Phase formation of nanolaminated Mo₂AuC and Mo₂(Au_1−xGa_x)₂C by a substitutional reaction within Au-capped Mo₂GaC and Mo₂Ga₂C thin films

Abstract

Au-containing nanolaminated carbides MoAuC and Mo(AuGa)C were synthesized by a thermally induced substitutional reaction in MoGaC and MoGaC, respectively. The Au substitution of the Ga layers in the structures was observed using cross-sectional high-resolution scanning transmission electron microscopy. Expansion of c lattice parameters was also observed in the Au-containing phases compared to the original phases. Energy dispersive spectroscopy detected residual Ga in Au-substituted layers of both phases with a peculiar Ga in-plane ordering for Au : Ga = 9 : 1 ratio along the Au-Ga layers in Mo(AuGa)C. These results indicate a generalization of the Au substitution reaction for the A elements in MAX phases.

Magnetic properties and structural characterization of layered (Cr0.5Mn0.5)2AuC synthesized by thermally induced substitutional reaction in (Cr0.5Mn0.5)2GaC

“…11 After annealing, the 000l diffraction peaks of the capped (Cr 0.5 Mn 0.5 ) 2 GaC phase are shifted to lower diffraction angles. This observation is similar to previously reported Ga substitution by Au in the Mo 2 GaC phase, 25 indicating the formation of Mo 2 AuC with an expansion in the c-axis (the [000l] direction). We therefore assign the shifted peaks to 000l diffraction peaks of the new nanolaminated phase, (Cr 0.5 Mn 0.5 ) 2 AuC, formed by substituting the Ga atoms in the A layers by Au atoms.…”

“…Substitution of Si, Al, and Ga by Au was observed using scanning transmission electron microscopy (STEM), where the chemical analysis showed the possibility of both partial substitution and full substitution. [23][24][25] Along with the discovery of a new A element and additional members of the MAX phase family, it was also shown that the Ti 3 Au 2 C 2 phase can further react with an Ir capping layer, forming yet another nanolaminated phase, Ti 3 IrC 2 . This points towards a fully reversible intercalation of Au in the MAX phase, with Au diffusing in and out of the A layer without any substantial change of the metal carbide, e.g., Ti 3 C 2 , layers.…”

“…The residual Ga signal, however, can still be detected in the (Cr 0.5 Mn 0.5 ) 2 AuC crystal, similarly to the previously reported Mo 2 AuC phase formed by annealing of an Au-capped Mo 2 GaC thin film. 25 Still, the intensity of the Ga signal is on the level of other non-film-forming impurities, e.g., Al, likely originating from sample preparation. Excess Ga likely diffuses to the surface to form oxides, similar to observations made for Si and Al.…”

“…19 Recently, several new MAX phase and related nanolaminated carbides have been reported, such as Ti 3 AuC 2 , Ti 3 Au 2 C 2 , Mo 2 AuC, and Mo 2 Au 2 C, synthesized by solid state reaction between the Au capping layers and the underlying nanolaminated carbide thin films (e.g., Ti 3 SiC 2 and Mo 2 Ga 2 C) at elevated temperature. [23][24][25] After the annealing, the interplanar spacing of the carbides expanded along the c-axis (denoted as out-of-plane direction in the following discussion). Substitution of Si, Al, and Ga by Au was observed using scanning transmission electron microscopy (STEM), where the chemical analysis showed the possibility of both partial substitution and full substitution.…”

Ti2Au2C and Ti3Au2C2 formed by solid state reaction of gold with Ti2AlC and Ti3AlC2

“…These phases can also be exfoliated to form a recently established class of two-dimensional materials labelled MXene. [11][12][13][14][15][16] The introduction of Au in M n+1 AX n phases by hightemperature-induced exchange can be used for Ga-containing MAX phases (Mo 2 GaC and Mo 2 Ga 2 C 17,18 ) to synthesize Mo 2 AuC and Mo 2 (Au 1Àx Ga x ) 2 through an Au substitution reaction with Ga. 19 The question remains whether this mechanism can be generally applied to other M n+1 AX n phases, and beyond. We chose thin films of Ti 2 AlC and Ti 3 AlC 2 as model system to address this question.…”