Experimental and theoretical characterization of ordered MAX phases Mo2TiAlC2 and Mo2Ti2AlC3

TLDR: In this article, the phase stabilities and crystal structures of two newly discovered ordered, quaternary MAX phases were reported by mixing and heating different elemental powder mixtures of mMo:(3m)Ti:1.1Al:2C with 1.5

Abstract: Herein, we report on the phase stabilities and crystal structures of two newly discovered ordered, quaternary MAX phases—Mo2TiAlC2 and Mo2Ti2AlC3—synthesized by mixing and heating different elemental powder mixtures of mMo:(3-m)Ti:1.1Al:2C with 1.5 ≤ m ≤ 2.2 and 2Mo: 2Ti:1.1Al:2.7C to 1600 °C for 4 h under Ar flow. In general, for m ≥ 2 an ordered 312 phase, (Mo2Ti)AlC2, was the majority phase; for m < 2, an ordered 413 phase (Mo2Ti2)AlC3, was the major product. The actual chemistries determined from X-ray photoelectron spectroscopy (XPS) are Mo2TiAlC1.7 and Mo2Ti1.9Al0.9C2.5, respectively. High resolution scanning transmission microscopy, XPS and Rietveld analysis of powder X-ray diffraction confirmed the general ordered stacking sequence to be Mo-Ti-Mo-Al-Mo-Ti-Mo for Mo2TiAlC2 and Mo-Ti-Ti-Mo-Al-Mo-Ti-Ti-Mo for Mo2Ti2AlC3, with the carbon atoms occupying the octahedral sites between the transition metal layers. Consistent with the experimental results, the theoretical calculations clearly show that M l...

Figures

FIG. 1. XRD patterns of 5 different starting compositions—noted below each pattern. Black squares represent Si peaks, added ( 10 wt. %) as an internal standard before the XRD measurements were taken. Bragg reflections’

TABLE I. Phase compositions and lattice parameters of the various starting compositions tested herein determined from Rietveld analysis of XRD data. Also, listed are the minor impurity phases and their wt. % and the v2 of the Rietveld refinements. The numbers in parentheses are the uncertainties on the last digit.

TABLE IV. Summary of unit cell volumes, a and c lattice parameters, the calculated total energy, E0, of quaternaries in the MomTi4-mAlC3 as a function of m. Also listed are the formation energies relative to those of the most competing phases, DHcp, listed in the last column. The letters in brackets in the first column refer to those shown in Fig. 7(b) and Fig. 8(b). SQS refers to quasi random structures.

FIG. 3. (a) HRSTEM of Mo2TiAlC2 along the [11 20] zone axis, EDS mapping on (a) for: (b) Mo, (c) Ti, (d) Al. (e) Overlap of (b) and (c). (f) Overlap of (a) and (e) showing the Mo atoms in red, Ti atoms in green and Al atoms in blue, (g) EDS line scan profile of Mo, Ti, and Al over the green arrow shown in g.

TABLE III. Summary of unit cell volumes, a and c lattice parameters, the calculated total energy E0, of quaternaries in the MomTi3-mAlC2 as a function of m. Also listed are the formation energies relative to those of the most competing phases, DHcp, listed in the last column. The letters in brackets in the first column refer to those shown in Fig. 7(a). SQS refers to quasi random structures.

FIG. 13. Electronic band structure of type A, (a) Mo2TiAlC2 and (b) Mo2Ti2AlC. Horizontal solid line at 0 represents the Fermi level Ef.

Frequently asked questions

Q1. What were the parameters used to analyze the XRD diffractograms?

29,30 Refinement parameters were: five background parameters, scale factors from which relative phase fractions are evaluated, X and Y profile parameters for peak width limited to the major phases, lattice parameters (LPs), isotropic global atomic displacement parameter for the major phases, Mo/Ti intermixing, and atomic positions for all phases. 

Q2. What contributions have the authors mentioned in the paper "Experimental and theoretical characterization of ordered max phases mo2tialc2 and mo2ti2alc3" ?

Anasori et al. this paper showed that despite the fact that Ti3AlC2 is stable, Mo2TiAlC 2 is not stable. 

Q3. What was the XRD method used to determine the composition of Mo2TiAl?

High-resolution scanning transmission electron microscope (HRSTEM) micrographs and EDS spectra were obtained on individual Mo2TiAlC2 and Mo2Ti2AlC3 particles using the Link€oping double corrected FEI Titan3 60–300 operated at 300 kV, equipped with the Super-X EDS system. 

Q4. How is the resistivity of the Mo2TiAlC2 sample measured?

At 150 lX cm, the measured resistivity value of the Mo2TiAlC2 sample at 10 K is lower than that of the Mo2Ti2AlC3 (817 lX cm) sample. 

Q5. What is the effect of Mo on the binding energy of the Ti species?

The XPS analysis shows almost no influence of Mo on the binding energy of the Ti species, whereas the binding energies of the Mo species are closer to those of Mo2C than to Mo metal. 

Q6. What are the elastic constants of type A Mo2TiAlC2 and Mo?

Calculated elastic constants Cij, elastic moduli BV , GV , E, Poisson 0s ratio, , anisotropy factor, A, and theoretical densities of Ti3AlC2, Mo3AlC2, Mo2TiAlC2 (type A) Ti4AlC3, Mo4AlC3, and Mo2Ti2AlC3 (type A). 

Q7. What was the intensity ratio of the peaks of the Ti and Mo components?

For the Ti 2p3/2 and 2p1/2, and Mo 3d5/2 3d3/2 components, the intensity ratios of the peaks were constrained to be 2:1 and 2:3, respectively. 

Q8. What is the criterion for the atomic distribution of the M-sites?

Solid solutions on the M-sites, full or partial, were modeled using the special quasi-random structure (SQS) method,37,38 where an appropriate supercell was chosen based on the criterion to mimic an atomic distribution in a random alloy, i.e., M-site correlation functions equal to zero. 

Q9. What is the lowest energy configuration of the quaternary Mo2TiAlC?

Consistent with the experimental results presented in this and previous work,27 its lowest energy configuration is when it is ordered, with the Molayers sandwiching both the Al and Ti-C layers.