Order and disorder in quaternary atomic laminates from first-principles calculations

Abstract

We report on the phase stability of chemically ordered and disordered quaternary MAX phases -TiMAlC, TiM2AlC2, MTi2AlC2, and Ti2M2AlC3 where M=Zr,Hf (group IV), M=V,Nb,Ta (group V), and M=Cr,Mo,W (group VI). At 0 K, layered chemically ordered structures are predicted to be stable for M from group V and VI. By taking into account the configurational entropy, an order-disorder temperature Tdisorder can be estimated. TiM2AlC2 (M=Cr,Mo,W) and Ti2M2AlC3 (M=Mo,W) are found with Tdisorder >1773 K and are hence predicted to be ordered at the typical bulk synthesis temperatures of 1773 K. Other ordered phases, even though metastable at elevated temperatures, may be synthesized by non-equilibrium methods such as thin film growth. Furthermore, phases predicted not to be stable in any form at 0 K can be stabilized at higher temperatures in a disordered form, being the case for group IV, for MTi2AlC2 (M=V,Cr,Mo), and for Ti2M2AlC3 (M=V,Ta). The stability of the layered ordered structures with M from group VI can primarily be explained by Ti breaking the energetically unfavorable stacking of M and C where M is surrounded by C in a face-centered cubic configuration, and by M having a larger electronegativity than Al resulting in fewer electrons available for populating antibonding Al-Al orbitals. The results show that these chemically ordered quaternary MAX phases allow for new elemental combinations in MAX phases, which can be used to add new properties to this family of atomic laminates and in turn prospects for tuning these properties.

Magnetic MAX phases from theory and experiments; a review

“…Even though they contain Cr, magnetic characteristics have to date only been explored theoretically [39]. For ternary magnetic MAX phases other than Cr 2 AC with A = Al, Ge, Ga, the first synthesized phase was with Mn as sole M element, Mn 2 GaC, based on guidance from stability calculations with ∆H = -30 meV/atom.…”

“…Examples are (Cr 2/3 Ti 1/3 ) 3 AlC 2 and (Cr 5/8 Ti 3/8 ) 4 AlC 3 [40], as well as the recently discovered Mo 2 Ga 2 C [87,119] which is essentially the same phase as the 211 MAX phase but with two subsequent Ga layers. Another example is (Cr 0.5 V 0.5 ) n+1 AlC n [83] which belongs to a group of materials which can be characterized as being composed of two different MAX phases [39]. The multitude of new phases sharing some of the attributes that make magnetic MAX phases an intriguing research subject indicates, that we are just starting to scratch the surface in this field.…”

Does aluminum play well with others? Intrinsic Al-A alloying behavior in 211/312 MAX phases

“…SQS work because they mimic (with a relatively small supercell) random configurations at least within a limited range around any given site. This approach has been previously used to look into mixing in the M, A and X sublattices of MAX phases [30][31][32][33][34][35][36]. SQS, however, are limited in that they assume a random(-like) state, precluding access to possibly ordered states.…”

Magnetic moment collapse induced axial alternative compressibility of Cr2TiAlC2 at 420 GPa from first principle

“…Formation energy8 computations on some titanium-doping stabilized structures confirm that the energy levels of Al/C species moving towards smaller-energy direction should be responsible for the synthesis of Mo 2 TiAlC 2 . Similarly, the novel Cr 2 TiAlC 2 might also exist special properties.…”