Theoretical study of thermodynamic and magnetic properties of transition metal carbide and nitride MAX phases
New publication
2023/04/24
![Thermodynamic (meta)stability of individual MAX phases is inferred by the sign and magnitude of the calculated maximum formation enthalpies, relative to all possible combinations of known competing phases, including unaries, binaries, and ternaries, that are present in the M-A-X phase diagram [top panel; see Eq. (1)]. Boundary phases are retrieved from MP [28] and OQMD [21, 22]. The bottom panel contains crystal structures of (3×3×1) supercells of n=1,2, and 3 MAX phases along with other exemplary unary, binary, and ternary phases.](/media-einrichtungen/sfb270/news_12/B07_Publ_Thermodynamic_MAX_phase_870x489.png)
A. M. Malik, J. Rohrer, and K. Albe
Phys. Rev. Materials 7, 044408 (2023).
We systematically perform density-functional theory (DFT) calculations for all possible Mn+1AXn (MAX) phases with transition metal M=Sc to Au (excluding Tc), A in group IIIA-IVA, X=C, N, and n=1,2,3, a total of about 1200 systems. The thermodynamic stability is determined by comparing the formation enthalpy (at 0 K) against all possible combinations of unary, binary, and ternary boundary phases (available from online DFT databases). Thereby, we identify 124 so far unknown phases (in terms of both experimental synthesis and other theoretical predictions), of which 54 are carbides and 70 are nitrides. Among all stable MAX phases, we identify nine with magnetic properties. In addition to already known and synthesized magnetic phases (Cr2AlC, Cr2GeC, Cr2GaN, and Mn2GaC), we predict five more MAX phases with magnetic ordering [Mn2A(=Ge, Sn)C, Cr3A(=Ga, Ge)N2, and Cr4A(=Ge)N3]. Evaluating previously suggested descriptors for the stability of MAX phases [valence electron concentrations (VECs), differences in atomic radius difference ΔRat, and differences in electronegativities Δχ], we find that ΔRat does not correlate with stability and stable phases are characterized by VEC<5.5, Δχ>1.5. The reverse is, however, not true; for example, a MAX phase with VEC<5.5 and Δχ>1.5 is not necessarily stable.
