M 5 X 4 : 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 MXT to five in MXT. Only one MX MXene, MoVC, has been reported. Herein, we report the synthesis and characterization of two MAX mixed transition metal MAX phases, TiTaAlC and TiNbAlC, and their successful topochemical transformation into TiTaCT and TiNbCT 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 MAX MAX phases and their corresponding MXenes. These materials were experimentally produced based on guidance from theoretical predictions, leading to more exciting applications for MXenes.