Synthesis of MAX phases Nb2CuC and Ti2(Al0.1Cu0.9)N by A-site replacement reaction in molten salts

ABSTRACT New MAX phases Ti2(AlxCu1−x)N and Nb2CuC were synthesized by A-site replacement by reacting Ti2AlN and Nb2AlC, respectively, with CuCl2 or CuI molten salt. X-ray diffraction, scanning electron microscopy, and atomically resolved scanning transmission electron microscopy showed complete A-site replacement in Nb2AlC, which lead to the formation of Nb2CuC. However, the replacement of Al in Ti2AlN phase was only close to complete at Ti2(Al0.1Cu0.9)N. Density-functional theory calculations corroborated the structural stability of Nb2CuC and Ti2CuN phases. Moreover, the calculated cleavage energy in these Cu-containing MAX phases are weaker than in their Al-containing counterparts. GRAPHICAL ABSTRACT IMPACT STATEMENT The preparation of MAX phases Nb2CuC and Ti2(Al0.1Cu0.9)N were realized by A-site replacement in Ti2AlN and Nb2AlN, respectively.


Introduction
The MAX phases constitute a family of ternary compounds with a hexagonal structure (space group P63/mmc, 194) and a molecular formula of Mn+1AXn, where M is an early transition metal, A mainly comes from groups 13-16, X is carbon and/or nitrogen, and n=1-3 [1,2].The MAX phases have potential applications in hightemperature electrodes, components with resistance to friction and wear, structural material in nuclear fuel cladding, and as precursor material for two-dimensional MXene [2][3][4].By now, more than 80 members of ternary MAX compositions have been discovered [5].Recent studies have also demonstrated that the A-site element in MAX phases can be a late transition metal (e.g., Au, Ir, Zn, Fe and Cu) [6][7][8][9][10][11][12][13].Transition metals have distinct properties different from other A-group elements due to their large d electron orbits.If late-transition metal elements can be introduced into the A layer of the MAX phase through a replacement reaction, there would be further prospects for tailoring the functionality of MAX phases.
In 2017, Ti3AuC2 and Ti3Au2C2 were synthesized by replacing Si with Au in Ti3SiC2, and Ti3IrC2 was identified by replacing Au with Ir in obtained Ti3Au2C2 [7].Recently, our group reported a series of new Zn-containing MAX phases (Ti3ZnC2, Ti2ZnC, Ti2ZnN, and V2ZnC) obtained by a replacement reaction between MAX phase precursors and ZnCl2 molten salt [10].In these phases, Zn atoms occupy the original Al position at the A site in the MAX phase structure.The key merit of this A-site replacement strategy is the prevention of competitive phases (such as M-Zn alloys) that can have lower Gibbs free energies than these new MAX phases and would thus be thermodynamically favored.In this synthesis methodology, the redox reaction between Al and Zn 2+ and simultaneous evaporation of AlCl3 accounts for the main driving force.Since Cu 2+ cations have higher oxidation potential than Zn 2+ cations, Cu atoms have 3 / 14 also been incorporated into Ti3AlC2, partially occupying the Al in resultant Ti3(Al1/3Cu2/3)C2 MAX phase through a similar replacement approach [14].The partial substitution behavior of Cu in Ti3(Al1/3Cu2/3)C2 was explained according to the Cu-Al binary phase diagram, in which intermediate Cu-Al alloys are in equilibrium with Al metal below the solidus line.When part of the Al is consumed in a redox reaction and driven out in the form of AlCl3, Cu and residual Al atoms occupy the A layer of asformed Ti3(Al1/3Cu2/3)C2.Although binary phase diagrams (Au-Si, Al-Zn, Al-Cu) have been used to describe the substitution/replacement behavior in these new MAX phases, the atom stacking or mutual atomic interaction in two-dimensional single-atomic A layer should be different from that in three-dimensional materials.
In order to expand this replacement strategy to other novel MAX phases, studies are needed on the incorporation of Cu into a range of MAX phases.Here, Nb2CuC and Ti2(Al0.1Cu0.9)Nwere synthesized by Cu-substitution for Al in Nb2AlC and Ti2AlN phases by reaction with CuI and CuCl2 molten salts.

Preparation of Ti2AlN and Nb2AlC
As in previous work, TiN/Ti/Al/NaCl/KCl powder mixture with a mole ratio of 1: 1: 1: 4: 4, and NbC/Nb/Al powder mixture with mole ratio of 1:1:1 was sintered in order to synthesize Ti2AlN and Nb2AlC MAX phase powders.For more details, refer to the Supplementary Information.

Preparation of Ti2(AlxCu1-x)N and Nb2CuC
The Ti2AlN powders were mixed with CuCl2 in stoichiometric molar ratios of 2:3 for Ti2(AlxCu1-x)N.The Nb2AlC and CuI (molar ratio=1:3) were used as the starting material to synthesize Nb2CuC.The material mixtures was heated in tube furnace to 600C at a rate of 2C/min for 7 h under the protection of argon, then cooled down to room temperature at a rate of 5C/min.Ammonium persulfate solution was used to remove the residual Cu in the reaction process.Finally, the product was filtered, washed, and dried at 50C.

Characterization and density functional theory calculations
The phase composition of the samples was analyzed by X-ray diffraction (XRD) with Cu K radiation.The microstructure and chemical composition were obtained in scanning electron microscopy with an energy-dispersive spectrometer (EDS).
Atomically-resolved structural analysis was also carried out by high-resolution scanning transmission electron microscopy (HRSTEM) capable of high angle annular dark field (HAADF) imaging and EDS Density functional theory (DFT) calculations were in the in the CASTEP code [15,16], using the generalized gradient approximation (GGA) as implemented in the Perdew-Breke-Ernzerhof (PBE) functional [17,18].Phonon calculations were carried out to evaluate the dynamical stability using the finite displacement approach, as implemented in CASTEP [19,20].The equation E=(Ebroken−Ebulk)/S [10] was adopted to calculate the cleavage energy E. In this equation, Ebulk and Ebroken represent the total energies of bulk MAX and the cleaving structures with a 10 Å vacuum separation in the corresponding M and A atomic layers, while S is the cross-sectional surface area of the MAX phase materials.
More details are provided in the Supplementary Information.The relative atomic ratio of (Al:Cu) at.% is about (1:9) at.% as identified by STEM-EDS, consistent with SEM-EDS results.Therefore, all above characterization results indicates that the Cu-incorporated MAX phase has a chemical formula of Ti2(Al0.1Cu0.9)N.The low amount of Al in Ti2(Al0.1Cu0.9)N is noteworthy.In our recent work on Ti3AlC2, only partial substitution of Cu in Ti3(Al1/3Cu2/3)C2 was achieved.Thus, the same reaction in other MAX phases should be investigated to understand the underlying mechanism.Figure 2 metal during the replacement reaction between Al (derived from Nb2AlC) and CuI.In addition, it can be observed that main the diffraction patterns of the Nb2AlC MAX phase and product after Cu replacement are similar, but the (002) diffraction peak of the Cu-incorporated MAX phase became significantly weaker, as in the case of Ti2(Al0.1Cu0.9)N.On the contrary, the (006) peak became stronger, which means that the Cu substitution in between Nb2C layers changes the stacking of atoms perpendicular to c plane [6,7,11].To confirm the lattice parameters of the resultant product, Rietveld refinement of the XRD pattern was carried out assuming phase-pure Nb2CuC, as shown 8 / 14 in Figure 2(b).The simulated pattern, with a reliability factor Rwp of 7.88%, is in good agreement with the experimental data.The previously reported lattice parameters of the Nb2AlC are a = 3.106 Å and c = 13.888Å [22], whereas the calculated lattice parameters of Nb2CuC are a = 3.153 Å and c = 13.587Å.The atomic positions of Nb2CuC determined from the Rietveld refinement are listed in Table 1.In order to further determine the structure of Nb2CuC, STEM was performed.The incorporation of transition elements (Zn, Cu) into A site of MAX phase has been discussed in our previous reports where chloride salts were used [10,14].Here, we used an alternative molten salt CuI, which has a melting point of 600°C.At 700°C, CuI is molten and ions of Cu + and I -can contact solid reactants [23].As a strong electron acceptor or Lewis acid [24,25]  which is why Ti3(Al1/3Cu2/3)C2 was formed in earlier work [14].Previous reports also indicated this alloying behavior of Cu into A site (Si [26] or Al [27]

Conclusion
In summary, the new MAX phases of Ti2(Al0.1Cu0.9)Nand Nb2CuC were synthesized by A-site replacement reaction in molten salt environment.Complete or partial occupancy of Cu at the original Al site in the final MAX phases was demonstrated

Figure 1 (
Figure 1(d-f) show STEM images of the resulting phase.The atomic positions perpendicular to the [112 _

Figure 2 .
Figure 2. (a) XRD patterns of the Nb2AlC and the Nb2CuC obtained from the reaction (a) shows XRD patterns of the raw Nb2AlC and the final product obtained through the same replacement methodology in CuI molten salt.Before treatment by ammonium persulfate solution, the characteristic peaks of Cu (2≈43, 2≈51 and 2≈75) are detected (Figure S(3)), which indicates the generation of Cu

Figure 3 .
Figure 3. (a) SEM image of Nb2AlC.(b) SEM image of the Nb2CuC obtained from the

Figure 4 (
a-b) show the atomic arrangements with the beam aligned along the [, respectively.As can be observed in both images, one darker layer (the A elements) interleaves two adjoining brighter layers of Nb with larger atomic mass.The presented images are similar to STEM images of other M2AX phases having the characteristic zig-zag stacking of the 211 Mn+1Xn layers [6,12].In Figure 4(c), the atomic-resolved EDS element mapping and line-scan of Nb-Lα and Cu-Kα further identified the atom position of Cu, all at A sites, corroborating the synthesis of Nb2CuC 10 / 14 MAX phase through a replacement reaction, to form a MAX phase with only Cu on the A site.
calculated lattice parameters of Ti2CuN and Nb2CuC show reduced c values compared to Ti2AlN and Nb2AlC, respectively, consistent with our experimental results.However, the experimental a value of Ti2(Al0.1Cu0.9)Nconflicts with the calculated a value, which may be due to the fact that A layers are not completely replaced.The large decrease in elastic constant C44 and shear modulus G indicate enhanced plasticity in the Cucontaining MAX phases.The phonon dispersion relations are plotted in Figure S4.The vibrational frequencies have no imaginary component, showing that all these MAX phases are dynamically stable.The variation of cleavage energy in the M-Al and M-Cu atomic layers suggests weaker bonding between M and Cu atoms, which implies that these Cu-containing MAX phases may be useful as precursors for two-dimensional MXene materials.
elements).However, the present work indicates that this reasoning based on phase-diagram-guidelines is incomplete, since an end member with only Cu on A sites (Nb2CuC) and one with very high Cu content (Ti2(Al0.1Cu0.9)N)were formed.The atomic arrangement in bulk materials, accurately predicted in phase diagrams, will change in a confined space due to the different crystal field strength exerted by other components.In the nanolaminated MAX-phase crystal structure, A atoms have relatively weak bonding with the nearest M atoms and negligible bonding with the next A layer.When an A atom (here Cu) occupies the original A-atom position, the crystal field exerted by M atoms modifies the arrangement of A in A atomic sites, which may explain why full replacement of A atoms can be achieved in the present work despite the thermodynamic tendency to form a Cu-Al mixture.

Table 1 .
Atomic positions in Nb2CuC determined from the Rietveld refinement.

Table 2
The lattice parameters, elastic constants and cleavage energies of Ti/Nb and Al/Cu atomic layer in the MAX phases are listed in Table2.The [7,10]ten salt, Cu + oxidizes Al a drive it out from Nb2AlC.The Al 3+ cation is then coordinated with Cl -to form AlCl3 which evaporates (boiling point ~360°C).However, the occupancy of copper in the final products is not simply determined by the phase diagram.In the Au-Si and Al-Zn binary phase diagrams, two end members of metal components are separated below the solidus line, which provided a predictable guideline for synthesis of new MAX phases, such as Ti3AuC2 and Mn+1ZnXn (M=Ti or V; X=C or N, n=1 or 2)[7,10].In contrast, based on the Cu-Al binary phase diagram, it does not appear possible to obtain a single-phase Cu end member from the Al-rich side, since equilibrium intermediate Al-Cu alloys should form,