Detailed Analysis of the Synthesis and Structure of MAX Phase (Mo 0.75 V 0.25 ) 5 AlC 4 and Its MXene Sibling (Mo 0.75 V 0.25 ) 5 C 4 .

MAX phases with the general formula M n +1 AX n are layered carbides, nitrides, and carbonitrides with varying stacking sequence of layers of M 6 X octahedra and the A element depending on n . While "211" MAXphases ( n = 1) are very common, MAX phases with higher n , especially n ≥ 3, have hardly been prepared. This work addresses open questions regarding the synthesis conditions, structure, and chemical composition of the "514" MAX phase. In contrast to literature reports, no oxide is needed to form the MAX phase, yet multiple heating steps at 1,600 °C are required. Using high-resolution X-ray diffraction, the structure of (Mo 1- x V x ) 5 AlC 4 is thoroughly investigated, and Rietveld refinement suggests P -6 c 2 as the most fitting space group. SEM/EDS and XPS show that the chemical composition of the MAX phase is (Mo 0.75 V 0.25 ) 5 AlC 4 . It was also exfoliated into its MXene sibling (Mo 0.75 V 0.25 ) 5 C 4 using two different techniques (using HF and an HF/HCl mixture) that lead to different surface terminations as shown by XPS/HAXPES measurements. Initial investigations of the electrocatalytic properties of both MXene versions show that, depending on the etchant, (Mo 0.75 V 0.25 ) 5 C 4 can reduce hydrogen at 10 mA cm -2 with an overpotential of 166 mV (HF only) or 425 mV (HF/HCl) after cycling the samples, which makes them a potential candidate as an HER catalyst.