M5X4: A Family of MXenes

MXenes are two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides typically synthesized from layered MAX-phase precursors. With over 50 experimentally reported MXenes and a near-infinite number of possible chemistries, MXenes make up the fastest-growing family of 2D materials. They offer a wide range of properties, which can be altered by their chemistry (M, X) and the number of metal layers in the structure, ranging from two in M₂XT_x to five in M₅X₄T_x. Only one M₅X₄ MXene, Mo₄VC₄, has been reported. Herein, we report the synthesis and characterization of two M₅AX₄ mixed transition metal MAX phases, Ti_2.5Ta_2.5AlC₄ and Ti_2.675Nb_2.325AlC₄, and their successful topochemical transformation into Ti_2.5Ta_2.5C₄T_x and Ti_2.675Nb_2.325C₄T_x MXenes. The resulting MXenes were delaminated into single-layer flakes, analyzed structurally, and characterized for their thermal and optical properties. This establishes a family of M₅AX₄ MAX phases and their corresponding MXenes. These materials were experimentally produced based on guidance from theoretical predictions, leading to more exciting applications for MXenes.