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Best Experts & Science about Max Phases


Barsoum, Michel W

On the Chemical Diversity of the max phases

The M n+1 AX n , or max, phases are nanolayered, hexagonal, machinable, early transition-metal carbides and nitrides, where n = 1, 2, or 3, M is an early transition metal, A is an A-group element (mostly groups 13 and 14), and X is C and/or N. These phases are characterized by a unique combination of both metallic and ceramic properties. The fact that these phases are precursors for MXenes and the dramatic increase in interest in the latter for a large host of applications render the former even more valuable. Herein we describe the structure of most, if not all, max phases known to date. This review covers ≈155 max compositions. Currently, 16 A elements and 14 M elements have been incorporated in these phases. The recent discovery of both quaternary in- and out-of-plane ordered max phases opens the door to the discovery of many more. The chemical diversity of the max phases holds the key to eventually optimizing properties for prospective applications. Since many of the newer quaternary (and higher)phases have yet to be characterized, much work remains to be done.

Crystallographic evolution of max phases in proton irradiating environments

This work represents the first use of proton irradiation to simulate in-core radiation damage in Ti3SiC2 and Ti3AlC2 max phases. Irradiation experiments were performed to 0.1 dpa at 350 °C, with a damage rate of 4.57 × 10−6 dpa s−1. The max phases displayed significant dimensional instabilities at the crystal level during irradiation leading to large anisotropic changes in lattice parameter, even at low damage levels. The instabilities were accompanied by a decomposition of the Ti-based max phases to their binary constituents, TiC. Experimentally observed changes in lattice parameter have been correlated with density functional theory modelling. The most energetically favourable and/or most difficult to recombine defects considered were an M-A antisite ({MA:AM}), and carbon Frenkel ({VC:Ci}). It is proposed that antisite defects, {MA:AM}, are the main contributor to the observed changes in lattice parameter. The proposed mechanism reported in this work potentially enables to design max phase compositions, which do not favour antisite defect accumulation. In addition, comparison between the experimental results and theoretical calculations shows that a greater amount of residual damage remains in Ti3AlC2 when compared to Ti3SiC2 after the same irradiation treatment.

Chroneos, Alexander

312 max phases: Elastic Properties and Lithiation

Interest in the Mn+1AXn phases (M = early transition metal; A = group 13–16 elements, and X = C or N) is driven by their ceramic and metallic properties, which make them attractive candidates for numerous applications. In the present study, we use the density functional theory to calculate the elastic properties and the incorporation of lithium atoms in the 312 max phases. It is shown that the energy to incorporate one Li atom in Mo3SiC2, Hf3AlC2, Zr3AlC2, and Zr3SiC2 is particularly low, and thus, theoretically, these materials should be considered for battery applications.

Electronic structures, bonding natures and defect processes in Sn-based 211 max phases

Abstract The electronic structure, bonding natures, and defect processes of the new superconducting max phase Lu2SnC are investigated by using density functional theory, and are compared to other existing M2SnC phases. The formation of M2SnC max phases is exothermic and these compounds are intrinsically stable in agreement with experiment. The finite value of DOS, in addition to the d-resonance at the vicinity of the Fermi level, indicates a metallic nature and conductivity of M2SnC max phases. The strength of the covalent M–C bond is higher than that of the covalent M–Sn bond. The calculated effective valence charge also indicates the dominance of covalency in the chemical bonding in the studied compounds. The charge transfer in M2SnC phases indicates the ionic nature of their chemical bonds. The ionic character of their chemical bonds can also be understood from the spherical nature of charge distribution in their contour maps of electron charge density. Therefore, the overall bonding nature in the studied M2SnC max phases is a combination of metallic, covalent, and, ionic. The bond length is directly proportional to the crystal radius, while bond covalency is inversely proportional to the crystal radius. Additionally, the Fermi surface topology is also investigated. Considering the intrinsic defect processes it is calculated that Nb2SnC is the material that is predicted to have better radiation tolerance.

Dahlqvist, Martin

Predictive theoretical screening of phase stability for chemical order and disorder in quaternary 312 and 413 max phases

In this work we systematically explore a class of atomically laminated materials, Mn+1AXn (max) phases upon alloying between two transition metals, M′ and M′′, from groups III to VI (Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W). The materials investigated focus on so called o-max phases with out-of-plane chemical ordering of M′ and M′′, and their disordered counterparts, for A = Al and X = C. Through use of predictive phase stability calculations, we confirm all experimentally known phases to date, and also suggest a range of stable ordered and disordered hypothetical elemental combinations. Ordered o-max is favoured when (i) M′ next to the Al-layer does not form a corresponding binary rock-salt MC structure, (ii) the size difference between M′ and M′′ is small, and (iii) the difference in electronegativity between M′ and Al is large. Preference for chemical disorder is favoured when the size and electronegativity of M′ and M′′ is similar, in combination with a minor difference in electronegativity of M′ and Al. We also propose guidelines to use in the search for novel o-max; to combine M′ from group 6 (Cr, Mo, W) with M′′ from groups 3 to 5 (Sc only for 312, Ti, Zr, Hf, V, Nb, Ta). Correspondingly, we suggest formation of disordered max phases by combing M′ and M′′ within groups 3 to 5 (Sc, Ti, Zr, Hf, V, Nb, Ta). The addition of novel elemental combinations in max phases, and in turn in their potential two-dimensional MXene derivatives, allow for property tuning of functional materials.

Atomically Layered and Ordered Rare-Earth i-max phases: A New Class of Magnetic Quaternary Compounds

In 2017, we discovered quaternary i-max phases – atomically layered solids, where M is an early transition metal, A is an A group element, and X is C – with a (M12/3M21/3)2AC chemistry, where the M1 and M2 atoms are in-plane ordered. Herein we report on the discovery of a new class of magnetic i-max phases in which bi-layers of a quasi-2D magnetic frustrated triangular lattice overlays a Mo honeycomb arrengement and an Al Kagomé lattice. The chemistry of this family is (Mo2/3RE1/3)2AlC, and the rare earth elements, RE, are Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, and Lu. The magnetic properties were characterized and found to display a plethora of ground states, resulting from an interplay of competing magnetic interactions in the presence of magnetocrystalline anisotropy.

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