i-MAX Phases Research Articles

Investigations on the crystallographic stacking disordering and properties of novel Ga-containing in-plane chemically ordered (Cr2/3R1/3)2GaC (R=Y, Tb - Lu) i-MAX phases

Eight novel transition metal carbides, nanolaminated (Cr2/3R1/3)2GaC (R=Y, Tb, Dy, Ho, Er, Tm, Yb and Lu) with R in-plane chemical ordered occupation, have been synthesized with mainly Cmcm orthorhombic structure and monotonously decreased lattice parameters due to lanthanide contraction. Stacking disorderings and high-density slidings within c plane have been observed. Two characteristic in-plane sliding, along [100] or [1¯30]o directions have been analyzed with former one leading into the structural evolution between Cmcm and C2/c symmetry. Such an atomic scale co-existence of Cmcm and C2/c structures is explained by more negative charge for Ga and anisotropic bonding with Ga compared with A=Al case. Thus, the Vickers hardness, fracture strength and increased wear depth are reduced in representative (Cr2/3Lu1/3)2GaC i-MAX bulk compared with the (Cr2/3Lu1/3)2AlC counterpart.

The crystallographic structure and properties of novel quaternary nanolaminated rare-earth-Cr-based i-MAX phases

Seven novel inherently nanolaminated transition metal carbides, (Cr2/3R1/3)2AlC (R = Gd, Tb, Dy, Ho, Er, Tm, Lu) with R in-plane chemical ordering in the carbide sheet, have been synthesized and found to be crystallized in Cmcm orthorhombic structure with monotonously decreased lattice parameters due to lanthanides constrictions. Linear antiferromagnetism is found for R = Gd and gradually changed to paramagnetism for R = Er, Tm and Lu through nonlinear magnetic configurations with gradually decreased ordering temperature. Compared with Cr2AlC benchmark, the Vickers hardness is significantly higher, leading to higher compressive fracture strength and better wear resistance. After compression at high temperature, nanoscale co-existence of the C2/c monoclinic and orthorhombic structure is observed, which is caused by the random stackings and high-density slidings within ab plane. The further first-principle calculations confirm the close thermal stability of both two structures.

Variety and complexity of magnetic structures of rare earth-based nano-lamellar i-MAX phases

Variety and complexity of magnetic structures of rare earth-based nano-lamellar i-MAX phases

Synthesis and characterizations of solid-solution i-MAX phase (W1/3Mo1/3R1/3)2AlC (R = Gd, Tb, Dy, Ho, Er and Y) and derivated i-MXene with improved electrochemical properties

In this work, a series of new solid-solution i-MAX phase (W1/3Mo1/3R1/3)2AlC (R = Gd, Tb, Dy, Ho, Er and Y) were synthesized and characterized. From R = Gd to Er, the lanthanide contract is observed while the magnetic behaviors are varied from nonlinear antiferromagnetism-like type for R = Gd to weak ferromagnetism-like one for R = Er with gradually weakened magnetic coupling and reduced Curie-Weiss temperature. The solid-solution i-MXene W2/3Mo2/3(R)CTx was synthesized by selective etching Al and R elements and the highest specific capacitance of 120 Fg−1 at 1 Ag−1 was obtained. By further HM (Hydrazine Monohydrate, N2H4•H2O) treatment and hydrothermal composite with rGO, the specific capacitance is further elevated to 249 Fg−1. This is explained by the increament of O-/N-containing terminals and improved conductivity due to the formation of multidimensional interactive microstructure.

The rise of MAX phase alloys - large-scale theoretical screening for the prediction of chemical order and disorder.

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.

Novel W-based in-plane chemically ordered (W2/3R1/3)2AlC (R = Gd, Tb, Dy, Ho, Er, Tm and Lu) MAX phases and their 2D W1.33C MXene derivatives

Novel rare-earth-contained W-based in-plane chemically ordered (W2/3R1/3)2AlC (R = Gd, Tb, Dy, Ho, Er, Tm and Lu) i-MAX phases have been firstly reported. All are identified to be C2/c monoclinic by structural analysis. Magnetic ground state is found to change from linear antiferromagnetism for R = Gd to paramagnetism for R = Lu with gradually decreased ordering temperature with the evolution on magnetic configurations. By exfoliation and delamination, brittle W1.33C MXene fragments have been obtained and demonstrated undesired specific capacitance of 76 F g−1 at 1 A g−1. By making W1.33C/Ti3C2 composite MXene, flexible free-standing paper has been obtained and the specific capacitance is substantially enhanced to 183 F g−1 with good retention, which is the highest performance realized in W1.33C MXene up to now.

In-plane ordered quaternary phases (i-MAB): electronic structure and mechanical properties from first-principles calculations

We have by means of first principles density functional theory calculations studied the mechanical and electronic properties of the so called i-MAB phases, M 4/3′M″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 (#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 Young’s 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).

Structural, magnetic properties of in-plane chemically ordered (Mo2/3R1/3)2AlC (R = Gd, Tb, Dy, Ho, Er and Y) MAX phase and enhanced capacitance of Mo1.33C MXene derivatives

The crystallographic structure of rare-earth-contained (Mo2/3R1/3)2AlC (R = Gd, Tb, Dy, Ho, Er and Y) i-MAX phases have been verified to be C2/c monoclinic. The magnetic behaviors are varied from antiferromagnetism to non-linear ferromagnetism from R = Gd to Ho in (Mo2/3R1/3)2AlC i-MAX phases due to variant R-R interactions. The capacitance of Mo1.33C@R MXene derived from different (Mo2/3R1/3)2AlC precursors varies from 170 F/g to 228 F/g but can be double by hydrazine monohydrate treatment. The highest one of 431 F/g up to now has been achieved in Mo1.33C@Y-HM MXene, which is caused by the increased fraction of critical -O group evidenced by XPS spectra. Our results illustrate the promising applications in supercapacitor of Mo1.33C MXene derived from R-contained (Mo, R)2AlC i-MAX phases.

Enhanced yield synthesis of bulk dense (M2/3Y1/3)2AlC (M = Cr, W, Mo) in-plane chemically ordered quaternary atomically laminated i-MAX phases and oxidation of (Cr2/3Y1/3)2AlC and (Mo2/3Y1/3)2AlC

Recently, a new family of MAX phases with in-plane chemical order, i-MAX, have been discovered which incorporate new elements expanding the family of MAX phases. i-MAX phases remain to be synthesized in single-phase bulk form for characterization. Herein, we show that by reactively hot pressing an intermetallic precursor, Y2.23Al, instead of elemental Y, in combination with excess overall Al and a sub-stoichiometric fraction of carbon, we enhance the yield of (Cr2/3Y1/3)2AlC to 85±3 wt% (86±3 mol%). Both the fractions of the impurity Y2O3 phase and the undesirable ternary Cr2AlC are reduced. Subsequent isothermal oxidation of (Cr2/3Y1/3)2AlC in natural air in the 1000–1400 °C temperature range reveals the formation of Y3Al2(AlO4)3 (YAG), Cr2O3 and Y2O3, without a continuous Cr7C3 sub-layer. Additionally, we show that by starting with Y2.23Al reagent we synthesize dense bulk (W2/3Y1/3)2AlC and (Mo2/3Y1/3)2AlC samples with enhanced i-MAX yields of 72±3 wt% (59±2 mol%) and 91±3 wt% (72±2 mol%), respectively. Oxidation of (Mo2/3Y1/3)2AlC at 1300 °C for 12 h leads to formation of a thick, porous oxide of Y2Mo3O12.

Synthesis and Characterizations of Solid-Solution i-MAX Phase (W <sub>1/3</sub>Mo <sub>1/3</sub>R <sub>1/3</sub>) <sub>2</sub>AIC (R = Gd, Tb, Dy, Ho Er and Y) and Derivated i-MXene with Improved Electrochemical Properties

Synthesis and Characterizations of Solid-Solution i-MAX Phase (W <sub>1/3</sub>Mo <sub>1/3</sub>R <sub>1/3</sub>) <sub>2</sub>AIC (R = Gd, Tb, Dy, Ho Er and Y) and Derivated i-MXene with Improved Electrochemical Properties

Theoretical Prediction and Synthesis of a Family of Atomic Laminate Metal Borides with In-Plane Chemical Ordering.

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 Mo4/3Y2/3AlB2 and Mo4/3Sc2/3AlB2 of space group R3̅m (no. 166), displaying the characteristic in-plane chemical order of Mo and Y/Sc and Kagomé 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 two-dimensional derivatives.

(Keynote) The Rise of MXenes

Numerous compounds, ranging from clays to boron nitride (BN) and transition metal dichalcogenides, have been produced as 2D sheets. Although many of these materials remain subjects of purely academic interest, others have jumped into the limelight due to their attractive properties, which have led to practical applications. Among the latter are carbides and nitrides of transition metals known as MXenes (pronounced “maxenes”), a fast-growing family of 2D materials. The family of 2D transition metal carbides and nitrides (MXenes) has been expanding rapidly since the discovery of Ti3C2 at Drexel University in 2011 [1]. More than 30 different MXenes have been reported, and the structure and properties of numerous other MXenes have been predicted using density functional theory (DFT) calculations [2,3]. Moreover, the availability of solid solutions on M and X sites, control of surface terminations, and the discovery of ordered double-M MXenes (e.g., Mo2TiC2), i-MAX phases and their MXenes offer the potential for producing dozens of new distinct structures.This presentation will describe the state of the art in the field. The manufacturing of MXenes, their delamination into single-layer 2D flakes and assembly into films, fibers and 3D structures will be briefly covered. Synthesis-structure-properties relations of MXenes will be addressed on the example of Ti3C2. The use of MXenes in ceramic- metal- and polymer-matrix composites, smart fibers and textiles will also be discussed. The versatile chemistry of the MXene family renders their properties tunable for a large variety of applications [3-5]. Oxygen or hydroxyl-terminated MXenes, such as Ti3C2O2, have been shown to have redox capable transition metals layers on the surface and offer a combination of high electronic conductivity with hydrophilicity, as well as fast ionic transport [4]. This, among many other advantageous properties, makes the material family promising candidates for energy storage and related electrochemical applications [4], but applications in plasmonics, electrocatalysis, biosensors, water purification/ desalination and other fields are equally exciting.

Theoretical Prediction and Experimental Verification of the Chemically Ordered Atomic-Laminate i-MAX Phases (Cr2/3Sc1/3)2GaC and (Mn2/3Sc1/3)2GaC

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)2GaC and (Mn2/3Sc1/3)2GaC, where the latter compound displays a two-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 (STEM). From density functional theory (DFT) 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.

Theoretical Analysis, Synthesis, and Characterization of 2D W1.33C (MXene) with Ordered Vacancies

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)2AlC and (W2/3Y1/3)2AlC. 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 μm 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.

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 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)2AlC, 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.

Synthesis of (V2/3Sc1/3)2AlC i-MAX phase and V2-xC MXene scrolls.

We report the synthesis and characterization of a new laminated i-MAX phase, (V2/3Sc1/3)2AlC, with in-plane chemical ordering between the M-elements. We also present evidence for the solid solution (V2-xScx)2AlC, where x ≤ 0.05. Chemical etching of the Al and Sc results in a two-dimensional (2D) MXene counterpart: V2-xC from the latter phase. Furthermore, etching with HF yields single-sheet MXene of flat morphology, while LiF + HCl gives MXene scrolls. We also show a 4× reduction in etching time for (V2-xScx)2AlC compared to V2AlC, suggesting that traces of Sc changes the phase stability, and make the material more susceptible to etching. The results show a path for improved control of MXene synthesis and morphology, which may be applicable also for other MAX/MXene systems.

Origin of Chemically Ordered Atomic Laminates ( i-MAX): Expanding the Elemental Space by a Theoretical/Experimental Approach.

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 (Mo2/3 M1/32)2 AC (where M2 = 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)2GaC and (Mo2/3Y1/3)2GaC were experimentally verified, displaying the characteristic in-plane chemical order of Mo and Sc/Y and Kagomé-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.

Electronic structures of iMAX phases and their two-dimensional derivatives: A family of piezoelectric materials

Recently, a group of MAX phases, (Mo$_{2/3}$Y$_{1/3}$)$_2$AlC, (Mo$_{2/3}$Sc$_{1/3}$)$_2$AlC, (W$_{2/3}$Sc$_{1/3}$)$_2$AlC, (W$_{2/3}$Y$_{1/3}$)$_2$AlC, and (V$_{2/3}$Zr$_{1/3}$)$_2$AlC, with in-plane ordered double transition metals, named iMAX phases, have been synthesized. Experimentally, some of these MAX phases can be chemically exfoliated into two-dimensional (2D) single- or multilayered transition metal carbides, so-called MXenes. Accordingly, the 2D nanostructures derived from iMAX phases are named iMXenes. Here, we investigate the structural stabilities and electronic structures of the experimentally discovered iMAX phases and their possible iMXene derivatives. We show that the iMAX phases and their pristine, F, or OH-terminated iMXenes are metallic. However, upon O termination, (Mo$_{2/3}$Y$_{1/3}$)$_2$C, (Mo$_{2/3}$Sc$_{1/3}$)$_2$C, (W$_{2/3}$Y$_{1/3}$)$_2$C, and (W$_{2/3}$Sc$_{1/3}$)$_2$C iMXenes turn into semiconductors. Owing to the absence of centrosymmetry, the semiconducting iMXenes may find applications in piezoelectricity. Our calculations reveal that the semiconducting iMXenes possess giant piezoelectric coefficients as large as 45$\times10^{-10}$~C/m.

Electronic structure, bonding characteristics, and mechanical properties in (W2/3Sc1/3)2AlC and (W2/3Y1/3)2AlC i-MAX phases from first-principles calculations

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.

Theoretical Prediction and Synthesis of (Cr2/3Zr1/3)2AlC i-MAX Phase.

Guided by predictive theory, a new compound with chemical composition (Cr2/3Zr1/3)2AlC was synthesized by hot pressing of Cr, ZrH2, Al, and C mixtures at 1300 °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)2AlC stoichiometry, with secondary Cr2AlC, AlZrC2, and ZrC phases and a small amount of Al-Cr intermetallics. A theoretical evaluation of the (Cr2/3Zr1/3)2AlC magnetic structure was performed, indicating an antiferromagnetic ground state. Also (Cr2/3Hf1/3)2AlC, of the same structure, was predicted to be stable.