• X-ray diffractograms of the Mo2ScAlC2 MAX powder and its corresponding MXene, Mo2ScC2, after etching and intercalation, is shown in Fig.
• The etching is not fully completed as the scan for the MXene also contains residual Mo2ScAlC2.

5. Conclusions

• The authors have theoretically predicted the existence of a new quaternary MAX phase alloy with out-of-plane chemical order, Mo2ScAlC2, with a Sc atomic layer sandwiched between two Mo-C layers.
• The prediction has been experimentally verified through bulk synthesis and materials characterization, primarily from high resolution STEM with EDX elemental mapping.
• The a and c lattice parameters determined using Rietveld refinement are 3.03 and 18.77 Å, respectively.
• Furthermore, the MAX phase has been converted into twodimensional MXene by selective etching of Al.
• The resulting MXene, Mo2ScC2, is the first MXene to date comprising Sc. 12.

Figures

Fig. 3. XRD scan of (a) Mo2ScAlC2 MAX powder (b) Mo2ScC2 MXene, and (c) MXene after intercalation in TBAOH solution. The c lattice parameters for these samples are 18.77, 24.6 and 34.8 Å, respectively.

Fig. 2. (a) HR(S)TEM micrograph shows the laminated structure of the MAX phase along the [112�0] zone axis. (b) The overlapping EDX elemental map for Mo, Sc and Al reveals the chemically ordered distribution of these elements within the sample.

Fig. 4. (a) An overview STEM micrograph of MoScC MXene sample; (b) left: HRSTEM image, middle: the corresponding EDX map and the HRSTEM image superimposed with the EDX map; (c) a line scan of the EDX map shown by the above light blue arrow.

Fig. 1. XRD analysis; measured scan (red), and the calculated pattern (black) using Rietveld refinement. The blue line is the difference between the two. The Bragg positions for the main phase, Mo2ScAlC2 (top) and the included phases, Mo3Al2C, Mo2C, Al2O3 and Mo3Al can also be seen in the figure.

Frequently asked questions

Q1. What are the future works in "Theoretical stability and materials synthesis of a chemically ordered max phase, mo2scalc2, and its two-dimensional derivate mo2scc2 mxene" ?

The authors have theoretically predicted the existence of a new quaternary MAX phase alloy with out-of-plane chemical order, Mo2ScAlC2, with a Sc atomic layer sandwiched between two Mo-C layers. 

Q2. What are the contributions in "Theoretical stability and materials synthesis of a chemically ordered max phase, mo2scalc2, and its two-dimensional derivate mo2scc2 mxene" ?

The authors 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.