# 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 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.

## Summary (2 min read)

### 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.

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...We adopted the non-spin polarized generalized gradient approximation (GGA) as parameterized by Perdew-Burke-Ernzerhof (PBE) [28] for treating electron exchange and correlation effects....

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###### 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.