Origin of Chemically Ordered Atomic Laminates (<i>i</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>i</i>-MAX phases. By probing (Mo_2/3<i>M</i>_1/3²)₂<i>A</i>C (where <i>M</i>² = Sc, Y and <i>A</i> = Al, Ga, In, Si, Ge, In), we predict seven stable <i>i</i>-MAX phases, five of which should have a retained stability at high temperatures. (Mo_2/3Sc_1/3)₂GaC and (Mo_2/3Y_1/3)₂GaC were experimentally verified, displaying the characteristic in-plane chemical order of Mo and Sc/Y and Kagomé-like ordering of the <i>A</i>-element. We suggest that the formation of <i>i</i>-MAX phases requires a significantly different size of the two metals, and a preferable smaller size of the <i>A</i>-element. Furthermore, the population of antibonding orbitals should be minimized, which for the metals herein (Mo and Sc/Y) means that <i>A</i>-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>i</i>-MAX phases and expect more phases to be synthesized, which will realize useful properties. Furthermore, based on <i>i</i>-MAX phases as parent materials for 2D MXenes, we also expect that the range of MXene compositions will be expanded