Chemical Conversions within the Mo–Ga–C System: Layered Solids with Variable Ga Content

Layered carbides are fascinating compounds due to their enormous structural and chemical diversity, as well as their potential to possess useful and tunable functional properties. Their preparation, however, is challenging and forces synthesis scientists to develop creative and innovative strategies to access high-quality materials. One unique compound among carbides is Mo₂Ga₂C. Its structure is related to the large and steadily growing family of 211 MAX phases that crystallize in a hexagonal structure (space group P6₃/mmc) with alternating layers of edge-sharing M₆X octahedra and layers of the A-element. Mo₂Ga₂C also crystallizes in the same space group, with the difference that the A-element layer is occupied by two A-elements, here Ga, that sit right on top of each other (hence named "221" compound). Here, we propose that the Ga content in this compound is variable between 2:2, 2:1, and 2: ≤1 (and 2:0) Mo/Ga ratios. We demonstrate that one Ga layer can be selectively removed from Mo₂Ga₂C without jeopardizing the hexagonal P6₃/mmc structure. This is realized by chemical treatment of the 221 phase Mo₂Ga₂C with a Lewis acid, leading to the "conventional" 211 MAX phase Mo₂GaC. Upon further reaction with CuCl₂, more Ga is removed and replaced with Cu (instead of fully exfoliating into the Ga-free Mo₂CT_x MXene), leading to Mo₂Ga_1-xCu_xC still crystallizing with space group P6₃/mmc, however, with a significantly larger c-lattice parameter. Furthermore, 211 Mo₂GaC can be reacted with Ga to recover the initial 221 Mo₂Ga₂C. All three reaction pathways have not been reported previously and are supported by powder X-ray diffraction (PXRD), electron microscopy, X-ray spectroscopy, and density functional theory (DFT) calculations.