Elsevier

Acta Materialia

Volume 125, 15 February 2017, Pages 476-480
Acta Materialia

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

https://doi.org/10.1016/j.actamat.2016.12.008Get rights and content

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 Mosingle bondC 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.

Introduction

It has been about six decades since Nowotny et al. discovered a family of laminated material called H-phases [1]. After their revival by Barsoum et al. some decades later [2], the family was expanded and given the nomenclature Mn+1AXn (MAX) phases, n = 1–3, being composed of an early transition metal (M), an A-group element primarily from group 13 and 14 (A), and carbon and/or nitrogen (X). These compounds are inherently laminated, and exhibit a combination of metallic and ceramic properties which stem from strong metallic-covalent M-X bonds in combination with weaker bonding between M-A atoms. Consequently, MAX phases display high electrical and thermal conductivity, good resistance to oxidation and thermal shock, and are elastically stiff and easily machinable. To date, more than 70 MAX phases have been synthesized in both bulk and thin film form.

Substitution of a fraction of M, A, or X atoms can be beneficial for property tuning, e.g., for increasing the hardness [3], or for introducing magnetic properties [4], [5]. MAX phase alloys to date are to a major extent solid solutions, and in particular alloys of 211 (n = 1) stoichiometry have not shown any tendency to order in atomic layers composed of one element only, possibly due to a high configurational entropy within these systems and only one crystallographic site for each M, A, and X element [6]. This is opposed to quaternary MAX phases of 312 (n = 2) or 413 (n = 3) stoichiometry and with M-site alloying, which can display an out-of-plane chemical order. Such examples are the recently reported Mo2TiAlC2 and Mo2Ti2AlC3, which were theoretically predicted and subsequently synthesized by Anasori et al. [7]. This is in addition to previously discovered Cr2TiAlC2 and V1.5Cr1.5AlC2, reported by Liu et al. [8] and Caspi et al. [9], respectively. Note that for V1.5Cr1.5AlC2, a partially ordered structure has been observed. In an explanatory and predictive theoretical study by Dahlqvist et al. [6], the authors have investigated the stability of TiMAlC, TiM2AlC2, MTi2AlC2, and Ti2M2AlC3 where M is from group 4–6 in the Periodic table of elements, trying to identify the origin behind the chemical ordering. Extending beyond that study, exploring a combination of M elements that can neither be found in a pure 312 MAX phase nor energetically promote a stacking in which M is surrounded by C in a face-centered cubic (fcc) configuration, we have here investigated quaternary MAX phases in the Mosingle bondScsingle bondAlsingle bondC system.

Interest in Al-containing MAX phases increased after evidence of their resistance to oxidation upon formation of protective oxide layers [10], [11], also used in studies focused towards crack healing [12], [13]. Moreover, selective etching of Al has been shown to produce MXenes, graphene analogous materials that are both electrically conducting and hydrophilic [14]. The quest for Mo-containing MXenes in particular was elevated after a number of theoretical studies, predicting these compounds as promising thermoelectric material [15], as catalyst [16] and also as efficient electrodes for Li-ion batteries [17]. 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].

There is only one previous report stating synthesis of a Sc-based MAX phase; Sc2InC [1], [21]. However, no information is presented on the specific synthesis conditions, and no experimental evidence of the resulting material or its properties. There is a theoretical report on the structural and elastic properties of a number of known and hypothetical M2InC phases with M = Sc, Ti, V, Nb, Zr, Hf and Ta. Beside the calculated crystal parameters, the authors have reported the theoretical Young's, shear, and bulk moduli, which for Sc2InC are well below the other phases investigated [22].

Consequently, exploring synthesis of a MAX phase based on Al, Mo and Sc is highly motivated from a fundamental as well as a property perspective. In the present study, we have theoretically predicted and experimentally verified the existence of Mo2ScAlC2 as a new chemically ordered MAX phase. Structural and compositional characterization show separation of the elements into individual atomic layers. Furthermore, we present evidence of the corresponding MXene; Mo2ScC2.

Section snippets

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 ab-initio simulation package (VASP) [25], [26], [27]. 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. A plane-wave energy cut-off of 400 eV was used and for sampling of the Brillouin zone

Experimental details

Elemental powders of graphite (99.999%), Mo (99.99%), (Sigma-Aldrich), Sc (99.99%, Stanford Advanced Material), Al (99.8%, ALFA AESAR) with mesh sizes of 200, 400, 200 and 200, respectively, were used for the materials synthesis. These powders were mixed in an agate mortar and placed in a covered Al2O3 crucible, which was inserted in a tube furnace. This was heated at a rate of 10 °C per minute up to 1700 °C, where it was kept for 30 min, with a resulting total duration of 4 h. After cooling

Results and discussion

For Mo2ScAlC2 and Sc2MoAlC2, six different layer sequences were considered, see Anasori et al. [37] for layer stacking definitions. In addition, a solid solution of Sc and Mo on the M-sites was also taken into account, see Data in Brief Table 1.

Only Mo2ScAlC2 of order A, i.e., with Sc at Wyckoff site 2a and Mo at site 4f, is predicted stable at 0 K with a calculated formation enthalpy of −24 meV/atom. A complete list of considered competing phases used for the phase stability evaluation can be

Conclusions

We 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 Mosingle bondC 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 XRD Rietveld refinement are 3.03 and 18.77 Å, respectively. Furthermore, the MAX phase has been

Acknowledgement

J. R. acknowledges funding from the Swedish Research Council (VR) under grant no. 621-2012-4425 and 642-2013-8020, from the the Knut and Alice Wallenberg (KAW) Foundation, and from the Swedish Foundation for Strategic Research (SSF) through the synergy grant FUNCASE. L.H. acknowledges the KAW Foundation for a Scholar Grant and support to the Linköping Ultra Electron Microscopy Laboratory. The simulations were carried out using supercomputer resources provided by the Swedish National

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