Dataset on the structure and thermodynamic and dynamic stability of Mo2ScAlC2 from experiments and first-principles calculations

The data presented in this paper are related to the research article entitled “Theoretical stability and materials synthesis of a chemically ordered MAX phase, Mo2ScAlC2, and its two-dimensional derivate Mo2ScC” (Meshkian et al. 2017) [1]. This paper describes theoretical phase stability calculations of the MAX phase alloy MoxSc3-xAlC2 (x=0, 1, 2, 3), including chemical disorder and out-of-plane order of Mo and Sc along with related phonon dispersion and Bader charges, and Rietveld refinement of Mo2ScAlC2. The data is made publicly available to enable critical or extended analyzes.


a b s t r a c t
The data presented in this paper are related to the research article entitled "Theoretical stability and materials synthesis of a chemically ordered MAX phase, Mo 2 ScAlC 2 , and its two-dimensional derivate Mo 2 ScC" (Meshkian et al. 2017) [1]. This paper describes theoretical phase stability calculations of the MAX phase alloy Mo x Sc 3-x AlC 2 (x ¼ 0, 1, 2, 3), including chemical disorder and outof-plane order of Mo and Sc along with related phonon dispersion and Bader charges, and Rietveld refinement of Mo 2 ScAlC 2 . The data is made publicly available to enable critical or extended analyzes.
& 2017 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Value of the data
This data allows other researchers to calculate and predict the phase stability of new compounds within the quaternary Mo-Sc-Al-C system and related subsystem.
The data presents refined/calculated structures that can be used as input for further theoretical evaluation of properties.
The structural information can also be used for interpretation and phase identification of, e.g., attained experimental XRD, (S)TEM, and electron diffraction data.

Data
The dataset of this paper provides information for calculated phases within the quaternary Mo-Sc-Al-C system and data obtained from refinement of the XRD pattern. Table 1 provides calculated lattice   Table 1 Calculated lattice parameters, equilibrium total energy E 0 in eV per formula unit, formation enthalpy ΔH cp in meV per atom, and identified equilibrium simplex for Mo 2 ScAlC 2 and Sc 2 MoAlC 2 . For comparison the corresponding end members Mo 3 AlC 2 and Sc 3 AlC 2 are also included.

Phase
Order parameters, formation enthalpy, and equilibrium simplex for the chemically ordered nanolaminates Mo 2 ScAlC 2 and Sc 2 MoAlC 2 with different atomic stacking sequences (described in detail in Fig. 7(a) in Ref. [2]). Table 2 provides information for all considered competing phases within the quaternary system. Fig. 1 show calculated phonon spectra for Mo 2 ScAlC 2 of order A and its corresponding end members Sc 3 AlC 2 and Mo 3 AlC 2 . Fig. 2 depicts calculated Bader charges of atoms in Mo x Sc 3-x AlC 2 (x ¼0, 2, 3). Table 3 shows the data obtained from refinement of the XRD pattern, see Ref. [1]; Lattice vectors a, b and c for the majority phase Mo 2 ScAlC 2 are 3.033, 3.033 and 18.775 Å, respectively.

Experimental design, materials and methods
First-principles calculations were performed by means of density functional theory (DFT) and the projector augmented wave method [3,4] as implemented within the Vienna ab-initio simulation package (VASP) 5.3.3 [5][6][7]. We adopted the non-spin polarized generalized gradient approximation (GGA) as parameterized by Perdew-Burke-Ernzerhof (PBE) [8] 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 we used the Monkhorst-Pack scheme [9]. The calculated total energy of all phases is converged to within 0.5 meV/atom with respect to k-point sampling and structurally optimized in terms of unitcell volumes, c/a ratios (when necessary), and internal parameters to minimize the total energy.
Chemically disordered of Sc and Mo in Mo x Sc 3-x AlC 2 have been modelled using the special quasirandom structure (SQS) method [10,11] on supercells of 4 Â 4 Â 1 M 3 AX 2 unit cells, with a total of 96 M-sites, respectively. Convergence tests with respect to total energy show that these sizes are appropriate to use, based on an energy of the 4 Â 4 Â 1 unit cells being within 2 meV/atom compared to larger supercells.
Evaluation of phase stability was performed by identifying the set of most competing phases at a given composition, i.e. equilibrium simplex, using a linear optimization procedure [11,12] including all competing phases in the system. A phase is considered thermodynamically stable when its energy is lower than the set of most competing phases, and when there is no imaginary frequencies in phonon spectra, i.e. an indicated dynamic stability. The approach has been proven successful to confirm already experimentally known MAX phases as well as to predict the existence of new ones [2,13,14].
Dynamical stability of the chemically ordered Mo x Sc 3-x AlC 2 (x ¼0, 2, 3) structures was evaluated by phonon calculations of 4 Â 4 Â 1 supercells using density functional perturbation theory and as implemented in the PHONOPY code, version 1.9.1 [15,16]. Calculated charges were obtained using Bader charge analysis, version 0.95a [17].  The synthesis of Mo2ScAlC2 were carried out by mixing elemental powders of Mo, Sc, Al and graphite in an agate mortar, put in an alumina crucible, and placed into a sintering furnace where it was heated up to 1700°C and kept at that temperature for 30 min.
θ-2θ X-ray diffraction (XRD) measurements were performed on the samples using a diffractometer