Abstract
With the recent discovery of in-plane chemically ordered MAX phases (i-MAX) of the general formula ()2AC comes addition of non-traditional MAX phase elements. In the present study, we use density functional theory calculations to investigate the electronic structure, bonding nature, and mechanical properties of the novel (W2/3Sc1/3)2AlC and (W2/3Y1/3)2AlC i-MAX phases. From analysis of the electronic structure and projected crystal orbital Hamilton populations, we show that the metallic i-MAX phases have significant hybridization between W and C, as well as Sc(Y) and C states, indicative of strong covalent bonding. Substitution of Sc for Y (M2) leads to reduced bonding strength for W–C and Al–Al interactions while M2–C and M2–Al interactions are strengthened. We also compare the Voigt–Reuss–Hill bulk, shear, and Young’s moduli along the series of M1 = Cr, Mo, and W, and relate these trends to the bonding interactions. Furthermore, we find overall larger moduli for Sc-based i-MAX phases.
Highlights
Mn+1AXn (MAX) phases, where M is an early transition metal, A is an A-group element and X is C and/or N, are a family of atomically layered ceramics that consist of Mn+1Xn sheets sandwiched in between one atom thick A-layers [1,2,3]
From analysis of the electronic structure and projected crystal orbital Hamilton populations, we show that the metallic in-plane chemically ordered MAX phases (i-MAX) phases have significant hybridization between W and C, as well as Sc(Y) and C states, indicative of strong covalent bonding
All calculations were performed within the framework of density functional theory as implemented in the Vienna ab initio simulation package [32,33,34], combined with the Perdew–Burke–Ernzerhof generalized gradient approx imation (PBE-GGA) [35] and the projector augmented wave (PAW) method [36, 37]
Summary
Mn+1AXn (MAX) phases, where M is an early transition metal, A is an A-group element (mostly fro m group 13 and 14) and X is C and/or N, are a family of atomically layered ceramics that consist of Mn+1Xn sheets sandwiched in between one atom thick A-layers [1,2,3]. We have used first-principles calculations to investigate the electronic, vibrational, and mechanical properties of (W2/3Sc1/3)2AlC and (W2/3Y1/3)2AlC i-MAX phases, motivated by their recent discovery, and being the first W-based MAX phase materials. Both (W2/3Sc1/3)2AlC and (W2/3Y1/3)2AlC are stable with a calculated formation enthalpy of −27 and −22 meV/atom, respectively [30]. The finding of i-MAX phases, allowing introduction of non-traditional MAX phase elements like Sc, Y and W, may alter or introduce new properties as compared to previously known MAX phases This motivates their exploration, for fundamental understanding, and potential future property tailoring. Schematics were produced with VESTA [42]
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