We report the synthesis of three out-of-plane chemically ordered quaternary transition metal borides (o-MAB phases) of the chemical formula M4CrSiB2 (M = Mo, W, Nb). The addition of these phases to the recently discovered o-MAB phase Ti4MoSiB2 shows that this is indeed a new family of chemically ordered atomic laminates. Furthermore, our results expand the attainable chemistry of the traditional M5SiB2 MAB phases to also include Cr. The crystal structure and chemical ordering of the produced materials were investigated using high-resolution scanning transmission electron microscopy and X-ray diffraction by applying Rietveld refinement. Additionally, calculations based on density functional theory were performed to investigate the Cr preference for occupying the minority 4c Wyckoff site, thereby inducing chemical order.

A desired prerequisite when performing a quantum mechanical calculation is to have an initial idea of the atomic positions within an approximate crystal structure. The atomic positions combined should result in a system located in, or close to, an energy minimum. However, designing low-energy structures may be a challenging task when prior knowledge is scarce, specifically for large multi-component systems where the degrees of freedom are close to infinite. In this paper, we propose a method for identification of low-energy crystal structures within multi-component systems by combining cluster expansion and crystal structure predictions with density-functional theory calculations. Crystal structure prediction searches are applied to the Mo2AlB2 and Sc2AlB2 ternary systems to identify candidate structures, which are subsequently used to explore the quaternary (pseudo-binary) (MoxSc1-x)(2)AlB2 system through the cluster expansion formalism utilizing the ground-state search approach. Furthermore, we show that utilizing low-energy structures found within the cluster expansion ground-state search as seed structures within crystal structure predictions of (MoxSc1-x)(2)AlB2 can significantly reduce the computational demands. With this combined approach, we not only correctly identified the recently discovered Mo(4/3)Sc(2/3)AlB(2)i-MAB phase, comprised of in-plane chemical ordering of Mo and Sc and with Al in a Kagome lattice, but also predict additional low-energy structures at various concentrations. This result demonstrates that combining crystal structure prediction with cluster expansion provides a path for identifying low-energy crystal structures in multi-component systems by employing the strengths from both frameworks.

MXenes are two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides typically synthesized from layered MAX-phase precursors. With over 50 experimentally reported MXenes and a near-infinite number of possible chemistries, MXenes make up the fastest-growing family of 2D materials. They offer a wide range of properties, which can be altered by their chemistry (M, X) and the number of metal layers in the structure, ranging from two in M2XTx to five in M5X4T x . Only one M5X4 MXene, Mo4VC4, has been reported. Herein, we report the synthesis and characterization of two M(5)AX(4) mixed transition metal MAX phases, Ti2.5Ta2.5AlC4 and Ti2.675Nb2.325AlC4, and their successful topochemical transformation into Ti2.5Ta2.5C4T x and Ti2.675Nb2.325C4Tx MXenes. The resulting MXenes were delaminated into single-layer flakes, analyzed structurally, and characterized for their thermal and optical properties. This establishes a family of M(5)AX(4) MAX phases and their corresponding MXenes. These materials were experimentally produced based on guidance from theoretical predictions, leading to more exciting applications for MXenes.

The recent discovery of chemical ordering in quaternary borides offers new ways of exploring properties and functionalities of these laminated phases. Here, we have synthesized and investigated chemical ordering of the laminated Mo4MnSiB2 (T2) phase, thereby introducing a magnetic element into the family of materials coined o-MAB phases. By X-ray diffraction and scanning transmission electron microscopy, we provide evidence for out-of-plane chemical ordering of Mo and Mn, with Mo occupying the 16l site and Mn preferentially residing in the 4c site. Mn and B constitute quasi-two-dimensional layers in the laminated material. We have therefore also studied the magnetic properties by magnetometry, and no sign of long-range magnetic order is observed. An initial assessment of the magnetic ordering has been further studied by density functional theory (DFT) calculations, and while we find an antiferromagnetic configuration to be the most stable one, ferromagnetic ordering is very close in energy.

Herein we report on the synthesis of a metastable (Cr,Y)(2)AlC MAX phase solid solution by co-sputtering from a composite Cr-Al-C and elemental Y target, at room temperature, followed by annealing. However, direct high-temperature synthesis resulted in multiphase films, as evidenced by X-ray diffraction analyses, room-temperature depositions, followed by annealing to 760 degrees C led to the formation of phase pure (Cr,Y)(2)AlC by diffusion. Higher annealing temperatures caused a decomposition of the metastable phase into Cr2AlC, Y5Al3, and Cr-carbides. In contrast to pure Cr2AlC, the Y-containing phase crystallizes directly in the MAX phase structure instead of first forming a disordered solid solution. Furthermore, the crystallization temperature was shown to be Y-content dependent and was increased by similar to 200 degrees C for 5 at.% Y compared to Cr2AlC. Calculations predicting the metastable phase formation of (Cr,Y)(2)AlC and its decomposition are in excellent agreement with the experimental findings.

Inspired by the recent discovery of Ti4MoSiB2, a quaternary phase with out-of-plane chemical order that we denote as o-MAB, we perform an extensive first-principles study to explore the attained chemical order and disorder (solid-solution) upon metal alloying of M(5)AB(2) (T2 phases), with M from Groups 3 to 9 and A = Al, Si, P, Ga, and Ge. We show that the attainable chemistries of T2 can be significantly expanded and predict 35 chemically ordered o-MAB phases and 121 solid solutions of an MM-4 AB(2) stoichiometry. The possibility of realizing o-MAB or solid solution MAB phases combined with multiple elemental combinations previously not observed in these borides suggests an increased property tuning potential. Furthermore, five ternary T2 phases, yet to be synthesized, are also predicted to be stable.

We perform a theoretical study of the electronic structure and magnetic properties of the prototypical magnetic MAX-phase Mn2GaC with the main focus given to the origin of magnetic interactions in this system. Using the density functional theory+dynamical mean-field theory (DFT+DMFT) method, we explore the effects of electron-electron interactions and magnetic correlations on the electronic properties, magnetic state, and spectral weight coherence of paramagnetic and magnetically ordered phases of Mn2GaC. We also benchmark the DFT-based disordered local moment approach for this system by comparing the obtained electronic and magnetic properties with that of the DFT+DMFT method. Our results reveal a complex magnetic behavior characterized by a near degeneracy of the ferro-and antiferromagnetic configurations of Mn2GaC, implying a high sensitivity of its magnetic state to fine details of the crystal structure and unit-cell volume, consistent with experimental observations. We observe robust local-moment behavior and orbital-selective incoherence of the spectral properties of Mn2GaC, implying the importance of orbital-dependent localization of the Mn 3d states. We find that Mn2GaC can be described in terms of local magnetic moments, which may be modeled by DFT with disordered local moments. However, the magnetic properties are dictated by the proximity to the regime of formation of local magnetic moments, in which the localization is in fact driven by Hunds exchange interaction, and not the Coulomb interaction.

Recent experiments [Barbero et al. Phys. Rev. B 95, 094505 (2017)] have established that bulk superconductivity (T_c ∼ 8.3-8.7 K) can be induced in AlB₂-type ZrB₂ and HfB₂, highly covalent refractory ceramics, by vanadium (V) doping. These AlB₂-structured phases provide an alternative to earlier diamon-like or diamond-based superconducting and superhard materials. However, the underlying mechanism for doping-induced superconductivity in these materials is yet to be addressed. In this paper, we have used first-principles calculations to probe electronic structure, lattice dynamics, and electron-phonon coupling (EPC) in V-doped ZrB₂ and consequently examine the origin of the superconductivity. We find that, while doping-induced stress weakens the EPC, the concurrently induced charges strengthen it. The calculated critical transition temperature (T_c) in electron (and V)-doped ZrB₂ is at least one order of magnitude lower than experiments, despite considering the weakest possible Coulomb repulsion between electrons in the Cooper pair, hinting a complex origin of superconductivity in it.

In this study, we have used density functional theory (DFT) calculations to characterize if and how defects influence the stability and electronic/mechanical properties of MB2 (AlB2-type) for different transition metal M. From a point defect analysis including vacancies, interstitials, and anti-sites, we identify vacancies to be most favored, or least unfavored. To provide insight into possible vacancy ordering, we focus on vacancies on M- and B-sublattices for nine metals (M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W), modelled both as disordered and ordered. We demonstrate and explain why vacancies have a significant impact for M from Group 4 (Ti, Zr, Hf), Group 5 (Nb, Ta) and 6 (Mo, W) with improved thermodynamical and dynamical stability as well as mechanical properties. This by diverging from the ideal composition through controlled off-stoichiometry in terms of vacancies in M- or B-deficient structures. Line compounds TiB2, ZrB2 and HfB2 account for B-poor or M-rich conditions by forming planar defects comprised of vacant B. This in contrast to the ordered M- and B vacancies identified for MoB2 and WB2, with an optimal result at 33.33% M- and 25% B-vacancies, respectively, which significantly improves the stability and concurrent properties through elimination of antibonding states and minimization of non-bonding states. Similar behavior with enhanced stability and properties is demonstrated for NbB2 and TaB2 with an optimum around 10% M- and 17% B-vacancies, respectively.

The laminated ternary boride Mo5SiB2 of T2 structure have two symmetrically inequivalent metallic sites, 16l and 4c, being occupied in a 4:1 ratio. The phase was recently shown to be stable for 80% substitution of Mo for Ti, at the majority site, forming an out-of-plane chemically ordered quaternary boride: Ti4MoSiB2. Considering that the hypothetical Ti5SiB2 is theoretically predicted as not stable, a key difference in bonding characteristics is indicated for full substitution of Mo for Ti at the metallic sites. To explore the origin of formation of Ti4MoSiB2, we here investigate the electronic properties and bonding characteristics of Mo5SiB2, Ti4MoSiB2 and Ti5SiB2 through their density of states, projected crystal orbital Hamilton population (pCOHP), Bader charge partitioning and second order force constants. The bond between the two different metallic sites is found to be key to the stability of the compounds, evident from the pCOHP of this bond showing a peak of bonding states close to the Fermi level, which is completely filled in Mo5SiB2 and Ti4MoSiB2, while only partially filled in Ti5SiB2. Furthermore, the lower electronegativity of Ti compared to Mo results in charge accumulation at the Si and B sites, which coincides with a reduced bond strength in Ti5SiB2 compared to Mo5SiB2 and Ti4MoSiB2. Bandstructure calculations show that all three structures are metallic. The calculated mechanical and elastic properties show reduced bulk (B) and elastic (E) moduli when introducing Ti in Mo5SiB2, from 279 and 365 GPa to 176 and 258 GPa, respectively. The Pugh criteria indicates also a slight reduction in ductility, with a G/B ratio increasing from 0.51 to 0.59.

Planar defect structures appearing in transition metal diboride (TMB2) thin films, grown by different magnetron sputtering-deposition approaches over a wide compositional and elemental range, were systematically investi-gated. Atomically resolved scanning transmission electron microscopy (STEM) imaging, electron energy loss spec-troscopy (EELS) elemental mapping, and first principles calculations have been applied to elucidate the atomic structures of the observed defects. Two distinct types of antiphase boundary (APB) defects reside on the {1(1) over bar 00} planes. These defects are without (named APB-1) or with (APB-2) local deviation from stoichiometry. APB-2 de-fects, in turn, appear in different variants. It is found that APB-2 defects are governed by the films composition, while APB-1 defects are endemic. The characteristic structures, interconnections, and circumstances leading to the formation of these APB-defects, together with their formation energies, are presented.

MAX phases (M = metal, A = A-group element, X = C and/or N) are layered materials, combining metallic and ceramic attributes. They are also parent materials for the two-dimensional (2D) derivative, MXene, realized from selective etching of the A-element. In this work, we present a historical survey of MAX phase alloying to date along with an extensive theoretical investigation of MAX phase alloys (M = Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Fe, Co, and Ni, A = Al, Ga, In, Si, Ge, Sn, Ni, Cu, Zn, Pd, Ag, Pt, and Au, and X = C). We assess both in-plane chemical ordering (in the so-called i-MAX phases) and solid solution. Out of the 2702 compositions, 92 i-MAX and 291 solid solution MAX phases are predicted to be thermodynamically stable. A majority of these have not yet been experimentally reported. In general, i-MAX is favored for a smaller size of A and a large difference in metal size, while solid solution is favored for a larger size of A and with comparable size of the metals. The results thus demonstrate avenues for a prospective and substantial expansion of the MAX phase and MXene chemistries.

In the quest for finding novel thermodynamically stable, layered, MAB phases promising for synthesis, we herein explore the phase stability of ternary MAB phases by considering both orthorhombic and hexagonal crystal symmetries for various compositions (MAB, M₂AB₂, M₃AB₄, M₄AB₄, and M₄AB₆ where M = Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Fe, and Co, A = Al, Ga, and In, and B is boron). The thermodynamic stability of seven previously synthesized MAB phases is confirmed, three additional phases are predicted to be stable, and 23 phases are found to be close to stable. Furthermore, the crystal symmetry preference for forming orthorhombic or hexagonal crystal structures is investigated where the considered Al-based MAB phases tend to favour orthorhombic structures whereas Ga- and In-based phases in general prefer hexagonal structures. The theoretically predicted stable MAB phases along with the structural preference is intended to both guide experimental efforts and to give an insight into the stability for different crystal symmetries of MAB phases.

Extensive research has been invested in two-dimensional (2D) materials, typically synthesized by exfoliation of van der Waals solids. One exception is MXenes, derived from the etching of constituent layers in transition metal carbides and nitrides. We report the experimental realization of boridene in the form of single-layer 2D molybdenum boride sheets with ordered metal vacancies, Mo4/3B2-xTz (where T-z is fluorine, oxygen, or hydroxide surface terminations), produced by selective etching of aluminum and yttrium or scandium atoms from 3D in-plane chemically ordered (Mo2/3Y1/3)(2)AlB2 and (Mo2/3Sc1/3)(2)AlB2 in aqueous hydrofluoric acid. The discovery of a 2D transition metal boride suggests a wealth of future 2D materials that can be obtained through the chemical exfoliation of laminated compounds.

We have by means of first principles density functional theory calculations studied the mechanical and electronic properties of the so called i-MAB phases, MM-4/3 2/3AlB2, where M = Cr, Mo, W and M = Sc, Y. These phases, experimentally verified for Mo4/3Sc2/3AlB2 and Mo4/3Y2/3AlB2, display an atomically laminated structure with in-plane chemical order between the M and M elements. Structural properties, along with elastic constants and moduli, are predicted for different structural symmetries, including the reported R (3) over barm (#166) space group. We find all considered i-MAB phases to be metallic with a significant peak in the electronic structure at the Fermi level and no significant anisotropy in the electronic band structure. The simulations also indicate that they are rather hard and stiff, in particular the Cr-based ones, with a Youngs modulus E of 325 GPa for M = Sc. The Mo-based phases are similar, with E = 299 GPa for M = Sc, which is higher than the corresponding laminated carbides (i-MAX phases).

Exploratory theoretical predictions in uncharted structural and compositional space are integral to materials discoveries. Inspired by M5SiB2 (T2) phases, the finding of a family of laminated quaternary metal borides, M M-4 SiB2, with out-of-plane chemical order is reported here. 11 chemically ordered phases as well as 40 solid solutions, introducing four elements previously not observed in these borides are predicted. The predictions are experimentally verified for Ti4MoSiB2, establishing Ti as part of the T2 boride compositional space. Chemical exfoliation of Ti4MoSiB2 and select removal of Si and MoB2 sub-layers is validated by derivation of a 2D material, TiOxCly, of high yield and in the form of delaminated sheets. These sheets have an experimentally determined direct band gap of approximate to 4.1 eV, and display characteristics suitable for supercapacitor applications. The results take the concept of chemical exfoliation beyond currently available 2D materials, and expands the envelope of 3D and 2D candidates, and their applications.

In this study, we model the chemical stability in the (Cr1-xFex)(2)AlC MAX phase system using density functional theory, predicting its phase stability for 0 < x < 0.2. Following the calculations, we have successfully synthesized nanolaminated (Cr1-xFex)(2)AlC MAX phase thin films with target Fe contents of x = 0.1 and x = 0.2 by pulsed laser deposition using elemental targets on MgO(111) and Al2O3 (0001) substrates at 600 degrees C. Structural investigations by X-ray diffraction and transmission electron microscopy reveal MAX phase epitaxial Iilms on both substrates with a coexisting (Fe,Cr)(5)Al-8 intermetallic secondary phase. Experiments suggest an actual maximum Fe solubility of 3.4 at %, corresponding to (Cr0.932Fe0.068)(2)AlC, which is the highest Fe doping level achieved so far in volume materials and thin films. Residual Fe is continuously distributed in the (Fe,Cr)(5)Al-8 intermetallic secondary phase. The incorporation of Fe results in the slight reduction of the c lattice parameter, while the a lattice parameter remains unchanged. The nanolaminated (Cr0.932Fe0.068)(2)AlC thin films show a metallic behavior and can serve as promising candidates for highly conductive coatings.

MAB phases are layered materials combining metallic and ceramic attributes. Their ternary compositions, however, have been limited to a few elemental combinations which makes controlled and tailored properties challenging. Inspired by the recent discovery of Mo4/3Y2/3AlB2 and Mo(4/3)Sc(2/3)AlB(2)i-MAB phases, i.e., quaternary layered MAB phases with in-plane chemical order, we perform an extensive first-principles study to explore formation of chemical order and solid-solutions upon metal alloying of M(2)AB(2) phases of 1092 compositions (M from group 3 to 9 and A = Al, Ga, In, Si, Ge, Sn). This large dataset provides 39 chemically ordered (i-MAB) and 52 solid solution (MAB) phases that are predicted to be thermodynamically stable at typical synthesis temperatures, of which a majority have not yet been experimentally reported. The possibility for realizing both i-MAB and solid solution MAB phases, combined with the multiple elemental combinations previously not observed in these boride-based materials, allows for an increased potential for property tuning and potential chemical exfoliation into 2D derivatives.

The MAB phases are a family of layered ternary transition metal borides, with atomically laminated crystal structures comprised of transition metal boride (M-B) layers interleaved by single, or double, Al (A) layers. Herein, density functional theory is implemented to evaluate the thermodynamic stability of disordered (Mn1-xCrx)(2)AlB2, and disordered and ordered (Mn1-xCrx)(3)AlB4 quaternaries. The (Mn1-xCrx)(2)AlB2 solid solutions were synthesized over the entire range of substitution. A (Mn1-xCrx)(3)AlB4 solid solution was produced, on the base of Cr3AlB4, to form (Mn0.33Cr0.66)(3)AlB4. Powder X-ray diffraction shows lattice parameter shifts and unit cell expansions indicative of successful solid solution formations.

The atomic structure and local composition of high quality epitaxial substoichiometric titanium diboride (TiB1.9) thin film, deposited by unbalanced magnetron sputtering, were studied using analytical high-resolution scanning transmission electron microscopy, density functional theory, and image simulations. The unpaired Ti is pinpointed to inclusion of Ti-based stacking faults within a few atomic layers, which terminates the {1 (1) over bar 00} prismatic planes of the crystal structure and attributed to the absence of B between Ti planes that locally relaxes the structure. This mechanism allows the line compound to accommodate off-stoichiometry and remain a line compound between defects. The planar defects are embedded in otherwise stoichiometric TiB2 and are delineated by insertion of dislocations. An accompanied decrease in Ti-Ti bond lengths along and across the faults is observed. (c) 2020ActaMaterialiaInc. PublishedbyElsevierLtd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

Using density functional theory, the phase stability and physical properties, including structural, electronic, mechanical, thermal and vibrational with defect processes, of a newly synthesized 211 MAX phase V2SnC are investigated for the first time. The obtained results are compared with those found in the literature for other existing M2SnC (M = Ti, Zr, Hf, Nb, and Lu) phases. The formation of V2SnC is exothermic and this compound is intrinsically stable in agreement with the experiment. V2SnC has potential to be etched into 2D MXene. The new phase V2SnC and existing phase Nb2SnC are damage tolerant. V2SnC is elastically more anisotropic than Ti2SnC and less than the other M2SnC phases. The electronic band structure and Fermi surface of V2SnC indicate the possibility of occurrence of its superconductivity. V2SnC is expected to be a promising TBC material like Lu2SnC. The radiation tolerance in V2SnC is better than that in Lu2SnC.

This work investigates the compatibility of Zr2AlC MAX phase-based ceramics with liquid LBE, and proposes a mechanism to explain the observed local Zr2AlC/LBE interaction. The ceramics were exposed to oxygen-poor (C-O <= 2.2 x 10(-10) mass%), static liquid LBE at 500 degrees C for 1000 h. A new Zr-2(Al,Bi,Pb)C MAX phase solid solution formed in-situ in the LBE-affected Zr2AlC grains. Out-of-plane ordering was favorable in the new solid solution, whereby A-layers with high and low-Bi/Pb contents alternated in the crystal structure, in agreement with first-principles calculations. Bulk Zr-2(Al,Bi,Pb)C was synthesized by reactive hot pressing to study the crystal structure of the solid solution by neutron diffraction.

The atomically laminated Mn2GaC has previously been synthesized as a heteroepitaxial thin film and found to be magnetic with structural changes linked to the magnetic anisotropy. Related theoretical studies only considered bulk conditions and thus neglected the influence from possible strain linked to the choice of substrate. Here we employ first principles calculations considering different exchange-correlation functionals (PBE, PW91, PBEsol, AM05, LDA) and effect from use of+U methods (or not) combined with a magnetic ground-state search using Heisenberg Monte Carlo simulations, to study influence from biaxial in-plane strain and external pressure on the magnetic and crystal structure of Mn2GaC. We find that PBE and PBE+U, with U-eff <= 0.25 eV, gives both structural and magnetic properties in quantitative agreement with available experimental data. Our results also indicate that strain related to choice of substrate or applied pressure is a route for accessing different spin configurations, including a ferromagnetic state. Moreover, the easy axis is parallel to the atomic planes and the magnetocrystalline anisotropy energy can be increased through strain engineering by expanding the in-plane lattice parameter a. Altogether, we show that a quantitative description of the structural and magnetic properties of Mn2GaC is possible using PBE, which opens the way for further computational studies of these and related materials.

In this work, we present high-pressure diffraction results of the Mo-based M-n (+) (1)AX(n) phase, Mo2GaC. A diamond anvil cell was used to compress the material up to 30 GPa, and x-ray diffraction was used to determine the structure and unit cell parameters as a function of pressure. Somewhat surprisingly, we find that, at 295 +/- 25 GPa, the bulk modulus of Mo2GaC is the highest reported of all the MAX phases measured to date. The c/a ratio increases with increasing pressure. At above 15 GPa, a splitting in the (1 0 0) reflection occurs. This result, coupled with new density functional theory calculations, suggests that a second order phase transition to possibly a mixture of hexagonal and monoclinic structures may explain this splitting. Such experimentally and theoretically supported phase transitions were not predicted in previously published calculations.

In this work we systematically explore a class of atomically laminated materials, M(n+1)AX(n) (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.

Nanolaminated materials including magnetic ele-ments are of special interest for commonly observed nontrivial magnetic characteristics and as potential precursors for 2D materials. Here, we explore the previously unknown layered phase M2Al2C3, where M = Sc and Er. Sc2Al2C3 was synthesized as single crystals of similar to mm(2) size, and its structure was determined by single crystal X-ray diffraction and scanning transmission electron microscopy. Evaluation of phase stability and possible vacancy formation based on first-principles calculations confirms the attained phase and suggests full occupancy on both the Al and C sites. Potential realization of the hypothetical phase Y2Al2C3 is also proposed. Furthermore, we also demonstrate that Er2Al2C3 can be synthesized in powder form, providing experimental evidence for stoichiometries based on rare earth elements, which, in turn, suggests possible incorporation of other lanthanides.

New bulk MAX phase-based ceramics were synthesized in the Ta-Hf-Al-C and Ta-Nb-Al-C systems. Specifically, (Ta1-x,Hf-x)(4)AlC3 and (Ta1-x,Nb-x)(4)AlC3 stoichiometries with x = 0.05, 0.1, 0.15, 0.2, 0.25 were targeted by reactive hot pressing of Ta2H, HfH2, NbH0.89, Al and C powder mixtures at 1550 degrees C in vacuum. The produced ceramics were characterized in terms of phase composition and microstructure by X-ray diffraction, scanning electron microscopy, electron probe microanalysis and scanning transmission electron microscopy. The investigation confirmed the existence of such M-site solid solutions with low solute concentrations, as predicted by first-principles calculations. These calculations also predicted a linear trend in lattice parameter evolution with increasing Hf concentration, in agreement with the experimental results. In order to increase the low phase purity of the produced ceramics, Sn was added to form (Ta1-x,Hf-x)(4)(Al-0.5,Sn-0.5)C-3 and (Ta1-x,Nb-x)(4)(Al-0.5,Sn-0.5)C-3 double solid solutions, thus resulting in a higher content of the 413 MAX phase compounds in the produced ceramics.

We combine predictive ab initio calculations with experimental verification of bulk materials synthesis for exploration of new and potentially magnetic atomically laminated i-MAX phases. Two such phases are discovered: (Cr2/3Sc1/3)(2)GaC and (Mn2/3Sc1/3)(2)GaC synthesized by the solid state reaction from elemental constituents. The latter compound displays a 2-fold increase in Mn content compared to previously reported bulk MAX phases. Both new compounds exhibit the characteristic in-plane chemical order of Cr(Mn) and Sc, and crystallize in an orthorhombic structure, space group Cmcm, as confirmed by scanning transmission electron microscopy. From density functional theory calculations of the magnetic ground state, including the electron-interaction parameter U, we suggest an antiferromagnetic ground state, close to degenerate with the ferromagnetic state.

All atomically laminated MAB phases (M = transition metal, A = A-group element, and B = boron) exhibit orthorhombic or tetragonal symmetry, with the only exception being hexagonal Ti2InB2. Inspired by the recent discovery of chemically ordered hexagonal carbides, i-MAX phases, we perform an extensive first-principles study to explore chemical ordering upon metal alloying of M2AlB2 (M from groups 3 to 9) in orthorhombic and hexagonal symmetry. Fifteen stable novel phases with in-plane chemical ordering are identified, coined i-MAB, along with 16 disordered stable alloys. The predictions are verified through the powder synthesis of Mo4/3Y2/3 AlB2 and Mo4/3Sc2/3AlB2 of space group R (3) over barm (no. 166), displaying the characteristic in-plane chemical order of Mo and Y/Sc and Kagome ordering of the Al atoms, as evident from X-ray diffraction and electron microscopy. The discovery of i-MAB phases expands the elemental space of these borides with M = Sc, Y, Zr, Hf, and Nb, realizing an increased property tuning potential of these phases as well as their suggested potential twodimensional derivatives.

Nanolamellar transition metal carbides are gaining increasing interests because of the recent developments of their twodimensional (2D) derivatives and promising performance for a variety of applications from energy storage, catalysis to transparent conductive coatings, and medicine. To develop more novel 2D materials, new nanolaminated structures are needed. Here we report on a tungsten based nanolaminated ternary phase, (W,Ti)(4)C4-x, synthesized by an Al catalyzed reaction of W, Ti, and C powders at 1600 degrees C for 4 h, under flowing argon. X-ray and neutron diffraction, along with Z-contrast scanning transmission electron microscopy, were used to determine the atomic structure, ordering, and occupancies. This phase has a layered hexagonal structure (P6(3)/mmc) with lattice parameters, a = 3.00880(7) angstrom, and c = 19.5633(6) angstrom and a nominal chemistry of (W,Ti)(4)C4-x (actual chemistry, W2.1(1)Ti1.6(1)C2.6(1)). The structure is comprised of layers of pure W that are also twin planes with two adjacent atomic layers of mixed W and Ti, on either side. The use of Al as a catalyst for synthesizing otherwise difficult to make phases, could in turn lead to the discovery of a large family of nonstoichiometric ternary transition metal carbides, synthesized at relatively low temperatures and shorter times.

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 ((M2/3M1/32)-M-1)(2)AC chemistry, where the M-1 and M-2 atoms are in-plane ordered. Herein, we report the discovery of a class of magnetic i-MAX phases in which bilayers of a quasi-2D magnetic frustrated triangular lattice overlay a Mo honeycomb arrangement and an Al Kagome lattice. The chemistry of this family is (Mo2/3RE1/3)(2)AlC, and the rare-earth, RE, elements 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.

The family of MXenes has expanded since the discovery of chemical order in parent quaternary MAX phases, displaying either out-of-plane (o-MAX) or in-plane (i-MAX) order upon alloying. Through selective chemical etching of these materials, corresponding MXenes can be derived, with out-of-plane and in-plane ordering of elements, as well as with ordering of vacancies. Both o-MAX and i-MAX phases have increased the number of metals that can be incorporated in these laminated carbides and nitrides. Examples of realized MXenes with out-of-plane order are Mo2Ti2C3 and Mo2ScC2, and for in-plane ordering of vacancies, there are Mo1.33C and W1.33C. Their versatile chemistry shows a high promise for a range of applications, including energy storage and catalysis

Research on low-dimensional materials has increased drastically in the last decade, with the discovery of two-dimensional transition metal carbides and nitrides (MXenes) produced by atom-selective chemical etching of laminated parent M(n+1)AX(n) (MAX) phases. Here, we apply density functional theory and subsequent materials synthesis and analysis to explore the phase stability and Mo/Sc intermixing on the M site in the chemically ordered quaternary i-MAX phase (MoxSc1-x)(2)AlC. Transmission electron microscopy confirms the theoretical predictions of preferential in-plane ordering of Mo and Sc, with the highest crystal quality obtained for the ideal Mo:Sc ratio of 2:1 (predicted as the most stable), as well as a retained i-MAX structure even for an increased relative Sc content, with Sc partially occupying Mo sites. The results are supported by refined neutron diffraction data, which show space group C2/c (no. 15), and a C occupancy of 1. Subsequent chemical etching produces MXene for x = 0.66, while for x = 0.33 and 0.5 no MXene is observed. These results demonstrate that a precise control of the i-MAX composition is crucial for derivation of MXene, with a MXene quality optimized for a Mo:Sc ratio of 2:1 with minimal intermixing between Mo and Sc.

Synthesis of delaminated 2D W1.33C (MXene) has been performed by selectively etching Al as well as Sc/Y from the recently discovered nanolaminated i-MAX phases (W2/3Sc1/3)(2)AlC and (W2/3Y1/3)(2)AlC. Both quaternary phases produce MXenes with similar flake morphology and with a skeletal structure due to formation of ordered vacancies. The measured O, OH, and F terminations, however, differ in amount as well as in relative ratios, depending on parent material, evident from X-ray photoelectron spectroscopy. These findings are correlated to theoretical simulations based on first-principles, investigating the W1.33C, and the effect of termination configurations on structure, formation energy, stability, and electronic structure. The theoretical results indicate a favored F-rich surface composition, though with a system going from insulating/semiconducting to metallic for different termination configurations, suggesting a high tuning potential of these materials. Additionally, free-standing W1.33C films of 2-4 mu m thickness and with up to 10 wt % polymer (PEDOT:PSS) were tested as electrodes in supercapacitors, showing capacitances up to 600 F cm(-3) in 1 M H2SO4 and high capacitance retention for at least 10000 cycles at 10 A g(-1). This is highly promising results compared to other W-based materials to date.

Interfaces containing misfit dislocations deteriorate electronic properties of heteroepitaxial wide bandgap III-nitride semiconductors grown on foreign substrates, as a result of lattice and thermal expansion mismatches and incompatible chemical bonding. We report grain-boundary-free AlN nucleation layers (NLs) grown by metalorganic chemical vapor deposition on SiC (0001) substrates mediated by an interface extending over two atomic layers L1 and L2 with composition (Al1/3Si2/3)(2/3)N and (Al2/3Si1/3)N, respectively. It is remarkable that the interfaces have ordered vacancies on one-third of the Al/Si position in L1, as shown here by analytical scanning transmission electron microscopy and ab initio calculations. This unique interface is coined the out-of-plane compositional-gradient with in-plane vacancy-ordering and can perfectly transform the in-plane lattice atomic configuration from the SiC substrate to the AlN NL within 1 nm thick transition. This transmorphic epitaxial scheme enables a critical breakdown field of similar to 2 MV/cm achieved in thin GaN-based transistor heterostructures grown on top. Lateral breakdown voltages of 900 V and 1800 V are demonstrated at contact distances of 5 and 20 mu m, respectively, and the vertical breakdown voltage is amp;gt;= 3 kV. These results suggest that the transmorphic epitaxially grown AlN layer on SiC may become the next paradigm for GaN power electronics. (C) 2019 Author(s).

With the recent discovery of in-plane chemically ordered MAX phases (i-MAX) of the general formula ((M2/3M1/32)-M-1)(2)AC 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)(2)AlC and (W2/3Y1/3)(2)AlC 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 (M-2) leads to reduced bonding strength for W-C and Al-Al interactions while M-2-C and M-2-Al interactions are strengthened. We also compare the Voigt-Reuss-Hill bulk, shear, and Youngs moduli along the series of M-1 = Cr, Mo, and W, and relate these trends to the bonding interactions. Furthermore, we find overall larger moduli for Sc-based i-MAX phases.

With increased chemical diversity and structural complexity comes the opportunities for innovative materials possessing advantageous properties. Herein, we combine predictive first-principles calculations with experimental synthesis, to explore the origin of formation of the atomically laminated i-MAX phases. By probing (Mo2/3M1/32)(2)AC (where M-2 = Sc, Y and A = Al, Ga, In, Si, Ge, In), we predict seven stable i-MAX phases, five of which should have a retained stability at high temperatures. (Mo2/3Sc1/3)(2)GaC and (Mo2/3Y1/3)(2)GaC were experimentally verified, displaying the characteristic in-plane chemical order of Mo and Sc/Y and Kagome-like ordering of the A-element. We suggest that the formation of i-MAX phases requires a significantly different size of the two metals, and a preferable smaller size of the A-element. Furthermore, the population of antibonding orbitals should be minimized, which for the metals herein (Mo and Sc/Y) means that A elements from Group 13 (Al, Ga, In) are favored over Group 14 (Si, Ge, Sn). Using these guidelines, we foresee a widening of elemental space for the family of i-MAX phases and expect more phases to be synthesized, which will realize useful properties. Furthermore, based on i-MAX phases as parent materials for 2D MXenes, we also expect that the range of MXene compositions will be expanded.

Guided by predictive theory, a new compound with chemical composition (Cr2/3Zr1/3)(2)AlC was synthesized by hot pressing of Cr, ZrH2, Al, and C mixtures at 1300 degrees C. The crystal structure is monoclinic of space group C2/c and displays in-plane chemical order in the metal layers, a so-called i-MAX phase. Quantitative chemical composition analyses confirmed that the primary phase had a (Cr2/3Zr1/3)(2)AlC stoichiometry, with secondary Cr2AlC, AlZrC2, and ZrC phases and a small amount of Al-Cr intermetallics. A theoretical evaluation of the (Cr2/3Zr1/3)(2)AlC magnetic structure was performed, indicating an antiferromagnetic ground state. Also (Cr2/3Zr1/3)(2)AlC, of the same structure, was predicted to be stable.

Structural design on the atomic level can provide novel chemistries of hybrid MAX phases and their MXenes. Herein, density functional theory is used to predict phase stability of quaternary i-MAX phases with in-plane chemical order and a general chemistry (W2/3M1/32)(2)AC, where M-2 = Sc, Y (W), and A = Al, Si, Ga, Ge, In, and Sn. Of over 18 compositions probed, only twowith a monoclinic C2/c structureare predicted to be stable: (W2/3Sc1/3)(2)AlC and (W2/3Y1/3)(2)AlC and indeed found to exist. Selectively etching the Al and Sc/Y atoms from these 3D laminates results in W1.33C-based MXene sheets with ordered metal divacancies. Using electrochemical experiments, this MXene is shown to be a new, promising catalyst for the hydrogen evolution reaction. The addition of yet one more element, W, to the stable of M elements known to form MAX phases, and the synthesis of a pure W-based MXene establishes that the etching of i-MAX phases is a fruitful path for creating new MXene chemistries that has hitherto been not possible, a fact that perforce increases the potential of tuning MXene properties for myriad applications.

The data presented in this paper are related to the research article entitled "Theoretical stability and materials synthesis of a chemically ordered MAX phase, Mo2ScAlC2, and its two-dimensional derivate Mo2ScC" (Meshkian et al. 2017) [1]. This paper describes theoretical phase stability calculations of the MAX phase alloy MoxSc3-xAlC2 (x=0, 1, 2, 3), including chemical disorder and out-of-plane order of Mo and Sc along with related phonon dispersion and Bader charges, and Rietveld refinement of Mo2ScAlC2. The data is made publicly available to enable critical or extended analyzes.

The enigma of MAX phases and their hybrids prevails. We probe transition metal (M) alloying in MAX phases for metal size, electronegativity, and electron configuration, and discover ordering in these MAX hybrids, namely, (V2/3Zr1/3)(2)AlC and (Mo2/3Y1/3)(2)AlC. Predictive theory and verifying materials synthesis, including a judicious choice of alloying M from groups III to VI and periods 4 and 5, indicate a potentially large family of thermodynamically stable phases, with Kagome-like and in-plane chemical ordering, and with incorporation of elements previously not known for MAX phases, including the common Y. We propose the structure to be monoclinic C2/c. As an extension of the work, we suggest a matching set of novel MXenes, from selective etching of the A-element. The demonstrated structural design on simultaneous two-dimensional (2D) and 3D atomic levels expands the property tuning potential of functional materials.

The large class of layered ceramics encompasses both van der Waals (vdW) and non-vdW solids. While intercalation of noble metals in vdW solids is known, formation of compounds by incorporation of noble-metal layers in non-vdW layered solids is largely unexplored. Here, we show formation of Ti3AuC2 and Ti3Au2C2 phases with up to 31% lattice swelling by a substitutional solid-state reaction of Au into Ti3SiC2 single-crystal thin films with simultaneous out-diffusion of Si. Ti3IrC2 is subsequently produced by a substitution reaction of Ir for Au in Ti3Au2C2. These phases form Ohmic electrical contacts to SiC and remain stable after 1,000 h of ageing at 600 degrees C in air. The present results, by combined analytical electron microscopy and ab initio calculations, open avenues for processing of noble-metal-containing layered ceramics that have not been synthesized from elemental sources, along with tunable properties such as stable electrical contacts for high-temperature power electronics or gas sensors.

We present theoretical prediction and experimental evidence of a new MAX phase alloy, Mo2ScAlC2, with out-of-plane chemical order. Evaluation of phase stability was performed by ab initio calculations based on Density Functional Theory, suggesting that chemical order in the alloy promotes a stable phase, with a formation enthalpy of -24 meV/atom, as opposed to the predicted unstable Mo3AlC2 and Sc3AlC2. Bulk synthesis of Mo2ScAlC2 is achieved by mixing elemental powders of Mo, Sc, Al and graphite which are heated to 1700 degrees C. High resolution transmission electron microscopy reveals a chemically ordered structure consistent with theoretical predictions with one Sc layer sandwiched between two Mo-C layers. The two-dimensional derivative, the MXene, is produced by selective etching of the Al-layers in hydrofluoric acid, resulting in the corresponding chemically ordered Mo2ScC2, i.e. the first Sc-containing MXene. The here presented results expands the attainable range of MXene compositions and widens the prospects for property tuning. (C)2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Incorporation of layers of noble metals in non-van der Waals layered materials may be used to form novel layered compounds. Recently, we demonstrated a high-temperature-induced exchange process of Au with Si in the layered phase Ti3SiC2, resulting in the formation of Ti3AuC2 and Ti3Au2C2. Here, we generalize this technique showing that Au/Ti2AlC and Au/Ti3AlC2 undergo an exchange reaction at 650 [degree]C to form Ti2Au2C and Ti3Au2C2 and determine their structures by electron microscopy, X-ray diffraction, and ab initio calculations. These results imply that noble-metal-containing layered phases should be possible to synthesize in many systems. The metal to be introduced should be inert to the transition-metal carbide layers, and exhibit negative heat of mixing with the initial A element in a liquid phase or two-phase liquid/solid region at the annealing temperature.

The exploration of two-dimensional solids is an active area of materials discovery. Research in this area has given us structures spanning graphene to dichalcogenides, and more recently 2D transition metal carbides (MXenes). One of the challenges now is to master ordering within the atomic sheets. Herein, we present a top-down, high-yield, facile route for the controlled introduction of ordered divacancies in MXenes. By designing a parent 3D atomic laminate, (Mo2/3Sc1/3)(2)AlC, with in-plane chemical ordering, and by selectively etching the Al and Sc atoms, we show evidence for 2D Mo1.33C sheets with ordered metal divacancies and high electrical conductivities. At similar to 1,100 F cm(-3), this 2D material exhibits a 65% higher volumetric capacitance than its counterpart, Mo2C, with no vacancies, and one of the highest volumetric capacitance values ever reported, to the best of our knowledge. This structural design on the atomic scale may alter and expand the concept of property-tailoring of 2D materials.

The nature of the magnetic structure in magnetic so-called MAX phases is a topic of some controversy. Here we present unpolarized neutron-diffraction data between 3.4 and 290.0 K and momentum transfer between Q = 0.0 and 1.1 angstrom(-1), as well as complementary x-ray-diffraction data on epitaxial thin films of the MAX phase material Mn2GaC. This inherently layered material exhibits neutron-diffraction peaks consistent with long-ranged antiferromagnetic order with a periodicity of two structural unit cells. The magnetic structure is present throughout the measured temperature range. The results are in agreement with first-principles calculations of antiferromagnetic structures for this material where the Mn-C-Mn atomic trilayers are found to be ferromagnetically coupled internally but spin flipped or rotated across the Ga layers. The present findings have significant bearing on the discussion regarding the nature of the magnetic structure in magnetic MAX phases.

In this work, we employ and critically evaluate a first-principles approach based on supercell calculations for predicting the magnetic critical order-disorder temperature 𝑇𝑐 . As a model material we use the recently discovered nanolaminate Mn₂GaC.

First, we derive the exchange interaction parameters 𝐽_𝑖𝑗 between pairs of Mn atoms on sites 𝑖 and 𝑗 of the bilinear Heisenberg Hamiltonian using the novel magnetic direct cluster averaging method (MDCA), and then compare the 𝐽’s from the MDCA calculations to the same parameters calculated using the Connolly-Williams method. We show that the two methods yield closely matching results, but observe that the MDCA method is computationally less effective when applied to highly ordered phases such as Mn₂GaC.

Secondly, Monte Carlo simulations are used to derive the magnetic energy, specific heat, and 𝑇_𝑐 . For Mn₂GaC, we find 𝑇_𝑐 = 660 K. The uncertainty in the calculated 𝑇_𝑐 caused by possible uncertainties in the 𝐽’s is discussed and exemplified in our case by an analysis of the impact of the statistical uncertainties of the MDCA-derived 𝐽’s, resulting in a 𝑇_𝑐 distribution with a standard deviation of 133 K.

This review presents MAX phases (M is a transition metal, A an A-group element, X is C or N), known for their unique combination of ceramic/metallic properties, as a recently uncovered family of novel magnetic nanolaminates. The first created magnetic MAX phases were predicted through evaluation of phase stability using density functional theory, and subsequently synthesized as heteroepitaxial thin films. All magnetic MAX phases reported to date, in bulk or thin film form, are based on Cr and/or Mn, and they include (Cr,Mn)(2)AlC, (Cr,Mn)(2)GeC, (Cr,Mn)(2)GaC, (Mo,Mn)(2)GaC, (V,Mn)(3)GaC2, Cr2AlC, Cr2GeC and Mn2GaC. A variety of magnetic properties have been found, such as ferromagnetic response well above room temperature and structural changes linked to magnetic anisotropy. In this paper, theoretical as well as experimental work performed on these materials to date is critically reviewed, in terms of methods used, results acquired, and conclusions drawn. Open questions concerning magnetic characteristics are discussed, and an outlook focused on new materials, superstructures, property tailoring and further synthesis and characterization is presented.

Inherently layered magnetic materials, such as magnetic M(n+1)AX(n) (MAX) phases, offer an intriguing perspective for use in spintronics applications and as ideal model systems for fundamental studies of complex magnetic phenomena. The MAX phase composition M(n+1)AX(n) consists of M(n+1)AX(n) blocks separated by atomically thin A-layers where M is a transition metal, A an A-group element, X refers to carbon and/or nitrogen, and n is typically 1, 2, or 3. Here, we show that the recently discovered magnetic Mn2GaC MAX phase displays structural changes linked to the magnetic anisotropy, and a rich magnetic phase diagram which can be manipulated through temperature and magnetic field. Using first-principles calculations and Monte Carlo simulations, an essentially one-dimensional (1D) interlayer plethora of two-dimensioanl (2D) Mn-C-Mn trilayers with robust intralayer ferromagnetic spin coupling was revealed. The complex transitions between them were observed to induce magnetically driven anisotropic structural changes. The magnetic behavior as well as structural changes dependent on the temperature and applied magnetic field are explained by the large number of low energy, i.e., close to degenerate, collinear and noncollinear spin configurations that become accessible to the system with a change in volume. These results indicate that the magnetic state can be directly controlled by an applied pressure or through the introduction of stress and show promise for the use of Mn2GaC MAX phases in future magnetoelectric and magnetocaloric applications.

We here use first-principles calculations to investigate the phase stability of the hypothetical laminated material V2Ga2C and the related alloy (Mo1-xVx)(2)Ga2C, the latter for a potential parent material for synthesis of (Mo1-xVx)(2)C, a new two-dimensional material in the family of so called MXenes. We predict that V2Ga2C is thermodynamically stable with respect to all identified competing phases in the ternary V-Ga-C phase diagram. We further calculate the stability of ordered and disordered configurations of Mo and V in (Mo1-xVx)(2)Ga2C and predict that ordered (Mo1-xVx)(2)Ga2C for x <= 0.25 is stable, with an order-disorder transition temperature of similar to 1000 K. Furthermore, (Mo1-xVx)(2)Ga2C for x = 0.5 and x >= 0.75 is suggested to be stable, but only for disordered Mo-V configurations, and only at elevated temperatures. We have also investigated the electronic and elastic properties of V2Ga2C; the calculated bulk, shear, and Youngs modulus are 141, 94, and 230 GPa, respectively.