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 MoGaC. Its structure is related to the large and steadily growing family of 211 MAX phases that crystallize in a hexagonal structure (space group 6/) with alternating layers of edge-sharing octahedra and layers of the -element. MoGaC also crystallizes in the same space group, with the difference that the element layer is occupied by two -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 MoGaC without jeopardizing the hexagonal 6/ structure. This is realized by chemical treatment of the 221 phase MoGaC with a Lewis acid, leading to the "conventional" 211 MAX phase MoGaC. Upon further reaction with CuCl, more Ga is removed and replaced with Cu (instead of fully exfoliating into the Ga-free MoC MXene), leading to MoGaCuC still crystallizing with space group 6/, however, with a significantly larger -lattice parameter. Furthermore, 211 MoGaC can be reacted with Ga to recover the initial 221 MoGaC. 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.