Trends in MXene structure due to elemental composition: A DFT study

MXenes are a tunable class of materials (M_n+1C_nT_z or M_n+1N_nT_z), where the transition metal (M) and termination group (T) can be modulated to change the material’s properties. Density functional theory (DFT) predictions are fundamental in understanding MXene properties and selecting the optimal composition for a given application. For years, MXene synthesis required a fluorinated solution, which limited termination groups to F, O, and OH. Recent developments in fluorine-free synthesis, however, have led to the discovery of more than six new termination groups. We present a DFT analysis of 90 MXenes, including 9 transition metals and 9 termination groups, the majority of which have not been previously studied. It was presumed that MXenes retain the trigonal symmetry of their parent materials, but recent studies showed that some MXenes are more stable with hexagonal symmetry. We consider a total of 8 possible structures for each MXene- four terminal group locations for each of the two symmetry types. We are the first to report on both preferred symmetry and termination group location for this many MXenes. Our results show that structural preferences of MXenes are highly dependent on the elemental properties of each metal atom and termination group, allowing us to make generalizations and predictions of how a given element may impact the structure of any MXene.