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 M 2 XT x to five in M 5 X 4 T x . Only one M 5 X 4 MXene, Mo 4 VC 4 , has been reported. Herein, we report the synthesis and characterization of two M 5 AX 4 mixed transition metal MAX phases, Ti 2.5 Ta 2.5 AlC 4 and Ti 2.675 Nb 2.325 AlC 4 , and their successful topochemical transformation into Ti 2.5 Ta 2.5 C 4 T x and Ti 2.675 Nb 2.325 C 4 T 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 M 5 AX 4 MAX phases and their corresponding MXenes. These materials were experimentally produced based on guidance from theoretical predictions, leading to more exciting applications for MXenes.